Data See, Data Do

A Custom Storyboard Component in Xaml

Mike Hillberg [MSFT] — Fri, 20 Nov 2009 23:19:00 GMT

In WPF & Silverlight, a Storyboard is a collection of animations running in parallel. Not everyone likes the name “Storyboard” though. The idea behind the name was that that list of timelines running in parallel are like a list of plot lines in the plan for a movie.

In any case, just like you can package up a tree of elements into a re-usable UserControl component, it’s nice to be able to package up a tree of timelines into a re-usable Storyboard component. Here’s an approach to do just that, using Xaml, and made simpler by using a new Xaml feature in WPF4.

For example, here’s a Storyboard that puts three animations on a Rectangle:

<Grid>

    <Grid.Resources>
        <BackEase Amplitude="0.5" x:Key="backEase" />
    </Grid.Resources>

    <Grid.Triggers>
        <EventTrigger RoutedEvent="Grid.Loaded">
            <BeginStoryboard>
                <Storyboard TargetName="Rectangle1">
                    <DoubleAnimation Storyboard.TargetProperty="Width"
                                     From="0"
                                     EasingFunction="{StaticResource backEase}"
                                     Duration="0:0:1" />

                    <DoubleAnimation Storyboard.TargetProperty="Height"
                                     From="0"
                                     EasingFunction="{StaticResource backEase}"
                                     Duration="0:0:1" />

                    <ColorAnimation Storyboard.TargetProperty="Fill.Color"
                                    From="LightGray"
                                    Duration="0:0:1" />
                </Storyboard>

            </BeginStoryboard>
        </EventTrigger>
    </Grid.Triggers>

    <Rectangle Name="Rectangle1" Width="100" Height="100" Fill="Red" />

</Grid>

The goal is to package up this Storyboard into a re-usable component that I can use on any Rectangle that has a SolidColorBrush Fill. To do that, I’ll just move that <Storyboard> markup into a separate Xaml file and parameterize it.

Unfortunately there’s no single menu item for this e.g. in Visual Studio, but it’s not hard to do. What I do is right click on the project, “Add”, “User Control”, name it something like “MyRectangleStoryboard.xaml”, and get something like this:

<UserControl x:Class="CustomStoryboard.MyRectangleStoryboard"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >
    <Grid>
            
    </Grid>
</UserControl>

… and this:

public partial class MyRectangleStoryboard : UserControl
{
    public MyRectangleStoryboard()
    {
        InitializeComponent();
    }
}

Now, just change “UserControl” to “Storyboard (and remove the <Grid> from the Xaml), and you’re all set:

<Storyboard x:Class="CustomStoryboard2.MyRectangleStoryboard"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >
    
</Storyboard>

public partial class MyRectangleStoryboard : Storyboard
{
    public MyRectangleStoryboard()
    {
        InitializeComponent();
    }
}

The goal is to use this component instead of the storyboard/animation markup:

<Grid>
    <Grid.Resources>
        <BackEase Amplitude="0.5" x:Key="backEase" />
    </Grid.Resources>

    <Grid.Triggers>
        <EventTrigger RoutedEvent="Grid.Loaded">
            <BeginStoryboard>
                <l:MyRectangleStoryboard 
                             TargetName="Rectangle1" 
                             Duration="0:0:1" 
                             EasingFunction="{StaticResource backEase}"/>
            </BeginStoryboard>
        </EventTrigger>
    </Grid.Triggers>

    <Rectangle Name="Rectangle1" Width="100" Height="100" Fill="Red" />

</Grid>

As for the implementation of the MyRectangleStoryboard here’s the markup, which is cut/paste from the original with a couple of modifications for parameterization (which I’ll explain):

<Storyboard x:Class="CustomStoryboard.MyRectangleStoryboard"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             xmlns:l="clr-namespace:CustomStoryboard" >

    <DoubleAnimation Storyboard.TargetProperty="Width"
                     From="0"
                     Duration="{Binding Duration, Source={l:XRoot}}"
                     EasingFunction="{Binding EasingFunction, Source={l:XRoot}}" />

    <DoubleAnimation Storyboard.TargetProperty="Height"
                     From="0"
                     Duration="{Binding Duration, Source={l:XRoot}}"
                     EasingFunction="{Binding EasingFunction, Source={l:XRoot}}" />

    <ColorAnimation Storyboard.TargetProperty="Fill.Color"
                    From="LightGray"
                    Duration="{Binding Duration, Source={l:XRoot}}" />

    
</Storyboard>

Notice that this custom storyboard is parameterized for the setting of the Duration and the EasingFunction properties on the animations. (Recall that these two properties were set when the MyRectangleStoryboard was used above).

Ordinarily, if you set the Duration on the storyboard it doesn’t inherit down to its children timelines, instead it clips its children timelines. But I want the MyRectangleStoryboard.Duration to actually be the Double/ColorAnimation.Duration, so I bound the Animation.Duration properties to the root’s Duration property.

The source of this binding is a custom “XRoot” markup extension that I wrote. You could equivalently establish this binding in code, and in fact you pretty much have to today. But in WPF4 (which is now in beta), you can write a custom MarkupExtension to access the root of a Xaml document, using the new IRootObjectProvider service. Here’s the implementation:

public class XRoot : MarkupExtension
{
    public override object ProvideValue(IServiceProvider serviceProvider)
    {
        var rootProvider = serviceProvider.GetService(typeof(IRootObjectProvider)) as IRootObjectProvider;
        if (rootProvider == null)
            return null;

        return rootProvider.RootObject;
    }
}

The other parameterization on this MyRectangleStoryboard component is the EasingFunction. This is a custom property on MyRectangleStoryboard so that I can set it any time I use it. The implementation is just a DependencyProperty:

public partial class MyRectangleStoryboard : Storyboard
{
    public MyRectangleStoryboard()
    {
        InitializeComponent();
    }

    public EasingFunctionBase EasingFunction
    {
        get { return (EasingFunctionBase)GetValue(EasingFunctionProperty); }
        set { SetValue(EasingFunctionProperty, value); }
    }

    public static readonly DependencyProperty EasingFunctionProperty =
        DependencyProperty.Register("EasingFunction", typeof(EasingFunctionBase), typeof(MyRectangleStoryboard));

}

So in the end there’s three takeaways from this:

You can create powerful custom animation sets by subclassing Storyboard.
Xaml is a useful component definition language, it’s not just for controls.
In WPF4, Xaml markup extensions can get explicit access to the lexical root of the Xaml document.

TreeView and HierarchicalDataTemplate, Step-by-Step

Mike Hillberg [MSFT] — Fri, 30 Oct 2009 19:40:00 GMT

I’ve never found TreeView to be terribly confusing by itself. But usually I want to data bind a TreeView to a collection with some hierarchy, which leads me to HierarchicalDataTemplate, which didn’t always just write itself for me. If you look at it in steps, though, there really is a pretty nice progression from ListBox to TreeView. Like a lot of my posts, this goes way too deep if you just want to create a quick TreeView. But it’s useful to look at it if you want the HierarchicalDataTemplate to just write itself …

First, start with the example of a simple ListBox:

<sys:String>Western Conference</sys:String>

<sys:String>Eastern Conference</sys:String>

</ListBox>

We can add a trivial ItemTemplate to it:

<ListBox.ItemTemplate>

</DataTemplate>

</ListBox.ItemTemplate>

<sys:String>Western Conference</sys:String>

<sys:String>Eastern Conference</sys:String>

</ListBox>

… and now the list box items are red.

A ListBox is an ItemsControl, and what ItemsControls like to do is wrap their items in a container (more on that here). So the above essentially becomes:

<ListBoxItem.ContentTemplate>

</DataTemplate>

</ListBoxItem.ContentTemplate>

</ListBoxItem>

<ListBoxItem Content="Western Conference"

ContentTemplate="{Binding ElementName=_template1}" />

</ListBox>

That is, for each item in the ListBox, a ListBoxItem is created. The ListBoxItem’s Content property is bound to the item, and the ListBoxItem’s ContentTemplate property is bound to the ListBox’s ItemTemplate.

On to TreeView …

A TreeView is also an ItemsControl; it generates TreeViewItem controls for its items. And actually, if you just take the original example and change “ListBox” to “TreeView”, it looks almost the same:

<sys:String>Eastern Conference</sys:String>

<sys:String>Western Conference</sys:String>

</TreeView>

You can similarly set an explicit item template:

<TreeView.ItemTemplate>

</DataTemplate>

</TreeView.ItemTemplate>

<sys:String>Western Conference</sys:String>

<sys:String>Eastern Conference</sys:String>

</TreeView>

The next step here caught be by surprise when I first saw it. The TreeView is going to create a TreeViewItem for each of its items (the two strings). A TreeViewItem is a HeaderedItemsControl, which is an ItemsControl (just like ListBox and TreeView), but also has a Header (and HeaderTemplate) property. (The Header is the part of the tree view item that you always see, right next to the expand/collapse button.) The Header/HeaderTemplate of a HeaderedItemsControl is analogous to the Content/ContentTemplate of a ContentControl. So, whereas the ListBox items were bound to each ListBoxItem’s Content property, in the TreeView case the items are bound to the TreeViewItem’s Header property. Similarly, the TreeView’s ItemTemplate property is bound to the TreeViewItem’s HeaderTemplate property. And in the end we essentially have:

<TreeViewItem.HeaderTemplate>

</DataTemplate>

</TreeViewItem.HeaderTemplate>

</TreeViewItem>

<TreeViewItem Header='Eastern Conference'

HeaderTemplate='{Binding ElementName=_template2}' />

</TreeView>

Now what we need is some hierarchy. Those items are the MLS conferences, and there’s supposed to be teams in those conferences. Here’s the full data:

var western = new Conference("Western")

{

Teams =

{

new Team("Club Deportivo Chivas USA"),

new Team("Colorado Rapids"),

new Team("FC Dallas"),

new Team("Houston Dynamo"),

new Team("Los Angeles Galaxy"),

new Team("Real Salt Lake"),

new Team("San Jose Earthquakes"),

new Team("Seattle Sounders FC"),

new Team("Portland 2011"),

new Team("Vancouver 2011")

}

};

var eastern = new Conference("Eastern")

{

Teams =

{

new Team("Chicago Fire"),

new Team("Columbus Crew"),

new Team("D.C. United"),

new Team("Kansas City Wizards"),

new Team("New York Red Bulls"),

new Team("New England Revolution"),

new Team("Toronto FC"),

new Team("Philadelphia Union 2010")

}

};

var league = new Collection<Conference>() { western, eastern };

DataContext = new

{

WesternConference = western,

EasternConference = eastern,

League = league

};

(Note that the DataContext now has this sample data.)

And now we can show all the teams with some explicit hierarchy:

<TreeViewItem Header='Western Conference'

ItemsSource="{Binding WesternConference.Teams}">

<TreeViewItem.ItemTemplate>

</DataTemplate>

</TreeViewItem.ItemTemplate>

</TreeViewItem>

<TreeViewItem Header='Eastern Conference'

ItemsSource="{Binding EasternConference.Teams}">

<TreeViewItem.ItemTemplate>

</DataTemplate>

</TreeViewItem.ItemTemplate>

</TreeViewItem>

</TreeView>

Of course, what you really want to do is bind the TreeView itself to the hierarchical collection (the League of the DataContext). I.e.:

As you can see, there’s two items, as you’d expect, just like a ListBox. Also just like a ListBox, it doesn’t show anything except for the ToString() of the Conference object. So we need to give it an ItemTemplate to show the conference Name:

<TreeView.ItemTemplate>

</DataTemplate>

</TreeView.ItemTemplate>

</TreeView>

Now, recall that the TreeView here is creating two TreeViewItems, binding the TreeViewItem’s Header to the Conference object, and setting the TreeViewItem’s HeaderTemplate to the TreeView’s ItemTemplate. Next question is, how do we get the TreeViewItem’s ItemsSource bound to Conference.Teams? That’s where the HierarchicalDataTemplate comes in.

A HierarchicalDataTemplate is a DataTemplate with some extra properties. But if you don’t use the extra properties, it’s no different than DataTemplate. For example, change the last markup from DataTemplate to HierarchicalDataTemplate, and nothing changes:

<TreeView.ItemTemplate>

</HierarchicalDataTemplate>

</TreeView.ItemTemplate>

</TreeView>

But HierarchicalDataTemplate adds two key properties: ItemsSource and ItemTemplate. The ItemsSource gets mapped to the TreeViewItem.ItemsSource, and the ItemTemplate gets mapped to the TreeViewItem.ItemTemplate. So now we can show the conferences and the teams:

<TreeView.ItemTemplate>

<HierarchicalDataTemplate.ItemTemplate>

</DataTemplate>

</HierarchicalDataTemplate.ItemTemplate>

</HierarchicalDataTemplate>

</TreeView.ItemTemplate>

</TreeView>

And if you want to show one level deeper in the hierarchy, you can change that team DataTemplate to a HierarchicalDataTemplate:

<TreeView.ItemTemplate>

<HierarchicalDataTemplate.ItemTemplate>

<HierarchicalDataTemplate.ItemTemplate>

</DataTemplate>

</HierarchicalDataTemplate.ItemTemplate>

</HierarchicalDataTemplate>

</HierarchicalDataTemplate.ItemTemplate>

</HierarchicalDataTemplate>

</TreeView.ItemTemplate>

</TreeView>

In the end, the bottom line that I keep in mind when I’m writing a HierarchicalDataTemplate, is that it’s the template for the TreeViewItem.Header, and it lets me set the TreeViewItem’s ItemsSource and ItemTemplate properties.

Why is it called a "DependencyProperty"?

Mike Hillberg [MSFT] — Fri, 27 Mar 2009 20:04:00 GMT

When we create new classes and members we spend a lot of time and effort to make them as usable, understandable, and discoverable as possible. We follow the .Net Design Guidelines in general, and in particular we constantly look at how this new class relates to other classes, future plans, etc.

So we went through all that when naming DependencyProperty and DependencyObject. All told we probably spent hours just on the name. What dependency properties (DPs) boil down to in the end is property calculation and dependency tracking. Property calculation isn’t terribly distinctive, lots of properties do that, so the essence of a DP is really the dependency tracking, thus “Dependency” properties.

Here’s an example of that, actually one piece of sample code that shows several examples of dependency tracking:

<StackPanel.Resources>

<Style.Triggers>

</Trigger>

</Style.Triggers>

</Style>

</StackPanel.Resources>

</StackPanel>

The properties of this TextBlock have quite a few dependencies:

· TextBlock.Text is dependent on the Binding, and the Binding is dependent on the DataContext in this case. The DataContext is inheriting down from the StackPanel, so the Text property is therefore also dependent on the shape of the tree; if the TextBlock is removed from the StackPanel, it’s Text property will update.

· TextBlock.FontSize is dependent on the tree as well. Here, you can see it’s inheriting from the StackPanel.

· All of the TextBlock properties depend on TextBlock.Style. For example, here TextBlock.FontWeight is coming from the Style.

· Similarly, TextBlock.Background depends on the Style, but here it’s being set in trigger. So TextBlock.Background in this case also depends on TextBlock.IsMouseOver.

Sometimes, if you write your own DP, you need to help with the dependency tracking. You do this by calling InvalidateProperty when a property needs to be re-calculated, usually because you reference it in a CoerceValueCallback.

For example, here’s a DP Foo and a companion (read-only) DP named FooPlus1. FooPlus1 just has a CoerceValueCallback that calculates “Foo+1”. Foo therefore has a PropertyChangedCallback that invalidates FooPlus1 whenever Foo changes.

// Foo property

public int Foo

{

get { return (int)GetValue(FooProperty); }

set { SetValue(FooProperty, value); }

}

public static readonly DependencyProperty FooProperty =

DependencyProperty.Register("Foo", typeof(int), typeof(Window1),

new PropertyMetadata(FooChangedCallback));

static void FooChangedCallback(DependencyObject d, DependencyPropertyChangedEventArgs args)

{

// Whenever Foo changes, we need to invalidate FooPlus1, so that

// the DependencyProperty system knows to update it (call its

// CoerceValueCallback again).

(d as Window1).InvalidateProperty(Window1.FooPlus1Property);

}

// FooPlus1 Property

public int FooPlus1

{

get { return (int)GetValue(FooPlus1Property); }

}

static readonly DependencyPropertyKey FooPlus1PropertyKey =

DependencyProperty.RegisterReadOnly("FooPlus1", typeof(int), typeof(Window1),

new PropertyMetadata(0, null, CoerceFooPlus1Callback));

static readonly DependencyProperty FooPlus1Property = FooPlus1PropertyKey.DependencyProperty;

static object CoerceFooPlus1Callback(DependencyObject d, object baseValue)

{

return (d as Window1).Foo + 1;

}

Implementing SelectedValue with the Silverlight ComboBox

Mike Hillberg [MSFT] — Fri, 27 Mar 2009 05:53:00 GMT

Here’s the scenario … You have a Customers collection and an Orders collection. In the Orders collection, an Order has a CustomerID property; this is the key to an item in the Customers collection. Your goal is a ComboBox that updates the CustomerID property of a Customer, but interacts with the end user in terms of customer names. If that doesn’t make sense, code & pictures help …

Here’s how you can accomplish this in markup in WPF (the l:Collection here is just a Collection<object>):

<Grid>

<Grid.Resources>

<l:Collection x:Key="Customers">

<l:Customer ID="1" CustomerName="Wilma" />

<l:Customer ID="2" CustomerName="Betty" />

<l:Customer ID="3" CustomerName="Fred" />

<l:Customer ID="4" CustomerName="Barney" />

</l:Collection>

<l:Collection x:Key="Orders">

<l:Order Description="Magic carpet" CustomerID="1" />

<l:Order Description="Blue suede shoes" CustomerID="4" />

<l:Order Description="Hanna Montana wig" CustomerID="1" />

</l:Collection>

</Grid.Resources>

</StackPanel>

<ComboBox ItemsSource="{StaticResource Customers}"

SelectedValuePath="ID"

SelectedValue="{Binding Text, ElementName=TextBlock1}"

DisplayMemberPath="CustomerName" />

</StackPanel>

</Grid>

More on that ComboBox after these pictures,

Initially we get this:

… and here I am opening the ComboBox, and changing the customer to Betty (her CustomerID is 2):

So that changed the “Customer ID” from 1 to 2, but I (the end user) interacted only with customer names via the ComboBox.

Now let’s analyze that ComboBox markup:

<ComboBox ItemsSource="{StaticResource Customers}"

SelectedValuePath="ID"

SelectedValue="{Binding Text, ElementName=TextBlock1}"

DisplayMemberPath="CustomerName" />

The ItemsSource is set to the Customers list. So when you open the ComboBox, you’ll see customers. Specifically, for each customer in that list, you’ll see the Customer.CustomerName, because the DisplayMemberPath is set to the “CustomerName” property.

Now whatever is selected in the ComboBox, I want it to show up in TextBlock1. If I just bind SelectedItem to TextBlock1, I’d be sending the whole Customer object over, but really I just want the TextBlock.Text to show the Customer.CustomerID. So, instead of binding SelectedItem to the TextBlock, I bind SelectedValue. SelectedValue behaves the same as SelectedItem, though, until you set a SelectedValuePath. When you set SelectedValuePath, then SelectedValue becomes that path into the SelectedItem. For example, if your SelectedItem is a Customer, and your SelectedValuePath is “ID”, then SelectedValue is going to be Customer.ID, which is just what we want.

(Note in this example I defaulted TextBlock1.Text to ‘1’. Otherwise the SelectedValue binding gets confused. Alternatively, I could have set the Binding.Mode to ‘OneWayToSource’.)

Next let’s do this with the Silverlight ComboBox, because it doesn’t have the SelectedValue or SelectedValuePath properties. But we can still get it to work with a little bit of code.

Update: The Silverlight 4 Beta does now have SelectedValue and SelectedValuePath properties.

Change the markup to this:

<ComboBox ItemsSource="{StaticResource Customers}"

DisplayMemberPath="CustomerName"

SelectionChanged="ComboBox_SelectionChanged" />

… and write a little code:

private void ComboBox_SelectionChanged(object sender, SelectionChangedEventArgs e)

{

var comboBox = sender as ComboBox;

TextBlock1.Text = (comboBox.SelectedItem as Customer).ID.ToString();

}

It’d be nice if we could do this all in markup, though, and in fact there have been some posts on the Silverlight forums to do just that with a Binding value converter. Also, I wish I could have this feature on a TextBlock too, not just on ComboBox. So below is a handy value converter that can be used with a Binding. (I created this on the Silverlight 3 beta, but most of this is applicable to Silverlight 2 as well.) The idea of this is to mimic the ComboBox mechanism. First, here’s an example of the converter being created:

<Grid.Resources>

<l:Collection x:Key="Customers">

...

</l:Collection>

<l:Collection x:Key="Orders">

...

</l:Collection>

<l:ValueToItemConverter x:Key="CustomerID2Name"

ItemsSource="{StaticResource Customers}"

ValuePath="ID"

DisplayMemberPath="CustomerName" />

</Grid.Resources>

… notice that it looks much like the ComboBox. The only thing it doesn’t have is a counterpart to the SelectedValue property, because the value is what comes into the IValueConverter methods. Now let’s use it in a trivial master/detail:

<Grid >

<Grid.ColumnDefinitions>

</Grid.ColumnDefinitions>

<ListBox.ItemTemplate>

</DataTemplate>

</ListBox.ItemTemplate>

</ListBox>

<StackPanel Grid.Column="1"

DataContext="{Binding SelectedItem, ElementName=ListBox1}">

<TextBlock Text="{Binding CustomerID,

Converter={StaticResource CustomerID2Name}}" />

</StackPanel>

</Grid>

… which gives us this (here I’ve selected the first order):

The key here is that in the detail, rather than showing CustomerID 1 from the Order, we see “Wilma”.

And finally, here’s an example using this value converter in the column of a DataGrid. Again, the items are Orders, and the column shows a CustomerID as a CustomerName, using a TextBlock when the cell isn’t being edited, and as a ComboBox when it is.

First, create a version of the value converter that doesn’t have DisplayMemberPath set (because ComboBox has its own DisplayMemberPath property):

<l:ValueToItemConverter x:Key="CustomerID2Customer"

ItemsSource="{StaticResource Customers}"

ValuePath="ID" />

… and then the DataGrid itself:

<d:DataGrid AutoGenerateColumns="False"

ItemsSource="{StaticResource Orders}">

<d:DataGrid.Columns>

<d:DataGridTextColumn Binding="{Binding Description}" />

<d:DataGridTemplateColumn>

<d:DataGridTemplateColumn.CellTemplate>

<TextBlock Text="{Binding CustomerID,

Converter={StaticResource CustomerID2Name}}"

Margin="4"/>

</DataTemplate>

</d:DataGridTemplateColumn.CellTemplate>

<d:DataGridTemplateColumn.CellEditingTemplate>

<ComboBox

DisplayMemberPath="CustomerName"

ItemsSource="{StaticResource Customers}"

SelectedItem="{Binding CustomerID, Mode=TwoWay,

Converter={StaticResource CustomerID2Customer}}" />

</DataTemplate>

</d:DataGridTemplateColumn.CellEditingTemplate>

</d:DataGridTemplateColumn>

</d:DataGrid.Columns>

</d:DataGrid>

An ICommand with IsEnabled

Mike Hillberg [MSFT] — Tue, 24 Mar 2009 08:13:00 GMT

ICommand is a simple interface with three members – Execute, CanExecute, and CanExecuteChanged (more on those here). You can write your own implementations of that interface, one for each command, but that gets a bit heavyweight. So there are several implementations of ICommand that are pluggable and re-usable, like DelegateCommand, RelayCommand, and RoutedCommand.

One tricky part of implementing ICommand is the CanExecuteChanged event, because you have to know when to raise it. (The easy alternative is to raise it whenever the CommandManager.RequerySuggested event is raised, but you need to use that with caution, because it can happen frequently and get expensive.)

Really, though, CanExecute and CanExecuteChanged look like a typical Foo/FooChanged property/event pair that we use in data binding. The reason that CanExecute isn’t actually a property, though, is that it takes an optional parameter. But if you’re not using that parameter, a property would work just as well.

So I’ve been playing with an ICommand implementation that takes a delegate for the Execute method (just like DelegateCommand and RelayCommand), but also has an IsEnabled property that wraps CanExecute/CanExecuteChanged. And since it’s all a DependencyObject, you can put a binding on IsEnabled.

I wasn’t sure what to call it though. The key is that it has an IsEnabled property, but DelegateCommandWithIsEnabled didn’t sound good. And it turns out that Enabled-able isn’t a word. The closest thing I could come up with that had “able” in it was AbilityCommand. Good enough for a blog post.

Anyway, here’s what the public members look like:

public class AbilityCommand : DependencyObject, ICommand

{

public AbilityCommand(

Action<object> executeDelegate,

Binding isEnabledBinding );

public AbilityCommand( Action<object> executeDelegate )

: this( executeDelegate, null ) { }

public bool IsEnabled {get; set; }

public static readonly DependencyProperty IsEnabledProperty ...;

}

Note that you can set IsEnabled however you want, including with a binding, and in fact the constructor lets you pass in the Binding to use. AbilityCommand’s ICommand.CanExecute simply returns IsEnabled. And any time IsEnabled’s value changes, AbilityCommand raises ICommand.CanExecuteChanged.

And here it is in practice. In this snippet, the (View)Model object is the first page of a wizard-type view (with a “Next” button on it). The SubmitCommand property binds the command’s IsEnabled to the object’s IsValid property:

public class FirstPage : INotifyPropertyChanged

{

public FirstPage()

{

SubmitCommand = new AbilityCommand(

(x) => OnSubmit(),

new Binding("IsValid") { Source=this } );

}

public ICommand SubmitCommand { get; private set; }

private void OnSubmit()

{

// ...

}

// INotifyPropertyChanged fires for this IsValid property if

// either the "FirstProperty" or "SecondProperty" property is updated

public bool IsValid

{

get

{

return !String.IsNullOrEmpty(FirstProperty)

&& !String.IsNullOrEmpty(SecondProperty);

}

And finally, here’s the whole AbilityCommand:

public class AbilityCommand : DependencyObject, ICommand

{

Action<object> _executeDelegate;

public AbilityCommand( Action<object> executeDelegate )

: this( executeDelegate, null ) { }

public AbilityCommand(

Action<object> executeDelegate,

Binding isEnabledBinding )

{

_executeDelegate = executeDelegate;

if( isEnabledBinding != null )

BindingOperations.SetBinding(this, IsEnabledProperty, isEnabledBinding);

}

void ICommand.Execute(object parameter)

{

_executeDelegate(parameter);

}

bool ICommand.CanExecute(object parameter)

{

return IsEnabled;

}

public event EventHandler CanExecuteChanged;

private void RaiseCanExecuteChanged()

{

if( CanExecuteChanged != null )

CanExecuteChanged( this, new EventArgs() );

}

public bool IsEnabled

{

get { return (bool)GetValue(IsEnabledProperty); }

set { SetValue(IsEnabledProperty, value); }

}

public static readonly DependencyProperty IsEnabledProperty =

DependencyProperty.Register("IsEnabled", typeof(bool), typeof(AbilityCommand),

new PropertyMetadata(OnIsEnabledChanged));

static void OnIsEnabledChanged(DependencyObject d, DependencyPropertyChangedEventArgs args)

{

(d as AbilityCommand).RaiseCanExecuteChanged();

}

ICommand is like a chocolate cake

Mike Hillberg [MSFT] — Sat, 21 Mar 2009 01:48:00 GMT

(This has been updated with some information about the origin of a routed command’s route, and of focus scopes.)

ICommand in WPF is a pretty simple thing at its core. But it gets more interesting and complicated as you build up functionality on top of it, and integrate it into the higher layers of the UI. So it’s either like a layer cake, or layers of an onion. But onion is an outside-in metaphor, and layer cake is a bottom-up metaphor, and ICommand is easier to think of bottom up, so I’m declaring ICommand to be a layer cake.

For example, commands provide a mechanism to abstract input (so “navigate back” means “navigate back”, whether it came from the keyboard’s back button or the mouse’s X1 button or anywhere else). And commands provide a mechanism for the View to update the Model in a Model/View separated application. And commands provide a way to search the element tree for a command handler, as well as a way for a command handler to say that it doesn’t want to be executed at the moment.

The mechanism for all those scenarios is all very similar, but the scenarios themselves seem so different that it all can get confusing. So here’s a step-by-step description of how it all fits together …

Start with ICommand

ICommand itself is very straightforward:

public interface ICommand

{

void Execute(object parameter);

bool CanExecute(object parameter);

event EventHandler CanExecuteChanged;

}

Given an instance of an ICommand, you just call Execute, and it does whatever it’s supposed to do. Except you shouldn’t call it if it’s CanExecute is false. If you want to know when CanExecute might be willing to give you a different answer, listen to the CanExecuteChanged event.

For example, here’s a super simple command:

public class HelloWorldCommand : ICommand

{

public void Execute(object parameter)

{

Debug.WriteLine("Hello, world");

}

public bool CanExecute(object parameter)

{

return true;

}

public event EventHandler CanExecuteChanged;

}

If you use that like this:

new HelloWorldCommand().Execute(null);

… you’ll see “Hello, world” in the debug output window.

You can make it more interesting with a parameter and by checking CanExecute. First change the command like this:

public class HelloWorldCommand : ICommand

{

public void Execute(object parameter)

{

Debug.WriteLine(parameter);

}

public bool CanExecute(object parameter)

{

return parameter != null;

}

public event EventHandler CanExecuteChanged;

}

… and call it like this:

var hwc = new HelloWorldCommand();

if (hwc.CanExecute(this))

hwc.Execute(this);

… and in my case I see “CommandCake.Window1” in the debugger output window.

Piece o’ cake.

Button (heart) ICommand

Once you have an ICommand instance handy, you can give it to a Button (on the Button.Command property), and Button knows what to do with it. As the simplest example, you can do this with the previous command:

<Grid>

<Grid.Resources>

<local:HelloWorldCommand x:Key="hwc"/>

</Grid.Resources>

Click

</Button>

</Grid>

But if you do that, you’ll notice that the Button is disabled. That’s because Button knows to call CanExecute, but we haven’t specified a parameter, and recall from above that if you pass null as an argument to CanExecute it returns false. So Button has a CommandParameter property that lets you specify what will be passed to CanExecute and Execute:

<Grid>

<Grid.Resources>

<local:HelloWorldCommand x:Key="hwc"/>

</Grid.Resources>

Click

</Button>

</Grid>

Now the button is enabled, and if you click on it, you’ll see “Hello, world” in the debug output window.

This actually isn’t just a Button feature, it’s actually in the base class ButtonBase. And MenuItem is a ButtonBase. So MenuItem (heart) ICommand too.

Update the Model from the View

Now we’ve got enough for the classic Model/View usage of ICommand. In the Model/View practice, your View is a bunch of elements, which is data-bound to your Model, which has your actual content. The View can modify the model with two-way bindings and with commands.

First, before showing an example of this, let’s make it easier to implement ICommand, by introducing a helper class (a more complete DelegateCommand helper can be found in Prism). This just creates an ICommand instance that takes a delegate which will be called by ICommand.Execute:

public class SimpleDelegateCommand : ICommand

{

Action<object> _executeDelegate;

public SimpleDelegateCommand(Action<object> executeDelegate)

{

_executeDelegate = executeDelegate;

}

public void Execute(object parameter)

{

_executeDelegate(parameter);

}

public bool CanExecute(object parameter) { return true; }

public event EventHandler CanExecuteChanged;

}

Now let’s define a Model of a simple Debug writer:

public class DebugWriter

{

ICommand _indentCommand = new SimpleDelegateCommand( (x) => Debug.Indent() );

ICommand _unindentCommand = new SimpleDelegateCommand( (x) => Debug.Unindent() );

ICommand _writeLineCommand = new SimpleDelegateCommand( (x) => Debug.WriteLine(x) );

public ICommand IndentCommand { get { return _indentCommand; } }

public ICommand UnindentCommand { get { return _unindentCommand; } }

public ICommand WriteLineCommand { get { return _writeLineCommand; } }

public int IndentSize

{

get { return Debug.IndentSize; }

set { Debug.IndentSize = value; }

}

… and use it from a View:

<StackPanel.DataContext>

<local:DebugWriter />

</StackPanel.DataContext>

<Grid>

<Grid.ColumnDefinitions>

</Grid.ColumnDefinitions>

<Grid.RowDefinitions>

</Grid.RowDefinitions>

<TextBlock Margin="3">Indent size:</TextBlock>

<TextBlock Grid.Row="1" Margin="3">Output string:</TextBlock>

</Grid>

<Button Command="{Binding IndentCommand}">Indent</Button>

<Button Command="{Binding UnindentCommand}">Unindent</Button>

<Button CommandParameter="{Binding Text,ElementName=OutputString}"

Command="{Binding WriteLineCommand}">WriteLine</Button>

</StackPanel>

Notice here that the Button bound to the WriteLine command (the third Button) has its CommandParameter bound to a TextBox.Text.

Routed commands: an ICommand.Execute implementation that searches for an execute handler

For the above example I used the SimpleDelegateCommand as my implementation of ICommand, which maps ICommand.Execute to a delegate call.

WPF similarly has a built-in ICommand implementation called RoutedCommand. You don’t give a delegate directly to the RoutedCommand, though. Instead, the RoutedCommand walks up the tree, looking for a delegate. It’s similar to a routed event, in fact it’s implemented as a routed event internally. So you can put your delegate anywhere higher in the tree, in the form of an event handler, using the CommandBinding class, and it will be called by Execute. You specify your delegate with a CommandBinding object.

So I add this to my Window1.xaml.cs:

public static RoutedCommand HelloWorldRoutedCommand = new RoutedCommand();

… and this to my Xaml:

<Window.CommandBindings>

<CommandBinding

Command="{x:Static local:Window1.HelloWorldRoutedCommand}"

Executed="CommandBinding_Executed" />

</Window.CommandBindings>

<Grid>

...

<Button Command="{x:Static local:Window1.HelloWorldRoutedCommand}">Hello, world </Button>

...

</Grid>

… and then when I click the Button, my CommandBinding_Executed method gets called:

private void CommandBinding_Executed(object sender, ExecutedRoutedEventArgs e)

{

if( e.Source is Button )

Debug.WriteLine((e.Source as Button).Content);

else

Debug.WriteLine("Hello, world");

}

… and once again I see “Hello, world” in the debug output.

Note that the event handler of the CommandBinding.Executed event can look at the Source property of the event args to see where the RoutedCommand started. You can also get the command itself, and the CommandParameter from the args.

In this example I put the CommandBinding in the tree ancestry of the Button. Alternatively, you can also register your CommandBinding globally, using the CommandManager.RegisterClassCommandBinding method. That CommandBinding’s CanExecute then gets called no matter where in the tree the routed command’s route is starting.

RoutedCommand also has support for CanExecute and CanExecuteChanged. That’s the most complicated part of routed commands, so I’m saving that for the end.

Mapping input to an ICommand (keyboard accelerators)

Beyond Button and MenuItem, there’s another way to get an ICommand to Execute, this one based on user input. As an example, with the following Xaml, pressing <Control>H on the keyboard executes our HelloWorldCommand, again showing “Hello, world” in the debugger:

<Grid>

<Grid.Resources>

<local:HelloWorldCommand x:Key="hwc"/>

</Grid.Resources>

<Grid.InputBindings>

</Grid.InputBindings>

...

Note that this key binding only takes effect if the keyboard focus is currently somewhere under that Grid, e.g. on a TextBox; otherwise the KeyBinding doesn’t see the <Control>H.

RoutedCommand also has an InputGestures property on it, where you can set the default gestures for a command. So you can get this same behavior for the HelloWorldRoutedCommand we created earlier by defining it like this:

public static RoutedCommand HelloWorldRoutedCommand = new RoutedCommand()

{

InputGestures = { new KeyGesture(Key.H, ModifierKeys.Control) }

};

… and then it will Execute no matter where your keyboard focus is.

Built-in routed commands in WPF

Anyone can create a RoutedCommand. But WPF has a set of them built-in. These are all defined as static fields in the ApplicationCommands, EditingCommands, MediaCommands, and NavigationCommands classes. E.g. NavigationCommands has the BrowseBack command, ApplicationCommands has cut/copy/paste, EditingCommands has ToggleBold, MoveRightByWord, etc. The Windows WM_APPCOMMAND commands get converted into these built-in routed commands automatically by the WPF input system.

So, for example, this markup:

<Window.CommandBindings>

<CommandBinding Command="{x:Static NavigationCommands.BrowseBack}"

Executed="BrowseBackExecuted" />

</Window.CommandBindings>

… will cause the BrowseBackExecuted method to be called when the NavigationCommands.BrowseBack command is executed, for example if you click the mouse X1 button (this is the button that makes a web browser navigate back).

From whence the command routes

If you call RoutedCommand’s ICommand.Execute, the route will start from the element that currently has keyboard focus. But RoutedCommand itself has a special overload of Execute that lets you pick where the route starts:

public class RoutedCommand

{

...

public void Execute( Object parameter, IInputElement target);

...

}

And in fact, RoutedCommand’s implementation of ICommand.Execute just calls this Execute, looking roughly like this:

void ICommand.Execute(object parameter)

{

this.Execute(parameter, Keyboard.FocusedElement );

}

Button (and MenuItem) actually understands RoutedCommand in particular, in addition to ICommand in general. So if Button.Command is a RoutedCommand, Button by default calls this RoutedCommand.Execute method, passing itself as the target. You can override this behavior, though, with the Button.CommandTarget property. For example, here is a Button that sends the Cut command to a TextBox:

<Button Command="{x:Static ApplicationCommands.Cut}"

CommandTarget="{Binding ElementName=TextBox1}">Click</Button>

This gets more complicated if you’re implementing a toolbar. Say you have a toolbar with cut/copy/paste buttons. You want that toolbar to work against whatever the currently focused text box is, so you don’t want to have to keep setting CommandTarget. E.g., in this:

<Button Command="{x:Static ApplicationCommands.Paste}">Paste</Button>

</ToolBar>

</StackPanel>

… the Cut/Copy/Paste should route to whichever TextBox has focus.

It doesn’t look like this should work, but it actually does, and here’s why … ToolBar is a “focus scope”. Focus scopes is a whole other post, but the important thing here is what happens with a routed command that’s executing and looking for a CommandBinding. As the execute request bubbles up to the ToolBar, it sees that it’s leaving a focus scope. At that point, rather than just continuing to walk up the visual tree, it goes next to the focused element in that focus scope. In this example, that means that when you click on the Cut button, the search for a handler starts at the Button, then the ToolBar, then the currently-focused TextBox, which is what we want.

Menu is also a focus scope, so what works for ToolBar works for Menu as well.

CanExecute and CanExecuteChanged for routed commands

Just like RoutedCommand allows you to define a command that walks up the tree, looking for someone to handle Execute, you also want to find someone to handle CanExecute. So RoutedCommand’s CanExecute implementation does that, and you can listen for it on CommandBinding, as you’d expect, e.g.:

<CommandBinding

Command="{x:Static local:Window1.HelloWorldRoutedCommand}"

CanExecute="CommandBinding_CanExecute"

Executed="CommandBinding_Executed" />

But here’s the trick: RoutedCommand similarly needs to raise CanExecuteChanged. But how does it know when to raise that event, when it doesn’t know what you’re going to do in CommandBinding_CanExecute?

The solution in WPF is a global event named CommandManager.RequerySuggested. This event is fired whenever the state of the routed command world might have changed. In fact, the implementation of ICommand.CanExecuteChanged in RoutedCommand is just a forwarder:

public event EventHandler CanExecuteChanged

{

add { CommandManager.RequerySuggested += value; }

remove { CommandManager.RequerySuggested -= value; }

}

Next problem: When should the RequerySuggested event fire? You make that happen by calling CommandManager.InvalidateRequerySuggested. But usually you don’t have to call it, it’s called automatically in several places, mostly during keyboard/mouse input.

That all creates a couple of interesting implications.

First of all, if you’re using routed commands, you’ve got some work taking place on every keystroke. On the one hand, even for a fast typist (I clocked in the other day at 88 words/minute on the high-difficulty test!), user input is very infrequent in CPU timeframes. On the other hand, it still takes time to find those CanExecute handlers, and if those handlers do anything non-trivial, and you have a lot of them, it can add up. We’ve seen that be a problem in some larger applications. You can mitigate that by reducing use of routed commands (just use ICommand instead), and by keeping the CanExecute implementations fast.

The second interesting implication is that CommandManager.RequerySuggested is a static (global) event. Usually such events can lead to leaks, because they hold the event handler delegate forever. But RequerySuggested instead only keeps weak references to its handler delegates. But now you’ve got a new problem; now the delegate can be garbage collected. You probably don’t have to worry about that, because most applications don’t listen to RequerySuggested; it’s really the Button doing this on your behalf when you set Button.Command. But Button (and MenuItem) deal with this problem by keeping their own strong reference on that delegate, so that it doesn’t get collected.

In summary

So the key points to remember here:

· ICommand is a simple definition with Execute, CanExecute, and CanExecuteChanged.

· You can point a Button or MenuItem at any ICommand with the Command property. The Button/MouseItem will then Execute that command if you click it, will disable itself if CanExecute is false, and will automatically listen for CanExecuteChanged.

· Routed commands are an ICommand implementation that searches the tree (usually starting with the focused element) for a CommandBinding that provides Execute/CanExecute handlers.

· You can also invoke an ICommand from input, e.g. using a KeyBinding.

· WPF pre-defines a set of routed commands in ApplicationCommands, EditingCommands, NavigationCommands, and MediaCommands.

· Be aware that routed commands can impact perf if you use a lot of them and/or your CanExecute handlers do non-trivial work.

A master/detail view with navigation

Mike Hillberg [MSFT] — Fri, 09 Jan 2009 21:50:00 GMT

I don’t remember what got me thinking about it, but somewhere along the line I wanted a master/detail view with a navigation bar. E.g., when you change selection in the master view, you can navigate back to the previous selection.

Here’s an example (here my selection started on “Wilma”, then I changed it to “Fred”, which is why the Back button of the navigation bar is enabled):

It looks like a Frame, and it is, but I just used the Frame for its navigation UI and its journaling functionality; I didn’t put anything (visible anyway) into the Content of the Frame.

Here’s the basic markup for the above Window sample:

<navbar:NavigationBar

JournaledValue="{Binding SelectedItem, ElementName=_listBox, Mode=TwoWay}"

NavigationUIVisibility="Visible"/>

<Grid>

<Grid.ColumnDefinitions>

</Grid.ColumnDefinitions>

<ListBox.ItemTemplate>

</DataTemplate>

</ListBox.ItemTemplate>

</ListBox>

...

</Grid>

</StackPanel>

What does this do? The NavigationBar control has the navigation UI (forward/backward button). It also journals whatever you put in the JournaledValue property. That means:

If you change NavigationBar.JournaledValue, the old value will be added to the navigation service’s “BackStack”, and the Back button will become enabled.
If you then click on the Back button, NavigationBar.JournaledValue will be restored to its previous value.

So the way the properties are bound in this example, if the ListBox.SelectedItem changes, the old value gets journaled by the NavigationBar. If you then click the Back button, the ListBox.SelectedItem gets restored to that previous value. And of course, in the full example, the details view is similarly bound to the ListBox.SelectedItem, so it updates as well.

The NavigationBar control just has a couple of properties and a default ControlTemplate. The properties are JournaledValue (which drives, and is driven by, the navigation bar UI), and NavigationUIVisibility (Automatic, Visible, or Hidden). Here’s the control code:

public class NavigationBar : Control

{

static NavigationBar()

{

DefaultStyleKeyProperty.OverrideMetadata(

typeof(NavigationBar),

new FrameworkPropertyMetadata(typeof(NavigationBar)));

}

// The JournaledValue property is the value that controls the navigation bar UI.

public object JournaledValue

{

get { return (object)GetValue(JournaledValueProperty); }

set { SetValue(JournaledValueProperty, value); }

}

public static readonly DependencyProperty JournaledValueProperty =

DependencyProperty.Register("JournaledValue", typeof(object), typeof(NavigationBar));

// The NavigationUIVisibility property is aliased to the

// Frame.NavigationUIVisibility property.

public NavigationUIVisibility NavigationUIVisibility

{

get { return (NavigationUIVisibility)GetValue(NavigationUIVisibilityProperty); }

set { SetValue(NavigationUIVisibilityProperty, value); }

}

public static readonly DependencyProperty NavigationUIVisibilityProperty =

Frame.NavigationUIVisibilityProperty.AddOwner(typeof(NavigationBar));

}

The NavigationBar’s ControlTemplate simplify forwards everything to a Frame. Frame has the UI for the navigation controls, and journals its Content property. So for journaling, I just bound the NavigationBar.JournaledValue to Frame.Content, and Frame takes care of the rest. (Note that NavigationBar.NavigationUIVisibility is also bound to the corresponding property on Frame.) The only trick is, I didn’t want the JournaledValue to actually get displayed by the Frame, so I put an empty ContentTemplate on it. This leads to:

<Frame NavigationUIVisibility

='{TemplateBinding local:NavigationBar.NavigationUIVisibility}'

Content='{Binding JournaledValue,

RelativeSource={RelativeSource TemplatedParent},

Mode=TwoWay}'

Background="{TemplateBinding Background}"

BorderBrush="{TemplateBinding BorderBrush}"

BorderThickness="{TemplateBinding BorderThickness}" >

<!-- Don't show the frame content, we're just using Frame

for it's navigation UI and journaling -->

<Frame.ContentTemplate>

</Frame.ContentTemplate>

</Frame>

</ControlTemplate>

That’s it for the NavigationBar control. Here’s the rest of the sample app too. For the details view, I used an ItemsControl to list the “fields” of the selected object, where each field is a HeaderedContentControl. To get the right look, I then templated HeaderedContentControl. Easier to show the markup. First, the sample data:

<Window.DataContext>

<x:Array Type="local:Person">

<local:Person FirstName="Wilma" LastName="Flintstone" Age="41" />

<local:Person FirstName="Fred" LastName="Flintstone" Age="42" />

<local:Person FirstName="Betty" LastName="Rubble" Age="36" />

<local:Person FirstName="Barney" LastName="Rubble" Age="35" />

</x:Array>

</Window.DataContext>

… and then the details view:

<ItemsControl

DataContext="{Binding SelectedItem, ElementName=_detailsView}"

Grid.IsSharedSizeScope="True"

Grid.Column="1"

Margin="5" >

<!-- Create a HeaderedContentControl template to display each item

as "Label: Value", e.g. "Age: 42" -->

<ItemsControl.Resources>

<Setter.Value>

<Grid>

<Grid.ColumnDefinitions>

<ColumnDefinition SharedSizeGroup="FirstColumn"

Width="Auto" />

</Grid.ColumnDefinitions>

<TextBlock Text="{TemplateBinding Header}" Grid.Column="0"

Margin="0,0,5,0" FontWeight="Bold"/>

<TextBlock Text="{TemplateBinding Content}" Grid.Column="1"

Margin="0,0,5,0"/>

</Grid>

</ControlTemplate>

</Setter.Value>

</Setter>

</Style>

</ItemsControl.Resources>

</ItemsControl>

A Comparable DataTrigger

Mike Hillberg [MSFT] — Tue, 30 Sep 2008 10:04:00 GMT

Property triggers today only check for equality. We’d like to add support for other comparison operators, but that hasn’t happened yet. But I needed them for a project, and wrote a workaround for it. It’s a bit hacky in a couple of places, but if you can get past that, it’s a handy way to simplify some coding.

Here’s a sample of what I ended up with:

The basics:

· You have to set the DataTrigger.Value to null. That’s the main hack.

· The supported comparison operators are GT, GTE, LT, LTE, and EQ.

· The comparand (“65” in the above example) is converted from string to the type of the target value (presumably Age is an int in the above example), using Compare.ChangeType or the target’s TypeConverter.

That’s all there is to use it. You have to remember to set DataTrigger.Value to null, otherwise it’s relatively straightforward.

And here’s the implementation:

// ComparisonBinding is a Binding that should be used in a DataTrigger.Binding.

// It supports a comparison operator and a comparand, so that you can use it as a

// conditional DataTrigger. The trick is to set {x:Null} as the DataTrigger.Value.

// E.g.:

// <DataTrigger Value={x:Null}

// Binding={h:ComparisonBinding Width, EQ, 100}"

// The operator can be EQ, LT, LTE, GT, GTE.

public class ComparisonBinding : Binding

{

// Default constructor

public ComparisonBinding()

: this(null, ComparisonOperators.EQ, null)

{

}

// Construction with an operator & comparand

public ComparisonBinding(string path, ComparisonOperators op, object comparand)

: base(path)

{

RelativeSource = RelativeSource.Self;

Comparand = comparand;

Operator = op;

Converter = new ComparisonConverter( this );

}

// Operator and comparand

public ComparisonOperators Operator { get; set; }

public object Comparand { get; set; }

}

// Supported types of comparisons

public enum ComparisonOperators

{

EQ = 0,

GT,

GTE,

LT,

LTE

}

// Thie IValueConverter is used by the StyleBinding to

// implement the logical comparisson. ConvertBack isn't supported.

// Convert returns null if the condition is met, non-null otherwise.

internal class ComparisonConverter : IValueConverter

{

// Keep a back reference to the StyleBinding

ComparisonBinding _styleBinding;

// Return this if the condition isn't met

static object _notNull = new Object();

// In construction, get a reference to the StyleBinding

public ComparisonConverter(ComparisonBinding styleBinding)

{

_styleBinding = styleBinding;

}

// IValueConverter.Convert

// Return null of the condition is met, non-null if not.

public object Convert(

object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)

{

// Simple check for null

if (value == null || _styleBinding.Comparand == null)

{

return ReturnHelper( value == _styleBinding.Comparand );

}

// Convert the comparand so that it matches the value

object convertedComparand = _styleBinding.Comparand;

try

{

// Only support simple conversions in here.

convertedComparand = System.Convert.ChangeType(_styleBinding.Comparand, value.GetType());

}

catch (InvalidCastException)

{

// If Convert.ChangeType didn't work, try a type converter

TypeConverter typeConverter = TypeDescriptor.GetConverter(value);

if (typeConverter != null)

{

if (typeConverter.CanConvertFrom(_styleBinding.Comparand.GetType()))

{

convertedComparand = typeConverter.ConvertFrom(_styleBinding.Comparand);

}

// Simple check for the equality case

if (_styleBinding.Operator == ComparisonOperators.EQ)

{

// Actually, equality is a little more interesting, so put it in

// a helper routine

return ReturnHelper(

CheckEquals(value.GetType(), value, convertedComparand) );

}

// For anything other than Equals, we need IComparable

if (!(value is IComparable) || !(convertedComparand is IComparable))

{

Trace(value, "One of the values was not an IComparable");

return ReturnHelper(false);

}

// Compare the values

int comparison = (value as IComparable).CompareTo(convertedComparand);

// And return the comparisson result

switch (_styleBinding.Operator)

{

case ComparisonOperators.GT:

return ReturnHelper( comparison > 0 );

case ComparisonOperators.GTE:

return ReturnHelper( comparison >= 0 );

case ComparisonOperators.LT:

return ReturnHelper( comparison < 0 );

case ComparisonOperators.LTE:

return ReturnHelper( comparison <= 0 );

}

return _notNull;

}

// This helper produces the return value; null if the values

// match, non-null otherwise.

object ReturnHelper(bool result)

{

return result ? null : _notNull;

}

// Trace output to the debugger

void Trace(object value, string message)

{

if (Debugger.IsAttached)

{

Debug.WriteLine("StyleBinding couldn't convert '"

+ value.GetType()

+ "' to '"

+ _styleBinding.Comparand.GetType()

+ "'");

Debug.WriteLine("(" + message + ")");

}

// Check for equality of two values

private bool CheckEquals(Type type, object value1, object value2)

{

if (type.IsValueType || type == typeof(string))

{

return Object.Equals(value1, value2);

}

else

{

return Object.ReferenceEquals(value1, value2);

}

// IValueConverter.ConvertBack isn't supported.

public object ConvertBack(

object value,

Type targetType,

object parameter,

System.Globalization.CultureInfo culture)

{

throw new NotImplementedException();

}

Compact CheckBox Sample

Mike Hillberg [MSFT] — Sat, 14 Jun 2008 01:17:00 GMT

I was creating a view of an object that had a bunch of boolean properties, but I wanted to keep the visual representation small. So I created a look for a compact CheckBox that I liked enough to post.

As an example scenario, say I’m visualizing a the attributes of a file (hidden, system, archive, and read-only). I could do it with a TextBlock and some CheckBoxes easy enough, to create a look like this:

… but I want it more compact. I want just a letter for each attribute, and I just want the letter to change when it’s checked. That got me this:

Here, the light gray italicized boxes are un-checked, and the un-italicized black letters with a gray background are checked. (So before capturing this screenshot, I had checked the read-only and archive bits on the “readme.txt” file.) This image doesn’t show it, but there’s also a tooltip for each CheckBox that shows the full name of the attribute.

Here’s the ControlTemplate for the CheckBox:

<Border BorderThickness="{TemplateBinding BorderThickness}"

BorderBrush="Gray" Background="LightGray" Padding="2"

Name="_border" >

<TextBlock Text="{TemplateBinding Content}"

FontStyle="Italic"

Foreground="LightGray"/>

<TextBlock Text="{TemplateBinding Content}"

Name="_checkedTextBlock" />

</Grid>

</Border>

<ControlTemplate.Triggers>

<Setter TargetName="_checkedTextBlock"

Property="Opacity" Value="0" />

<Setter TargetName="_border"

Property="Background" Value="Transparent" />

</Trigger>

</ControlTemplate.Triggers>

</ControlTemplate>

The interesting thing here is that I created two TextBlocks to show the content of the CheckBox (e.g. the “R” for the read-only attribute CheckBox). The first one shows the content in “unchecked” form, and the second one shows it in “checked” form. The Trigger then picks which one shows up. I had started with a single TextBlock, and had the Trigger set the FontStyle and Foreground. The problem with this is that the italicized text is slightly wider than the normal text, and that caused the checkboxes to jiggle a little bit when you click on them. There’s probably other ways to solve this, but a simple solution was to have both TextBlocks there and taking up space in layout, and then let the Trigger decide which one wins for render.

And here’s a full template with sample usage, again with some of the more interesting parts highlighted. (I changed the trigger so that instead of a Setter to update the Background when IsChecked changes, it animates the change.)

<Page

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">

<Grid>

<Grid.ColumnDefinitions>

</Grid.ColumnDefinitions>

<Grid.Resources>

<!-- Implicit style to be applied to all CheckBoxs in this Window

(this sets the new template) -->

<Setter.Value>

<Border BorderThickness="{TemplateBinding BorderThickness}"

BorderBrush="Gray" Background="LightGray" Padding="2"

Name="_border" >

<TextBlock Text="{TemplateBinding Content}"

FontStyle="Italic"

Foreground="LightGray"/>

<TextBlock Text="{TemplateBinding Content}"

Name="_checkedTextBlock" />

</Grid>

</Border>

<ControlTemplate.Triggers>

<Setter TargetName="_checkedTextBlock"

Property="Opacity" Value="0" />

<!-- Instead of a Setter to update the background color,

use Trigger.EnterActions/ExitActions to animate it

<Setter TargetName="_border"

Property="Background" Value="Transparent" /> -->

<Trigger.EnterActions>

</Storyboard>

</BeginStoryboard>

</Trigger.EnterActions>

<Trigger.ExitActions>

</Storyboard>

</BeginStoryboard>

</Trigger.ExitActions>

</Trigger>

</ControlTemplate.Triggers>

</ControlTemplate>

</Setter.Value>

</Setter>

</Style>

<Border BorderThickness="1" BorderBrush="Black"

Margin="4" CornerRadius="5,5,0,0" Background="#FF83AFEA">

</Border>

<CheckBox BorderThickness="0" ToolTip="Hidden"

IsChecked="{Binding XPath=@Hidden, Mode=TwoWay}">

H</CheckBox>

<CheckBox ToolTip="System"

IsChecked="{Binding XPath=@System, Mode=TwoWay}">

S</CheckBox>

<CheckBox ToolTip="Read only"

IsChecked="{Binding XPath=@ReadOnly, Mode=TwoWay}">

R</CheckBox>

<CheckBox BorderThickness="1,0,1,0" ToolTip="Archive"

IsChecked="{Binding XPath=@Archive, Mode=TwoWay}">

A</CheckBox>

</StackPanel>

</Border>

</DataTemplate>

<x:XData >

<File Name="autoexec.bat" Archive="False" System="False"

ReadOnly="True" Hidden="True" />

<File Name="readme.txt" Archive="False" System="True"

ReadOnly="False" Hidden="False" />

</Files>

</x:XData>

</XmlDataProvider>

</Grid.Resources>

<ItemsControl ItemsSource="{Binding XPath=File, Source={StaticResource SampleData}}"

ItemTemplate="{StaticResource FileTemplate}" />

</Grid>

</Page>

Trying out Binding.StringFormat

Mike Hillberg [MSFT] — Fri, 30 May 2008 00:15:00 GMT

StringFormat is a new property in .Net 3.5 SP1, which is currently in Beta. See Scott’s blog for more info on the beta.

When you bind data into a property on an element, it’s automatically type converted for you. For example, this markup:

<StackPanel.DataContext>

<sys:Int32>123</sys:Int32>

</StackPanel.DataContext>

</StackPanel>

… shows a TextBlock with the text “123”.

If you want to have more control over the conversion of the value during binding, you can write a value converter for it. For example, with this code:

public class TestConverter : IValueConverter

{

public object Convert(object value, Type targetType, object parameter,

System.Globalization.CultureInfo culture)

{

return String.Format(culture, "Cost: {0:C}", value);

}

public object ConvertBack(object value, Type targetType, object parameter,

System.Globalization.CultureInfo culture)

{

throw new NotImplementedException();

}

… and this markup:

<StackPanel.Resources>

<local:TestConverter x:Key="testConverter" />

</StackPanel.Resources>

<StackPanel.DataContext>

<sys:Int32>123</sys:Int32>

</StackPanel.DataContext>

</StackPanel>

… my TextBlock shows “Cost: $123.00”.

The New StringFormat Property

That’s great, but it’s too bad that you have to write a value converter to get that. To make it simpler, we’ve added the StringFormat property to Binding, which causes the value to be run through, not surprisingly, the String.Format method.

So, the above can now be accomplished with this markup, and no code:

<StackPanel.DataContext>

<sys:Int32>123</sys:Int32>

</StackPanel.DataContext>

</StackPanel>

If you just want to add the format specifier, e.g. just “{0:C}” rather than “Cost: {0:C}”, there’s nothing unusual if you’re in code, but in Xaml the syntax gets confused with the markup extension syntax. That is, a property value that starts with a “{“ (as the first character) is interpreted to be a markup extension, such as {Binding}, {DynamicResource}, {x:Null}, etc. The way to really have a “{“ as the first character of a property value is to escape it with a “{}”, so the above example becomes:

Not Just on Bindings

StringFormat shows up on some controls too. For example, just like ContentControl (the base class of Button) lets you template your content with the ContentTemplate property, you can now format your content with the ContentFormatString property, such as:

<sys:Int32>123</sys:Int32>

</Button>

The ContentStringFormat property is available on ContentControl, ContentPresenter, and TabControl.

There’s an analogous HeaderStringFormat for HeaderedContentControl, GridViewColumn, GroupStyle, and HeaderedItemsControl. Again, these are all cases where there is an analogous HeaderTemplate property.

Finally, there’s an ItemStringFormat on ItemsControl (the base class for ListBox) that allows you to format items. For example, this produces a ListBox with two entries, “1,234.00” and “5,6789.00”:

<sys:Int32>1234</sys:Int32>

<sys:Int32>5678</sys:Int32>

</ListBox>

Multi Bindings

All of the examples so far reference the bound value with some form of “{0}”, which is the zero-th index into the array of sources. With a MultiBinding, you can actually get more than one input. For example, the following has a TextBlock that composes the value in two other TextBlocks (in a real-world example, those would probably be TextBox’s for user input):

<TextBlock Name="Last">Flintstone</TextBlock>

<TextBlock.Text>

</MultiBinding>

</TextBlock.Text>

</TextBlock>

This produces “Name: Flintstone, Fred”.

Culture

All of the examples so far have been in US English (“en-US”). But you can pick the culture for StringFormat, the same way that you pick culture for the Binding.Converter property. That is, you can set it explicitly on the Binding, as in:

… which gives me “123.00” (note the period for the decimal separator), or as in:

… which gives me “123,00” (note the comma for the decimal separator).

Or you can specify it on the element in the Language property, as in:

</StackPanel>

As an aside, note that the xml:lang attribute is mapped to the Language property. So the above is equivalent to:

</StackPanel>

More Samples

Finally, see Lester’s post for more StringFormat examples.

Of logical and visual trees in WPF

Mike Hillberg [MSFT] — Sat, 24 May 2008 02:46:00 GMT

This post is about the “logical tree” in WPF, and how it differs from the visual tree. For the most part you don’t need to understand this. But if you want to understand some of the nit-like details of property inheritance, {DynamicResource} references, and ElementName bindings, this may be interesting.

A long time ago, I was trying to arrange all my files in a directory hierarchy (on DOS). I had a directory for different project files, a directory for design documents, etc. Then I hit the classic problem that reminded me of the 3 drawer metal filing cabinet – where do you put a project design document? So I was delighted when Windows95 came out and had shortcut files. A shortcut (similar to symbolic links) is a file that points to another file. (One way to create a shortcut in Windows is to drag a file from one folder to another, using the right mouse button.) So now I could put my project design document in my project directory, then create a shortcut to it in my design directory. My document was in both directories. It was cross-categorized. My row had two key fields. I was happy probably to a fault.

I flashed back to this early in WPF, with a hierarchy of another sort. In this case, the first hierarchy is the visual tree. The visual tree makes great sense to some developers, not as much to others. So we created a cross-categorization called the logical tree to help things to be more … logical. The rest of this post compares the two, and explains the motivation for their existence.

The visual tree always there

The visual tree in WPF is the core of all things rendering. An element doesn’t show up on the screen until it’s in a rendered visual tree. The visual tree is made up, not surprisingly, of Visual objects; Visual is an ancestor class for e.g. controls – like Button and ListBox – and panels – like Grid, StackPanel, and Canvas.

For example, in:

<Grid>

<Button>Click</Button>

</Grid>

… the Grid is the visual parent of the Button, plain as day.

The visual tree affects all things visual, including input. For example, setting the opacity to 50% on a visual parent makes all its children 50% as well (if you also set the child’s opacity to 50%, it is a cumulative 75% transparent). Similarly, setting a transform on the parent, transforms the child as well. Hit-testing goes through the visual tree. Disabling the parent (setting IsEnabled to false) disables visual descendents, etc.

Now look at another example, change the button’s content to a StackPanel:

<Grid>

<TextBlock>Clack</TextBlock>

</StackPanel>

</Button>

</Grid>

… and again the Button’s visual parent is the Grid. But the StackPanel’s visual parent isn’t the Button; it’s a ContentPresenter. Plain as night.

The reason for this is that the Button has a template that describes how the Button really is rendered; that template defines the visual tree under the Button. To make that more explicit, here’s an example of the above that actually shows the template, and therefore what the visual tree will be:

<TextBlock>Clack</TextBlock>

</StackPanel>

<Button.Template>

</ControlTemplate>

</Button.Template>

</Button>

The visual tree in this case, from Button on down, is Button : ContentPresenter : StackPanel : Image/TextBlock.

Here’s another interesting example, this time of elements embedded in text flow:

Hello <Rectangle Width='10' Height='10' Fill='Red' />

</Paragraph>

</FlowDocument>

</RichTextBox>

In this case, you would expect the Rectangle’s “parent” to be the Paragraph. But Paragraph, like Bold and Italic, isn’t a visual. So the Rectangle’s visual parent is actually the RichTextBox.

Enter the logical tree

So we set out to make expectations come true – the StackPanel’s “parent” above should be the Button, and the Rectangle’s should be the Paragraph. To do that, we created some supplementary helper references, like my shortcut files. We gave elements the option to have a second parent, in addition to the visual parent, called the logical parent.

The StackPanel’s logical parent above is the Button, and the Rectangle’s logical parent is the Paragraph. Since that is the “parent” we thought that most developers & designers would interact with, we gave it the better name in the APIs; the logical parent is simply the Parent property on elements, whereas to get to the visual parent you have to call VisualTreeHelper.GetParent static method.

Often, an element’s logical parent is the same as its visual parent. This is the case with elements in a Panel, such as a Grid; the Panel.Children property is a UIElementCollection type, and UIElementCollection has a feature that all items get the Panel as both the logical parent and the visual parent. Other times, such as for the content of a Button (or any ContentControl), the two parents are different.

When does the logical tree matter?

Before going into more detail, let’s talk about where the logical tree is used. The logical parent is relevant for several tree-based features:

· The Parent property

As I just said above, the FrameworkElement.Parent property returns the logical parent. So StackPanel.Parent above returns the Button, and Rectangle.Parent returns the Paragraph.

· Property inheritance

Inheritable properties, such as FontFamily and DataContext, come from an element’s logical parent (or logical ancestor). The exception is that if an element has a visual parent but not a logical parent, the visual parent will be used.

· {DynamicResource} references

If a property has a {DynamicResource} set on it, it will search the .Resources of logical ancestors. Just like property inheritance, though, if there’s only a visual parent, that link is followed instead.

· Name

When looking up a name, such as in {Binding ElementName=Foo}, the search walks up the ancestry looking for a name scope, again just as it does for inheritable properties.

· Routed events

When an event is routing up the tree, such as the MouseLeftButtonDownEvent, the event goes up both the visual parent and the logical parent, if they’re different.

Let’s look at another example, taking an earlier example and adding an inheritable FontWeight property:

<TextBlock>Clack</TextBlock>

</StackPanel>

<Button.Template>

</ControlTemplate>

</Button.Template>

</Button>

Is the FontWeight on the TextBlock bold (inherited from the Button) or normal (inherited from the ContentPresenter)? The StackPanel’s visual parent is the ContentPresenter, and its logical parent is the Button. So in searching for an inheritable property we walk up from the TextBlock to the StackPanel, from there to the Button, and find the property value. So the TextBlock is bold.

Logical tree mechanics

An element doesn’t actually pick its logical parent; instead, a parent “adopts” children. For example, when you set a property value to the ContentControl.Content property, ContentControl takes that value as its logical child. To adopt a new logical child, an element simply calls AddLogicalChild. Similarly, to remove a logical child, an element calls RemoveLogicalChild. Note that you can’t adopt a child that already has a logical parent.

There is also actually an API to walk the logical tree – LogicalTreeHelper.GetParent and LogicalTreeHelper.GetChildren.

Logical tree in WPF controls

WPF controls already do the work to make the logical tree operate correctly. And actually it’s more general than that – Panel, ItemsControl, ContentControl, and Decorator, which are the most common base classes, all do the work to make the logical tree operate already. For example, if you create a custom Button that subclasses ContentControl, your Content property will pick up your button as the logical parent without you doing anything.

Here’s most of the elements in WPF that support the logical tree. Recall that the logical parent is established by the parent, and should be in response to getting/losing property values. Consequently this table lists the classes that have special support, and under which property they implement that support.

Class	Property(ies)
*System.Windows.Controls*
AdornedElementPlaceholder	Child property
ContentControl	Content
Decorator	Child
Grid	Children (inherited from Panel), Columns, Rows
HeaderedContentControl	Content (inherited from ContentControl), Header
HeaderedItemsControl	Items (inherited from ItemsControl), Header
InkCanvas	Children
ItemsControl	Items
Page	Content
Panel	Children
RichTextBox	Document
TextBlock	Text
TextBox	Text
ToolBarTray	ToolBars
ViewBox	Child
*System.Windows.Controls.Primitives*
BulletDecorator	Bullet and Child
DocumentViewerBase	Document
Popup	Child
*System.Windows.Documents*
FixedDocument	Pages
FixedPage	Children
FlowDocument	Blocks
FlowDocumentReader	Document
FlowDocumentScrollViewer	Document
PageContent	Child
Table	RowGroups, Columns
Span	Inlines

Edge cases

There’s a few additional special cases for special circumstances. For example, a ContentControl won’t adopt its Content value as its logical child if the ContentControl is part of a ControlTemplate, and if that new value already has a logical parent. That’s to avoid the scenario of a template part trying to steal the logical child of the outer (“templated parent”) control. That’s getting more detailed, and deserves some more words and a picture. I’ll add that detail soon to complete this story …

In conclusion …

The main take-away from this is that the logical tree supplements the visual tree, to enable things like the Parent property and like property inheritance to behave more intuitively. It’s interesting to go deep and understand the details and the mechanism, but the bottom line is that you rarely need to understand it; base element classes like ContentControl, ItemsControl, Decorator, and Panel already do the work, so you don’t have to.

Model-see, Model-do, and the Poo is Optional

Mike Hillberg [MSFT] — Thu, 22 May 2008 01:05:00 GMT

Like a lot of people, I’ve developed software professionally for a lot of different environments: PC systems and embedded systems; high- and low-level languages; kernel mode, user mode, real mode, and protected mode; system services; domain controllers; bootstrappers; image processors; a debugger; a compiler; a search engine; small systems and really big systems; file systems; COM; real-time and internet time. I’ve never quite developed professionally for a mainframe, but I’ve worked on clients for mainframe services, and I sat next to a guy that wrote the services (I also sat next to a guy once who would only use a Commodore 64).

But I’ve been spending the last bunch of years helping to create WPF and Silverlight. So in a model/view world, I look at a lot of things from the View point of view. And from that point of view, you come to realize that all computer applications are really just data visualization applications. There’s certainly other software – services, device drivers, pi calculators (I’m still waiting for that one to respond) – but end-user applications are all about data viz.

Except that’s not quite the right, because you don’t just visualize data; you manipulate it too. E.g. Netflix.com lets me both see and modify my queue. And the word “data” isn’t great either, because it makes you think just of the bits stored in a database. So that got me to all applications being some form of “model visualization and manipulation”.

That doesn’t roll of the tongue either though. So now I just consider all applications to be a case of “model see model do”. Which makes me a code monkey.

Models, Views, and Poo

As for models and views, in my current View point of view, there’s always a view, and there’s optionally some amount (and maybe a large amount) of model. The view visualizes the model, and the end-user interacts with that visualization, which propagates back as changes to the model.

A lot of simple apps just need a view – a bunch of UI controls created in Xaml, with event handlers hooked up to some code-behind logic. But I usually end up splitting up my application logic into some model objects and a view object, with the controls in the view bound to model properties.

For more complex applications, you could have a more complicated view. But then you can’t change the look of your application easily, and views are more difficult to test. The solution to that is more sophisticated models. I like the Model/View/ViewModel approach (MVVM), which is similar to the Model/View/Presenter (MVP) pattern.

But I think it’s really useful to be able to pick the right number of layers and sophistication behind my view as is appropriate for the application; it’s all just some form of model in the end. Dr. WPF and Josh Smith talk about this too. Dr. WPF put a name on this agnosticism towards the particular model – Model/View/Poo. I like it. (There’s a joke in there somewhere about code monkeys and poo.)

Another thing I find interesting about the layers of models is that we continue to work to narrow the gap between the model and the view, by making the model more view-friendly:

· For example, tools generate classes with IEditableObject implementations, which enables view controls to perform cancelable updates to the model objects.

· For example, Linq makes it easier to convert data sources into objects, or to re-shape existing objects into objects more appropriate for the view.

Similarly, we’re making the view more model-friendly:

· For example, it’s easy to bind to any model object, or to lists of objects, or to find visualizations for model objects, etc.

· For example, the CollectionView class provides built-in support for the classic problem of synchronizing two parts of the view, such as the master list and the details pane.

Updates to the Model, from the View, in Xaml

Along the general lines of the “model do” side of things, it’s common to hook events in the view and write code to update the model, or to define commands in the model that are invoked by e.g. buttons in the view. As an experiment, I played with one way of somewhat marrying the two approaches – allow the view to phrase a model method in the form of a command.

The example is a view that’s bound to a File object; the view shows the file name, but as TextBox so that you can change the name. When you click the “Rename” button, it fires a command which calls the Rename method on the File object with the new name from the TextBox.

I.e., here’s the File object, which just has a Name property and a Rename method:

public class File : INotifyPropertyChanged

{

string _name;

public string Name

{

get { return _name; }

set

{

_name = value;

if (PropertyChanged != null)

PropertyChanged(this, new PropertyChangedEventArgs("Name"));

}

public void Rename(string newName)

{

// Perform the file rename here

}

public event PropertyChangedEventHandler PropertyChanged;

}

… here’s some sample data getting hooked up as the DataContext:

public Window1()

{

InitializeComponent();

DataContext = new File() { Name = "TestName" };

}

… and here’s the view, which has a TextBox and a Button, a command on the Button that calls the Rename method, and the argument to the Rename coming from the TextBox:

<Button.Command>

<mc:MethodCommand MethodName="Rename">

<mc:MethodArgument Value="{Binding Text, ElementName=_renameTextBox}" />

</mc:MethodCommand>

</Button.Command>

</Button>

</StackPanel>

The key here is that when the Button is clicked, the MethodCommand is invoked. That MethodCommand calls the Rename method on the File object, passing in the current value of the TextBox as the newName parameter. The File object is picked up by the MethodCommand by inheriting the DataContext.

The rest of this post is the MethodCommand implementation. The reason I made the MethodCommand class and the Arguments property of type Freezable was so that the bindings would be fully functional (so that the DataContext would inherit, and so that ElementName bindings could work).

(I haven’t tested this on Commodore 64 yet.)

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Windows.Input;

using System.Windows;

using System.Reflection;

using System.Collections;

using System.Windows.Data;

using System.ComponentModel;

using System.Windows.Markup;

namespace MethodCommandNS

{

[ContentProperty("Arguments")]

public class MethodCommand

: Freezable, // Enable ElementName and DataContext bindings

ICommand,

INotifyPropertyChanged

{

// The name of the method to call on Invoke

public string MethodName { get; set; }

// When this is set, exceptions during invoke are caught, and the exception

// is set as the Exception property

[DefaultValue(true)]

public bool CatchExceptions { get; set; }

// If there is an exception during a command invoke, and CatchExceptions

// is set, this will have the exception object.

public Exception Exception

{

get { return _exception; }

private set

{

_exception = value;

FirePropertyChanged("Exception");

}

Exception _exception;

// This holds the arguments to be passed to the method.

// This is Freezable so that the DataContext and ElementName bindings

// can work correctly.

public FreezableCollection<MethodArgument> Arguments

{

get { return (FreezableCollection<MethodArgument>)GetValue(ArgumentsProperty); }

}

public static readonly DependencyProperty ArgumentsProperty =

DependencyProperty.Register("Arguments", typeof(FreezableCollection<MethodArgument>), typeof(MethodCommand), null);

// This is a private DP that's used to get the inherited DataContext

// (see it used in the MethodCommand constructor).

private static readonly DependencyProperty ElementDataContextProperty =

DependencyProperty.Register("ElementDataContext", typeof(object), typeof(MethodCommand), null);

// The Target property specifies the object on which to invoke the method.

// If this is null, invoke the method on the DataContext

public object Target

{

get { return (object)GetValue(TargetProperty); }

set { SetValue(TargetProperty, value); }

}

public static readonly DependencyProperty TargetProperty =

DependencyProperty.Register("Target", typeof(object), typeof(MethodCommand), null);

// Constructor

public MethodCommand()

{

// The Arguments property is read-only and never null

SetValue(ArgumentsProperty, new FreezableCollection<MethodArgument>());

// Set a default Binding onto the private ElementDataContextProperty.

// A default binding just binds to the inherited DataContext. This is how

// MethodCommand typically gets the object on which to invoke the method.

BindingOperations.SetBinding(

this,

ElementDataContextProperty,

new Binding());

// By default, catch exceptions that are raised by the method.

CatchExceptions = true;

}

// Fire the PropertyChanged event when the Exception property changes.

public event PropertyChangedEventHandler PropertyChanged;

void FirePropertyChanged(string propertyName)

{

if (PropertyChanged != null)

{

PropertyChanged(this, new PropertyChangedEventArgs(propertyName));

}

// We need to implement this method because we're a Freezzable subtype.

protected override Freezable CreateInstanceCore()

{

throw new NotImplementedException();

}

// Implement the CanExecute members of ICommand

public event EventHandler CanExecuteChanged;

public bool CanExecute(object parameter)

{

// We'll always be enabled to be invokes.

return true;

}

// ICommand.Execute implementation. This version calls ExecuteImpl, which does the

// the actual call. But this method wraps that call in a try/finally, depending on

// the value of CatchExceptions.

public void Execute(object parameter)

{

// Clear out any exception of a former invocation.

Exception = null;

if (CatchExceptions)

{

// Invoke the method in a try/finally

try

{

ExecuteImpl(parameter);

}

catch (Exception e)

{

// Any exceptions will likely come back as a

// TargetInvocationException, but the original exception

// is more interesting.

if (e is TargetInvocationException)

{

if (e.InnerException != null)

Exception = e.InnerException;

else

Exception = e;

}

else

Exception = e;

}

// Otherwise, CatchExceptions isn't set, so just forward the call

else

{

ExecuteImpl(parameter);

}

// ExecuteImpl is where we actually invoke the method.

void ExecuteImpl(object parameter)

{

// See if the Target property is set

object target = Target;

if (target == null)

{

// If not, look for an inherited DataContext

target = GetValue(ElementDataContextProperty);

}

// We must have a target, either from Target or DataContext

if (target == null)

{

throw new InvalidOperationException("MethodCommand target not found (must set either Target or DataContext)");

}

// Get the method to be called. Note that this doesn't support

// overloaded methods, but it could be updated to do so.

MethodInfo methodInfo = target.GetType().GetMethod(MethodName);

if (methodInfo == null)

{

throw new InvalidOperationException("Method '" + MethodName + "' couldn't be found on type '" + target.GetType().Name + "'");

}

// Copy the Arguments to an array

object[] arguments = new object[Arguments.Count];

for (int i = 0; i < Arguments.Count; i++)

arguments[i] = Arguments[i].Value;

// Invoke the method

methodInfo.Invoke(target, arguments);

}

// The MethodArgument class plugs into MethodCommand.Arguments

public class MethodArgument

: Freezable // Enable ElementName and DataContext bindings

{

public MethodArgument() { }

// The value of a method argument

public object Value

{

get { return (object)GetValue(ValueProperty); }

set { SetValue(ValueProperty, value); }

}

public static readonly DependencyProperty ValueProperty =

DependencyProperty.Register("Value", typeof(object), typeof(MethodArgument), null);

// We need to implement this method since we are a subtype of Freezable. But since

// we don't need to support cloning, we won't implement it.

protected override Freezable CreateInstanceCore()

{

throw new NotImplementedException();

}

Binding to anonymous types in an Xbap or Silverlight application

Mike Hillberg [MSFT] — Mon, 05 May 2008 19:56:00 GMT

It's easy to use Linq queries to create objects, and to use {Binding}s to bind properties of those objects into your view. If you're doing this for an application that will run as an Xbap ("WPF Browser Application") or as a Silverlight app, just note that you need to generate nominal types rather than anonymous types.

For example, say you have a bound ListBox like:

<ListBox.ItemTemplate>

</DataTemplate>

</ListBox.ItemTemplate>

</ListBox>

... where the ListBox ItemsSource is established with the following code at initialization-time:

public partial class Window1 : Window

{

private Collection<Person> _collection = new Collection<Person>()

{

new Person() { FirstName = "John", LastName = "Doe" },

new Person() { FirstName = "Jane", LastName = "Doe" },

new Person() { FirstName = "Fred", LastName = "Flinstone" }

};

public Window1()

{

InitializeComponent();

ListBox1.ItemsSource = from person in _collection

where person.LastName == "Doe"

select new { First = person.FirstName, Last = person.LastName };

}

This won’t show anything in the ListBox. (You might see debug output like “System.Windows.Data Error: 12 : Cannot get 'First' value (type 'String') from '' (type '<>f__AnonymousType0`2')”).

The issue here is that the select statement in the query (select new { First = person.FirstName, Last = person.LastName }) generates an internal (anonymous) type, and with an Xbap or Silverlight app you can’t bind to internal types. The solution is to use a nominal type. E.g., add this class definition:

public class SelectedPerson

{

public string First { get; set; }

public string Last { get; set; }

}

... and use it in the select:

ListBox1.ItemsSource = from person in _collection

where person.LastName == "Doe"

select new SelectedPerson { First = person.FirstName, Last = person.LastName };

... and your list box is populated with “John” and “Jane”.

Simple way to create a gradient brush in Xaml

Mike Hillberg [MSFT] — Fri, 25 Apr 2008 09:06:00 GMT

Rob, Nikhil & I were talking today about the early days of Xaml when you could create linear gradient brushes as an attribute value. E.g. (borrowing from Rob’s post on this subject) instead of creating a LinearGradientBrush for the fill of a rectangle with this Xaml:

<Rectangle.Fill>

</LinearGradientBrush>

</Rectangle.Fill>

</Rectangle>

… you could write:

<Rectangle Width="300" Height="200"

Fill="HorizontalGradient Blue White" >

</Rectangle>

The up side of the latter syntax obviously is that it’s terser. The drawback is that it’s difficult for a tool to understand it; it has no idea what “HorizontalGradient” means, nor that it has two arguments (nor how those arguments should be interpreted).

For that reason, we created the markup extension primitive for Xaml. A markup extension is any type that derives from MarkupExtension, and it can be specified in an attribute using “squiggle” syntax. The most common markup extension you see in Xaml is the binding extension, e.g.:

So we caution against “mini languages” in Xaml, such as the HorizontalGradient example above, because of the lack of toolability and compile-time validation, and we recommend markup extensions instead.

And so, here’s an example of a definition for such an extension, in this case to create a LinearGradientBrush:

[MarkupExtensionReturnType(typeof(LinearGradientBrush))]

public class LinearGradientBrushExtension : MarkupExtension

{

public LinearGradientBrushExtension()

{

StartColor = EndColor = Colors.White;

}

public LinearGradientBrushExtension( Color startColor, Color endColor, double angle)

{

StartColor = startColor;

EndColor = endColor;

Angle = angle;

}

public Color StartColor { get; set; }

public Color EndColor { get; set; }

public double Angle { get; set; }

public override object ProvideValue(IServiceProvider serviceProvider)

{

return new LinearGradientBrush(StartColor, EndColor, Angle);

}

… which can be used in Xaml like this:

<Rectangle Width="300" Height="200"

Fill="{helper:LinearGradientBrush StartColor=red, EndColor=blue, Angle=0}" />

… or, since the LinearGradientBrushExtension has a convenience constructor for setting the properties, the Xaml can simply be:

<Rectangle Width="300" Height="200"

Fill="{helper:LinearGradientBrush red, blue, 0}" />

Similarly, here’s a markup extension for a RadialGradientBrush:

[MarkupExtensionReturnType(typeof(RadialGradientBrush))]

public class RadialGradientBrushExtension : MarkupExtension

{

public RadialGradientBrushExtension()

{

StartColor = EndColor = Colors.White;

}

public RadialGradientBrushExtension( Color startColor, Color endColor )

{

StartColor = startColor;

EndColor = endColor;

}

public Color StartColor { get; set; }

public Color EndColor { get; set; }

public override object ProvideValue(IServiceProvider serviceProvider)

{

return new RadialGradientBrush(StartColor, EndColor);

}

… which enables this Xaml:

<Rectangle Width="300" Height="200"

Fill="{local:RadialGradientBrush red, blue}" />

A Context-Sensitive Help Provider in Wpf

Mike Hillberg [MSFT] — Fri, 27 Jul 2007 05:56:00 GMT

Here's an example of a way to add context-sensitive help to your application.

The main idea is to simply use the built-in ApplicationCommands.Help command. This command is already tied to the F1 key, and so executes when you hit F1, and tells your command handler what element the user was on when it was hit.

First, start with this little application:

<Window x:Class="ContextSensitiveHelp.Window1"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

Title="Window1" Height="300" Width="300" >

<Grid>

<Grid.ColumnDefinitions>

</Grid.ColumnDefinitions>

<Grid.RowDefinitions>

</Grid.RowDefinitions>

<TextBlock Grid.Column='0' Grid.Row='1' >Address</TextBlock>

</Grid>

</Window>

... which looks like this:

Now add some help text. I'll add specific help text to the NameField text box, and general help text to the Window, but nothing to the AddressField, just to show how that behaves:

<Window x:Class="ContextSensitiveHelp.Window1"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

Title="Window1" Height="300" Width="300"

xmlns:h="clr-namespace:ContextSensitiveHelp"

h:HelpProvider.HelpString='This form is for your name and address' >

<Grid>

<Grid.ColumnDefinitions>

</Grid.ColumnDefinitions>

<Grid.RowDefinitions>

</Grid.RowDefinitions>

<TextBlock Grid.Column='0' Grid.Row='1' >Address</TextBlock>

<TextBox Grid.Column='1' Grid.Row='0' Name='NameField'

h:HelpProvider.HelpString="Enter your name here"/>

</Grid>

</Window>

So you see here that the help text is being provided by attached properties defined by a class named HelpProvider. Here's the attached property definition:

static class HelpProvider

{

public static string GetHelpString(DependencyObject obj)

{

return (string)obj.GetValue(HelpStringProperty);

}

public static void SetHelpString(DependencyObject obj, string value)

{

obj.SetValue(HelpStringProperty, value);

}

public static readonly DependencyProperty HelpStringProperty =

DependencyProperty.RegisterAttached(

"HelpString", typeof(string), typeof(HelpProvider));

}

I made this a help string property, but you could have a HelpKeyword property that held a help topic ID, or a HelpURI property that referenced online documentation, etc.

Now we just need to respond to the command. The way you respond to commands is with event listeners registered to handle the command. You can simply set up a command handler on a particular element by setting a CommandBinding like this:

<Window x:Class="ContextSensitiveHelp.Window1"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

Title="Window1" Height="300" Width="300"

xmlns:h="clr-namespace:ContextSensitiveHelp"

h:HelpProvider.HelpString='This form is for your name and address' >

<Window.CommandBindings>

</Window.CommandBindings>

<Grid>

<Grid.ColumnDefinitions>

</Grid.ColumnDefinitions>

<Grid.RowDefinitions>

</Grid.RowDefinitions>

<TextBlock Grid.Column='0' Grid.Row='1' >Address</TextBlock>

<TextBox Grid.Column='1' Grid.Row='0' Name='NameField'

h:HelpProvider.HelpString="Enter your name here"/>

</Grid>

</Window>

... and then respond to the command like this:

private void Window_Help_Executed(object sender, ExecutedRoutedEventArgs e)

{

FrameworkElement source = e.Source as FrameworkElement;

if (source != null)

{

string helpString = HelpProvider.GetHelpString(source);

if (helpString != null)

{

System.Windows.MessageBox.Show("Help: " + helpString);

}

The problem with that is that you'd really rather not implement the handling as part of your application, but rather as part of the HelpProvider (following the WinForms model). Also, the code above finds the HelpString on NameField, which has a HelpString set, but it doesn't find anything for the AddressField, because it doesn't have a good way to find the more general HelpString set on the Window.

So instead of putting the command handler in the Window, we'll put it in the HelpProvider itself. We can do this by registering a command handler directly against the FrameworkElement class; now we'll get called for any FrameworkElement instance in the application. We'll also register for the command's CanExecute handler. This allows us to decide if we should handle a command at an element, or let it bubble up to a higher handler. That is, this will allow the Help command to bubble up from AddressField, which has no HelpString, to Window which does.

The HelpProvider class is then updated with these new members:

static class HelpProvider

{

static HelpProvider()

{

CommandManager.RegisterClassCommandBinding(

typeof(FrameworkElement),

new CommandBinding(

ApplicationCommands.Help,

new ExecutedRoutedEventHandler(Executed),

new CanExecuteRoutedEventHandler(CanExecute)));

}

static private void CanExecute(object sender, CanExecuteRoutedEventArgs e)

{

FrameworkElement senderElement = sender as FrameworkElement;

if (HelpProvider.GetHelpString(senderElement) != null)

e.CanExecute = true;

}

static private void Executed(object sender, ExecutedRoutedEventArgs e)

{

MessageBox.Show("Help: " + HelpProvider.GetHelpString(sender as FrameworkElement));

}

...

... and we can remove the CommandBindings from the Window:

<Window x:Class="ContextSensitiveHelp.Window1"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

Title="Window1" Height="300" Width="300"

xmlns:h="clr-namespace:ContextSensitiveHelp"

h:HelpProvider.HelpString='This form is for your name and address' >

<~~Window.CommandBindings~~>

<~~CommandBinding~~ ~~Command~~=~~'ApplicationCommands.Help' Executed='Window_Help_Executed' />~~

</~~Window.CommandBindings~~>

<Grid>

<Grid.ColumnDefinitions>

</Grid.ColumnDefinitions>

<Grid.RowDefinitions>

</Grid.RowDefinitions>

<TextBlock Grid.Column='0' Grid.Row='1' >Address</TextBlock>

<TextBox Grid.Column='1' Grid.Row='0' Name='NameField'

h:HelpProvider.HelpString="Enter your name here"/>

</Grid>

</Window>

... and now F1 on NameField displays "Enter your name here", and F1 on the AddressField displays "This form is for your name and address".

Finally, note that you can also do more exciting things in the help handler, like load a chm file with the System.Windows.Forms.Help class:

static private void Executed(object sender, ExecutedRoutedEventArgs e)

{

System.Windows.Forms.Help.ShowHelp(null, @"MyHelp.chm");

}

A look at stretchy elements

Mike Hillberg [MSFT] — Fri, 06 Jul 2007 03:31:00 GMT

There are several elements in Wpf that have a "Stretch" property. The basic idea of these properties is to allow you to specify how an element adapts itself to a layout container, such as a Grid. For example, if you have a 100x200 pixel image, and it's being positioned in a 150x300 pixel cell in a Grid, should the image be centered and leave some blank space? Or stretched to fit? And if stretched, should aspect ratio be preserved?

I'll use Image, Viewbox, and Shape (particularly Rectangle and Ellipse) as examples.

Background

First, the basic setup for these examples. Here is a Grid with 9 cells (a 3 by 3 Grid). The center cell (the Border named Content) is where we'll put things that need to be stretched.

<Grid.ColumnDefinitions>

</Grid.ColumnDefinitions>

<Grid.RowDefinitions>

</Grid.RowDefinitions>

</Border>

</Grid>

It looks like this so far (the white box in the middle is the 'Content' Border):

Second, why do we need to stretch at all? The answer is that not every element knows how to size itself to its parent's layout. For example, a Button does know how to size it self; putting a button inside Content gives us:

<Button>Click</Button>

</Border>

and looks like:

That is, the button sized itself to fit the 'layout slot' provided by the container (the center cell of the Grid). (Specifically, the Button doesn't size itself, but the root of the Button's template is a Border, which does size itself.)

Other elements, such as Image, however, need help to determine how or if they fit into their layout slot. Thus the Image.Stretch property.

Stretching an Image

If we set an image into our content, likely its natural size won't be exactly the same as the layout slot provided by the parent. How the image stretches is specified by the Stretch property, which is an enumerator also named Stretch, and has 4 values:

None: The element preserves its "natural size", it's not scaled in any way.
Fill: The element is stretched as necessary to fit the layout slot.
Uniform: The element is stretched to exactly fill either the height or the width of the layout slot, but the "shape" or "aspect ratio" of the element is not altered, and none of the element is clipped.
UniformToFill: The element is stretched so that it completely fills both the height and the width of the layout slot, again the aspect ratio is not altered, and the element is clipped as necessary.

For Image, the default value of Stretch is 'Uniform', so here's an example of that first:

</Border>

Figure: 'Uniform' stretch on an image

Here, the image was scaled to fit the layout slot, but it preserves the aspect ratio of the picture. That avoids distortion of the image, but doesn't fill the entire slot either, resulting in the white bands on the left/right of the image in this case.

To prevent any scaling of the image, set the Stretch to 'None' and you get:

Figure: 'None' stretch on an image

Here we see the image in its natural size, which is bigger than the space allotted to the image by the Grid. As a result, we're only seeing part of the image. (In this case, the part we're seeing is the upper left corner, but you can control that by setting VerticalAlignment and/or HorizontalAlignment on the image.)

To ensure the image exactly fills the layout slot, set Stretch to 'Fill':

Figure: 'Fill' stretch on an image

But notice that this comes at the expense of distortion.

Finally, setting stretch to 'UniformToFill' does a combination of scaling an clipping:

Figure: 'UniformToFill' stretch of an image

In this case, the image is scaled until it's big enough to fit the width of the layout slot, which meant that some of the height had to be clipped.

Stretching any element

You can actually stretch any element tree using the Viewbox decorator. For example, start by replacing the image in our Content with a Viewbox, and start with Stretch set to 'None' so that it has no effect (like Image, the default value for Viewbox.Stretch is 'Uniform'):

<Button>Click</Button>

</Viewbox>

</Border>

Figure: 'None' stretch on a Viewbox

The only difference between this and the same Button example earlier without the Viewbox is that the Viewbox tells the button to be the size it wants to be, rather than growing itself to fit the larger layout slot in the Grid. That is, the Button here takes on it's "natural size".

Next, to see the effect of stretch, change Stretch to 'Uniform' (the default value) and get:

Figure: 'Uniform' stretch on a Viewbox

Here, just like the image example above, the button is stretched until it fills either the height or the width of the layout slot (the height in this example), but the aspect ration is preserved, leaving un-rendered bands on the left/right.

If filling the space is more important than the aspect ration, then you can use 'Fill' again:

Figure: 'Fill' stretch on a Viewbox

And similarly, 'UniformToFill' fills the whole layout slot and preserves the aspect ratio, but clips one of the dimensions:

Figure: 'UniformToFill' stretch on a Viewbox

Stretching shapes

Shapes are mostly similar to Image and Viewbox. Rectangle and Ellipse shapes require an understand of their natural size to make sense of, which is covered later in a separate section. For other shapes, such as Path, the behavior is pretty much the same as Image/Viewbox/etc, but there are a few differences which I'll describe here.

Start with this Path as our 'Content':

<Path.Data>

<PathGeometry.Figures>

<PathFigure.Segments>

</PathSegmentCollection>

</PathFigure.Segments>

</PathFigure>

</PathFigureCollection>

</PathGeometry.Figures>

</PathGeometry>

</Path.Data>

</Path>

One way that Shape.Stretch is different from other type's Stretch properties is that it defaults to 'None' rather than 'Uniform. So, with the stretch set to 'None' we get the Path in its natural size, which is 150x200 in this case:

Figure: 'None' stretch on a path

Now try 'Uniform', where as usual we preserve aspect ration and fill one of the dimensions, causing white bands on the left/right in this example:

Figure: 'Uniform' stretch on a path

Other than the default value of Stretch being 'None', that's all the same as the Stretch examples above for Image & Viewbox. One thing that shapes do differently than other types is that not all of the pixels are scaled; if you look closely at the last picture, the overall path was scaled to be larger, but the stroke was not, it is still 10 pixels wide.

The other difference in Shape's Stretch behavior is in an un-constrained layout, it takes up the same space as it does under 'Fill'. To use some pictures as examples, here's the above Path in a StackPanel, with a Gray rectangle above and below it in the stack, and with the width constrained enough to make the Stretch of 'Uniform' interesting:

... which looks like:

Figure: Path in a StackPanel with over-constrained in width, with Uniform stretch

Since the stretch is Uniform, the Path is scaled down to fit the layout slot while preserving aspect ratio. But notice that there are white bands on the top and bottom. That accounts for the space that the Path would take up under a 'Fill' stretch.

To make that more clear, here's the same Path, next to a Path with a stretch of 'Fill'.

Figure: Same Path in a StackPanel, with 'Uniform' and 'Fill' stretches

Notice that the Path takes up just as much space in the StackPanel in either case.

In contrast, Image and Viewbox only size themselves as necessary, such as in this Image example (still in a StackPanel, and with a Stretch of 'Uniform'):

Figure: An Image in a StackPanel with 'Uniform' stretch

Effect of Width and Height (and other layout properties) on stretching

In the examples so far, the width and height of the layout slot was determined by the root Grid (whose size, in turn, is determined by the Window it lives in). If you specify layout properties such as Width/Height, MaxWidth/MaxHeight, Margin, etc., you might alter the layout slot.

For example, take the image again where we had UniformToFill:

... which looks like:

Figure: 'UniformToFill' stretch on an image (again)

Now, force the Width to be 100:

... and we get:

Figure: 'UniformToFill' stretch on the same image with Width=100

In the first case (where Width isn't specified), the layout slot for the image is 500 pixels wide by 250 high in this example. In that case, by the time the image is scaled down enough to get down to 500 wide, the height hasn't shrunk to 250, causing the height to be clipped. In the second case, setting the Width to 100 reduced the layout slot to 100x250. Now, by the time the image is scaled down enough to meet the height of 250, the width is still much more than 100, causing the width to get clipped.

Rectangle and Ellipse

Rectangle and Ellipse are different from other Path types, because they don't have a natural width and height. That's significant, because all of the stretching behavior depends on that. For example, 'None' stretching means just use the natural width/height, 'Uniform' stretching means preserve the natural width/height.

For example, in the Path example above, the natural size of the Path is determined by the PathGeometry. In a Line, the natural size is determined by the start and end points.

Of course, the Rectangle and Ellipse elements do have properties named "Width" and "Height", but recall that these are layout properties that all elements have (along with other layout properties such as Margin).

As a result, the natural size of any Rectangle or Ellipse is 0x0. Let's see what that does under the different stretches ...

Rectangle with 'Fill' stretch

First, for the examples, change the 'Content' Border to contain a Rectangle:

<Rectangle.Fill>

</LinearGradientBrush>

</Rectangle.Fill>

</Rectangle>

</Border>

... which looks like:

Figure: Rectangle with no Width/Height and default stretch ('Fill')

Notice that the Rectangle has no Width or Height specified. Also note that the default value of the Stretch property on a Rectangle (and Ellipse) is 'Fill', which differs from Image, Viewbox, MediaElement, and Shape, which all default Stretch to 'Uniform'. As a result, the rectangle simply fills the layout slot.

Now specify a Width, say 200, and you get:

Figure: Rectangle with Width and 'Fill' stretch

Here the width of the layout slot is constrained by setting the Width property, but the height is determined by the parent layout. Again, that's all the same as the earlier Image/Viewbox 'Fill' examples.

Rectangle with 'Uniform' stretch

Where Rectangle starts to get interesting is in uniformity. For a Rectangle/Ellipse, "uniform" means "make it a square/circle". So, set our Rectangle with a Width and 'Uniform' stretch (but still no Height):

...

... and we get:

Figure: Rectangle with Width and 'Uniform' stretch

Here we have the Width constrained to be within 200, and the height constrained (by the layout slot) to be within 250. Since the stretch is 'Uniform', the width and height are calculated to be min(200,250)=200, and we get a square that is skinnier and shorter than the layout slot offered by the Grid.

An interesting thing to note here is the positioning of the square; it's centered horizontally but not vertically. The horizontal centering is coming from the Border (recall the parent of this Rectangle is a Border named 'Content'). And the Border actually thinks that it's centering the Rectangle vertically as well; the final height of the rendered Rectangle in this example is reported to be 250 pixels, though it actually only renders 200.

Let's make it more interesting and specify a height of 100:

...

... this gives us:

Figure: Rectangle with Width and Height and 'Uniform' stretch

Here, since this Rectangle is constrained to be a square by the 'Uniform' stretch, and since the Height (100) is more constrained than the Width (200), we get a 100x100 square.

Again, here the positioning of the Rectangle within the 'Content' Border results from the Rectangle having a final size of 200x100, but with the 'Uniform' stretch it only renders 100x100.

Rectangle with 'None' and 'UniformToFill' stretch

So far we've looked at Rectangle with 'Fill' and 'Uniform'. A stretch of 'None' means use the natural size of the object. Since the natural size of a Rectangle or Ellipse is 0x0, setting the stretch to 'None' gives you a size of 0x0.

So the only thing left to look at is UniformToFill. For Rectangle and Ellipse, 'UniformToFill' produces a square/circle, just as is the case with 'Uniform'. Like the other stretch behaviors (e.g. Image and Viewbox above), 'UniformToFill' clips as necessary to fill the layout slot without altering the aspect ratio.

For example, the following shows an Ellipse with no Width or Height specified, and the Stretch set to 'Fill' and then 'UniformToFill':

Figure: Ellipse with no Width/Height and stretch of 'Fill' then 'UniformToFill'

The first picture shows the 'Fill' case, where the Ellipse simply scales to the height and width of the layout slot. In the second picture, the UniformToFill causes the Ellipse to be round, fill the 500x250 pixel layout slot (which is determined by the Grid), and preserve aspect ratio. What this means is that the Ellipse ends up being a 500x500 circle, and the bottom half of it gets clipped.

To sum it up ...

Stretching is about filling your layout slot:

Your layout slot is a function of your parent element, Width/Height, MinWidth/MinHeight, Margin, etc. (The usual layout system features.)
'None' means use your natural size
'Fill' means to scale to the slots width and height, at the expense of shape distortion.
'Uniform' means fill the slot in one dimension, but not the other, in order to preserve aspect ratio.
'UniformToFill' means fill the slot in both dimensions, clipping as necessary, to again preserve aspect ratio.
The default value for Stretch is usually 'Uniform'

When stretching Shape types:

The default value of Stretch is usually 'None'
In unconstrained layouts, such as a StackPanel, the Shape takes up the amount of space that it would under 'Fill' stretch.

Rectangle and Ellipse have some custom handling of stretching:

The default value of Stretch is 'Fill'
'Uniform' means constrain the rectangle/ellipse to be a square/circle
'None' is interpreted as a size of 0x0.

Building read-only objects in Xaml

Mike Hillberg [MSFT] — Mon, 02 Jul 2007 20:32:00 GMT

We often use Xaml to instantiate and initialize objects. For example, given “<Foo Bar=’1’/>”, a Xaml loader creates a Foo object, and sets the Bar property to 1. That works when the Bar property is settable, but what can you do if it isn’t?

An example of this scenario in .Net today shows up with strings; the String type is immutable, it doesn’t have any settable properties. But sometimes you want to build a string with formats, concatenation, etc. Thus the invention of the StringBuilder class, where the properties have setters (and the methods can mutate an instance).

You can follow the same idea in Xaml by creating builder types, using markup extensions.

Here’s an example of a type whose properties can’t be set except internally, since it has read-only properties:

<Assembly: XmlnsDefinition("http://blogs.msdn.com/mikehillberg/BuilderPattern", "PeopleNamespace")>

Namespace PeopleNamespace

Public Class Person

Private _name As String

Public ReadOnly Property Name() As String

Get

Return _name

End Get

End Property

Private _age As Integer

Public ReadOnly Property Age() As Integer

Get

Return _age

End Get

End Property

Public Shared Function Create(ByVal Name As String, ByVal Age As Integer) As Person

Dim person As New Person

person._name = Name

person._age = Age

Return person

End Function

End Class

End Namespace

(I’m using the XmlnsDefinition assembly attribute here so that I can have a normal-looking Uri in my Xaml, but you can also make references directly to an assembly using the “clr-namespace” syntax, as described here.)

For demonstration, also define a List(Of Person) named “People”:

Namespace PeopleNamespace

Public Class People

Inherits List(Of Person)

End Class

End Namespace

… and then, for example, you can’t load this Xaml, because of the read-only properties:

</People>

(This gives an error of “'Name' property is read-only and cannot be set from markup”).

But we can create a markup extension that follows the builder pattern to create the object.

First, define the markup extension, named PersonExtension. It looks just like the Person type, except it has setters for the properties, and overrides the ProvideValue function:

<Assembly: XmlnsDefinition("http://blogs.msdn.com/mikehillberg/BuilderPatternBuilder", "PeopleBuilderNamespace")>

Namespace PeopleBuilderNamespace

Public Class PersonExtension

Inherits MarkupExtension

Private _name As String

Public Property Name() As String

Get

Return _name

End Get

Set(ByVal value As String)

_name = value

End Set

End Property

Private _age As Integer

Public Property Age() As Integer

Get

Return _age

End Get

Set(ByVal value As Integer)

_age = value

End Set

End Property

Public Overrides Function ProvideValue(ByVal serviceProvider As IServiceProvider) As Object

Return Person.Create(Name, Age)

End Function

End Class

End Namespace

And now we can modify the markup slightly so that it successfully loads:

<People

xmlns="http://blogs.msdn.com/mikehillberg/BuilderPattern"

xmlns:builder="http://blogs.msdn.com/mikehillberg/BuilderPatternBuilder" >

<builder:Person Name='Mike' Age='5' />

<builder:Person Name='Dwayne' Age='7' />

</People>

To test it out, run this code:

Sub Main()

Dim people As People

people = XamlReader.Load(File.Open("..\..\Test.xaml", FileMode.Open))

For Each person As Person In people

Console.WriteLine("Name = " + person.Name + ", Age = " + person.Age.ToString())

End Sub

… and we get this output:

Name = Mike, Age = 5

Name = Dwayne, Age = 7

One last note about markup extensions … Notice that I named the builder class “PersonExtension”, but I referred to it from Xaml as “Person”. That’s possible because when Xaml doesn’t find “Person”, it automatically appends the “Extension” suffix, and looks for “PersonExtension”. If Person and PersonExtension had been in the same Clr namespace, that would have caused confusion, so I put them in different namespaces (“PeopleNamespace” and “PeopleBuilderNamespace”). Alternatively, I could have put them both in the “PeopleNamespace”, and used the specific <PersonExtension> tag (or maybe I would have named it “PersonBuilder”).

Two ProgressBar Tricks

Mike Hillberg [MSFT] — Fri, 18 May 2007 01:57:00 GMT

Here’s a couple of handy ProgressBar tricks …

The first trick is to use a negative Minimum value, so that as soon as a ProgressBar starts, you give the user the visual feel that the progress has already begun. This is especially useful if, in your scenario, it takes a couple of seconds to get the Value moving. So simply:

The second trick allows you to get text or other content overlaid on top of a ProgressBar, which is a look you see a lot of nowadays. All you need to do is put the ProgressBar and text in the same cell of a Grid, causing them to render on top of each other. And if you put the text in a ViewBox to stretch it as necessary, everyone will size appropriately to whatever size the Grid is.

For example, this markup:

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

</Viewbox>

</Grid>

</Page>

… produces this look:

With a little more work, you can modify this so that the text changes itself when progress is complete. E.g., make the ProgressBar show “Loading …” until it’s finished, then change the text to “Complete”. Here's an example. (For convenience of trying this out, I also added a Loaded animation on the ProgressBar.Value, so that you can simply load it into IE or XamlPad and see it run.):

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

<Page.Triggers>

</Storyboard>

</BeginStoryboard>

</EventTrigger>

</Page.Triggers>

<Grid >

<TextBlock.Style>

<Style.Triggers>

</DataTrigger>

</Style.Triggers>

</Style>

</TextBlock.Style>

</TextBlock>

</Viewbox>

</Grid>

</Page>

Namespaces in Xaml

Mike Hillberg [MSFT] — Wed, 16 May 2007 19:51:00 GMT

If you’re really into Xml conformance, and you’ve really wondered how Xaml uses Xml namespaces, read on; I can cover the most relevant details in 1160 words or less …

Namespaces on tags

(I’m using the term “tag” here instead of the more correct term “element”, so that I don’t get confused with WPF element objects.)

In the W3C Namespaces for XML recommendation, it’s valid for a tag not to be in any namespace; to have no “namespace name”. In Xaml, it’s not. Or rather it’s mostly not; it’s OK if the XML is hiding in a CData section, or in an XData section such as:

<Page

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

<Page.DataContext>

<x:XData>

</BookList>

</x:XData>

</XmlDataProvider>

</Page.DataContext>

<ListBox.ItemTemplate>

</DataTemplate>

</ListBox.ItemTemplate>

</ListBox>

</Page>

... But generally tags must be in a namespace. That because, when Xaml is mapping tags to types, it needs to identify and locate the exact Type. There’s two ways this mapping is done. The really obvious way is for the namespace name URI to use the “clr-namespace” scheme. In this case, the CLR namespace and assembly of the type are specified in the Xaml, such as in the following mappings to the mscorlib.dll assembly for the System.String and System.Collections.ArrayList types:

<Page

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

xmlns:sys="clr-namespace:System;assembly=mscorlib"

xmlns:collections="clr-namespace:System.Collections;assembly=mscorlib" >

<Page.DataContext>

<collections:ArrayList>

<sys:String>Hello</sys:String>

<sys:String>World</sys:String>

</collections:ArrayList>

</Page.DataContext>

</Page>

The other way to map tags to types is to match up the namespace name with an assembly that has a corresponding [XmlnsDefinition] assembly attribute. For example, in another post, I put the XmlnsDefinition attribute in my code like this:

[assembly: System.Windows.Markup.XmlnsDefinition(

"http://blogs.msdn.com/mikehillberg/archive/2006/09/27/XamlInspiredMarkup.aspx",

"EmployeeDatabase")]

namespace EmployeeDatabase

{

public class Employee

{

private AddressInformation _contactInformation;

public AddressInformation ContactInformation

{

get { return _contactInformation; }

set { _contactInformation = value; }

}

...

… and then I referenced that type by using the matching namespace name in my markup:

<Employee Name="Rob" Title="President"

xmlns="http://blogs.msdn.com/mikehillberg/archive/2006/09/27/XamlInspiredMarkup.aspx">

<Employee.ContactInformation>

</Employee.ContactInformation>

...

Namespaces on other type references

More generally than just tags, anywhere in Xaml where a string refers to a type, it’s interpreted in the same way. Specifically, any value that references a Type is treated as a QName. A QName is a name that may or may not have a namespace prefix on it. If a QName doesn’t have a prefix, it’s usually interpreted to have the default namespace name. (If an attribute name doesn’t have a prefix it’s not in the default namespace, but more on that later.)

For example, in the following markup, the x:Static markup extension refers to a Class.Property value, so it interprets ‘SystemColors’ (the Class) to be in the default namespace:

<Page

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

<Button Background='{x:Static SystemColors.ControlBrush}'>Click</Button>

</Page>

Or, using a non-default namespace, you can have the equivalent:

<wpf:Page

xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

<wpf:Button Background='{x:Static wpf:SystemColors.ControlBrush}'>Click</wpf:Button>

</wpf:Page>

Unprefixed attribute names

Attributes are a bit more interesting and non-obvious than tags or other QNames.

Whereas for tags, an unprefixed tag name is interpreted to be in the default namespace, unprefixed attributes are by definition not in any namespace. The first time I encountered that was when I was watching some XML get parsed by XmlReader. When an unprefixed attribute went by, the NamespaceURI property of the reader was null. I thought it was a bug. But it actually makes sense if you think about this example:

</RandomThings>

Here you have two unprefixed attributes named “State”, but they have different semantics, and more importantly they have distinct sets of valid values: one has to be either liquid, solid, or gas; the other has to be CA, WA, NY, MI, FL, OH, etc. So while the Molecule and Address tags both have the same namespace name (“http://blogs.msdn.com/mikehillberg/NamespacesInXaml”), it wouldn’t make sense to consider the two State attributes to be in the same namespace. And that’s why in the XML namespaces recommendation the State attributes have no namespace name; “interpretation of unprefixed attributes is determined by the element on which they appear”.

And when you think about Xaml being used to map XML to types and properties, that makes even more sense. That’s because you encounter the same rules in a class definition; two different classes can have properties with the same name that have different semantics and types. (A “Molecule” class and an “Address” class could both have un-related “State” properties.)

Thus in this example there’s no namespace prefix on the Background attribute, but we know it references the Button.Background property:

<wpf:Page xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation">

<wpf:Button Background='Red'>Click</wpf:Button>

</wpf:Page>

That’s a “standard” property, though. “Attached” properties are more interesting. For example, in the following, Canvas.Top is an attached property; it’s defined by Canvas, not by Button, despite being set on the Button tag:

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

<Button Canvas.Top='100'>Click</Button>

</Canvas>

</Page>

Since ‘Canvas’ is a reference to a type, it's expanded name uses the default namespace when no prefix is specified. Therefore in this case it has the default namespace name (“http://schemas.microsoft.com/winfx/2006/xaml/presentation”). Or, to make everything explicit again:

<wpf:Page xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

<wpf:Canvas>

<wpf:Button wpf:Canvas.Top='100' >Click</wpf:Button>

</wpf:Canvas>

</wpf:Page>

Prefixed attribute names

If you do put a prefix on an attribute for a standard property, and it maps to the same namespace name, then it behaves functionally equivalent to the attribute without the prefix. For example, this markup:

<wpf:Page xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation">

<wpf:Button Background='Red'>Click</wpf:Button>

</wpf:Page>

… is different only in syntax (not function) from this markup, where the Background attribute now has a prefix:

<wpf:Page xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation">

<wpf:Button wpf:Background='Red'>Click</wpf:Button>

</wpf:Page>

You can even give the attribute a different prefix, as long as it has the same namespace name (the URI):

<wpf:Page

xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:wpf2="http://schemas.microsoft.com/winfx/2006/xaml/presentation" >

<wpf:Button wpf2:Background='Red'>Click</wpf:Button>

</wpf:Page>

… But if the namespace names are different, you’ll get an error:

<wpf:Page

xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:wpf2="SomethingElse" >

<wpf:Button wpf2:Background='Red'>Click</wpf:Button>

</wpf:Page>

For attached properties, as noted above, the class part of an attached property attribute name is interpreted as a QName, and so the prefix is used to determine the namespace name. The namespace name of the tag is irrelevant.

Built-in Xaml attributes

Finally, in Xaml there are a few built-in attributes: Key, Name, and Uid. For example, Key is used to provide the key for a dictionary item, such as the ‘myBrush’ here (Page.Resources is a dictionary):

<Page

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" >

<Page.Resources>

</Page.Resources>

</Page>

These attributes are global and always must have a prefix. For example, if you modify that example so that ‘x:’ is now the default namespace name, the example becomes invalid:

<wpf:Page

xmlns:wpf="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml" >

<wpf:Page.Resources>

<wpf:SolidColorBrush Key='myBrush'>Red</wpf:SolidColorBrush>

</wpf:Page.Resources>

<wpf:Rectangle Width='100' Height='100' Fill='{wpf:StaticResource myBrush}' />

</wpf:Page>

… This is invalid because Key is an unprefixed attribute, so it’s interpreted to be a standard property on SolidColorBrush, rather than being interpreted as in the default namespace (and there is no SolidColorBrush.Key property).

One last thing

One last place where Xaml uses XML namespaces is in the Markup Compatibility feature. But this post is already long enough. See Rob’s blog or MSDN for interesting reading.

Expandos in xaml

Mike Hillberg [MSFT] — Tue, 15 May 2007 03:36:00 GMT

This post has a couple of suggestions on ways to accomplish expandos in Xaml. First some background …

On an HTML page you can define your own new “expando” properties on the fly, such as in this example:

<HTML>

</BODY>

</HTML>

Here I invented a new property named ‘testing’. I can both set that property and get it (the two parts in yellow above).

That’s a good feature when you want it. It’s a frustrating feature when you don’t. For example, in the following markup, I misspelled the “bgcolor” attribute. I’d like to be told that this is invalid, but instead it’s assumed to simply be an expando property.

<HTML>

<P>Hello</P>

</BODY>

</HTML>

It’s for this reason that the Xaml language is strongly-typed. E.g. the following is a compile/load error (the “BGround” should be “Background”):

<TextBlock BGround='Red'>Hello</TextBlock>

But then if you do want an expando, how do you do it?

One way is to actually define the properties you want to use, as attached properties. For example, this markup uses the ‘MyProperties.Testing’ attached property to set a property onto an unsuspecting TextBlock:

<Window x:Class="WPFApplication1.Window1"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:local="clr-namespace:MyNamespace" >

<TextBlock local:MyProperties.Testing='1, 2, 3' >Hello</TextBlock>

</Window>

.. where the code for MyProperties looks like this:

namespace MyNamespace

{

public static class MyProperties

{

public static string GetTesting(FrameworkElement obj)

{

return (string)obj.GetValue(TestingProperty);

}

public static void SetTesting(FrameworkElement obj, string value)

{

obj.SetValue(TestingProperty, value);

}

// Using a DependencyProperty as the backing store for Testing. This enables animation, styling, binding, etc...

public static readonly DependencyProperty TestingProperty =

DependencyProperty.RegisterAttached("Testing", typeof(string), typeof(MyProperties) );

}

If you don’t want to write any code, you can do it all in markup, though it takes more markup than the HTML case.

First of all, recognize that expandos are really just a property bag of key/value pairs. So in the first HTML example above the P element has a dictionary item that maps the key “testing” to the value ”1,2,3”. In WPF you can get a real dictionary using the built-in Hashtable class. (You can’t use the Dictionary<K,T> class, because Generics aren’t supported in Xaml.) There’s also a Tag property on all elements to hold your random data. Put those two together and you can do something like the following:

<Page

xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'

xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'

xmlns:col='clr-namespace:System.Collections;assembly=mscorlib.dll'

xmlns:sys='clr-namespace:System;assembly=mscorlib'

Name='Page1' >

<Page.Tag>

<col:Hashtable>

<sys:String x:Key='testing'>1, 2, 3</sys:String>

<sys:String x:Key='anotherOne'>foo</sys:String>

<sys:String x:Key='yetAnotherOne'>bar</sys:String>

</col:Hashtable>

</Page.Tag>

<!-- This TextBlock.Text Binding queries a property out

of the dictionary -->

</Page>

Update: Another common use of expando properties is to “comment out” an attribute, since XML doesn’t provide a built-in means to do so. I was just reading an old post on Chuck’s blog, and he shows there a way to do this in Xaml by using the markup compatibility feature.

Where does a Binding find its data?

Mike Hillberg [MSFT] — Mon, 14 May 2007 20:04:00 GMT

If you’ve look at much WPF Xaml you’ve probably seen bindings like this:

… which binds the Text property of the TextBlock to the Name property of some data object.

The question that begets is: where does the data come from? The rest of this post looks at the answer.

Properties on Binding

The Binding class has a few properties that provide ways to let you set the source of the data onto the Binding, depending on what works best for your situation. Binding looks like this:

public class Binding : BindingBase

{

...

public object Source { get; set; }

public string ElementName { get; set; }

public RelativeSource RelativeSource { get; set; }

...

}

The Source Property

The most straightforward way to set the source of the Binding is to use the Source property. One of the more common ways to do that is to reference the data out of a resource dictionary:

<Grid>

<Grid.Resources>

</Grid.Resources>

</Grid>

(This example creates a ListBox with 3 entries in it, labeled “1”, “2”, and “3”.)

You can be even more explicit by setting the Source directly, specifying the Binding in full Xml syntax (rather than the above markup extension syntax):

<Grid>

<ListBox.ItemsSource>

<Binding.Source>

</Binding.Source>

</Binding>

</ListBox.ItemsSource>

</ListBox>

</Grid>

You can also set the Binding.Source by referencing a static member:

namespace MyNamespace

{

class MyClass

{

public static List<int> MyData;

static MyClass()

{

MyData = new List<int>();

MyData.Add(1);

MyData.Add(2);

MyData.Add(3);

}

…

<Page

xmlns='http://schemas.microsoft.com/winfx/2006/xaml/presentation'

xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'

xmlns:my="clr-namespace:MyNamespace" >

</Page>

The ElementName Property

Aside from the Binding.Source property, you can also reference your data by name, using Binding.ElementName, such as:

</StackPanel>

(This example causes the button label to be whatever you type into the text box.)

The RelativeSource Property

The RelativeSource property provides a way to tell the Binding to look around where it’s used to find its source. For example, the ‘Self’ RelativeSource binds to the object on which the Binding is placed, such as in the following (which creates a TextBlock that says “Hi”):

As another example, the following binds the TextBlock.Text property to the Grid in its ancestry (again displaying “Hi” in this case):

<TextBlock

Text='{Binding Tag, RelativeSource={RelativeSource FindAncestor, AncestorType=Grid}}' />

</Border>

</Grid>

The other two modes of a RelativeSource are TemplatedParent and PreviousData.

Explicit DataContext Property

A common way the Binding can get its source is via a DataContext property set on an element (on the element itself, not on the Binding). For (a rather boring) example, this creates a Button that says “Click”:

<Button.DataContext>

<sys:String>Click</sys:String>

</Button.DataContext>

</Button>

This gets more interesting and typical if you set the DataContext somewhere else, usually on the root of the page/window. That works because the DataContext property inherits. The following example shows that by setting the DataContext onto the root. In this case, it is set to be an XmlDataProvider which is querying a Yahoo web service for the weather in Barrow, Alaska. Since the DataContext inherits, Bindings throughout the page have access to it automatically. Note also in this case that since we’re binding to XML, the Bindings are using XPath syntax.

<Page

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

<Page.DataContext>

<XmlDataProvider

Source="http://xml.weather.yahoo.com/forecastrss?p=99723"

XPath="/rss/channel"/>

</Page.DataContext>

<Border BorderBrush="Black"

BorderThickness="1" Width="370" Height="170" CornerRadius="6">

<Image Margin="15,15,0,0"

Stretch="None"

HorizontalAlignment="Left"

Source="{Binding XPath=image/url}" />

<!-- Text to say e.g. "Yahoo! Weather - Bellevue, WA", with a hyperlink to a

detailed weather page -->

</Hyperlink>

</TextBlock>

<TextBlock FontWeight="Bold" Margin="15,15,0,0"

Text="{Binding XPath=item[1]/title}"/>

<TextBlock.Text>

<Binding.XPath>

item[1]/*[local-name()="condition" and

namespace-uri()="http://xml.weather.yahoo.com/ns/rss/1.0"]/@text

</Binding.XPath>

</Binding>

</TextBlock.Text>

</TextBlock>,

<TextBlock.Text>

<Binding.XPath>

item[1]/*[local-name()="condition" and

namespace-uri()="http://xml.weather.yahoo.com/ns/rss/1.0"]/@temp

</Binding.XPath>

</Binding>

</TextBlock.Text>

</TextBlock>°

<TextBlock.Text>

<Binding.XPath>

*[local-name()="units" and

namespace-uri()="http://xml.weather.yahoo.com/ns/rss/1.0"]/@temperature

</Binding.XPath>

</Binding>

</TextBlock.Text>

</TextBlock>

Sunrise:

<TextBlock.Text>

<Binding.XPath>

*[local-name()="astronomy" and

namespace-uri()="http://xml.weather.yahoo.com/ns/rss/1.0"]/@sunrise

</Binding.XPath>

</Binding>

</TextBlock.Text>

</TextBlock>, sunset:

<TextBlock.Text>

<Binding.XPath>

*[local-name()="astronomy" and

namespace-uri()="http://xml.weather.yahoo.com/ns/rss/1.0"]/@sunset

</Binding.XPath>

</Binding>

</TextBlock.Text>

</TextBlock>

</StackPanel>

</Border>

</Page>

… Sunrise at midnight, sunset at noon:

Implicit DataContext

Finally, there are a couple of places where the DataContext gets set automatically for you. That usually happens with ContentControl (e.g. Button) and ItemsControl (e.g. ListBox).

Here’s a ContentControl example. In this markup we have a DataTemplate set up in a ResourceDictionary so that it will be used for any ContentControl that has a String as its content. So it’s automatically picked up in the subsequent Button. In that DataTemplate, we have a binding to the data item, which in this case is the ContentControl.Content. Therefore what we get here is a Button that says “Click” in bold italic.

<Page

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

xmlns:sys="clr-namespace:System;assembly=mscorlib" >

<Page.Resources>

<!-- The Text property is bound to the Content property of

whatever ContentControl with which this DataTemplate is used -->

</DataTemplate>

</Page.Resources>

<Button>Click</Button>

</Page>

And here’s an ItemsControl example. In this markup, we have a ListBox bound to an integer collection again (using the inherited DataContext as the source), but this time we have an item template, which is a template to use when displaying the items in the ListBox. And within that item template, the DataContext is again automatically set to be the item

<Page

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x=http://schemas.microsoft.com/winfx/2006/xaml >

<Page.DataContext>

</Page.DataContext>

<ListBox.ItemTemplate>

<DataTemplate.Triggers>

<!-- Within the DataTemplate, the DataContext is set to be the data item.

So this DataTrigger.Binding property's DataContext is going to be

the item (1, 2, or 3) -->

</DataTrigger>

</DataTemplate.Triggers>

</Border>

</DataTemplate>

</ListBox.ItemTemplate>

</ListBox>

</Page>

How to target a template setter at non-element content

Mike Hillberg [MSFT] — Tue, 08 May 2007 03:04:00 GMT

Here’s a technique you can follow to use property triggers in a template on non-element type objects. First, though, some background on what that means …

Take this example of a Button with a custom template which is simply a rectangle:

<Button.Template>

</Rectangle>

<ControlTemplate.Triggers>

</Trigger>

</ControlTemplate.Triggers>

</ControlTemplate>

</Button.Template>

</Button>

… the rectangle is Red, but changes to Green when the mouse goes over it.

Now fill the Rectangle with a LinearGradientBrush, rather than the SolidColorBrush:

<Button.Template>

<Rectangle.Fill>

</LinearGradientBrush>

</Rectangle.Fill>

</Rectangle>

<ControlTemplate.Triggers>

</Trigger>

</ControlTemplate.Triggers>

</ControlTemplate>

</Button.Template>

</Button>

… and now the Rectangle is red-to-blue by default, but still the Rectangle changes to green when the mouse is over it.

Now say you want the mouse-over behavior to keep LinearGradientBrush, but to instead just modify the second stop to green. One way you could do that would be to have two copies of the LinearGradientBrush:

<Button.Template>

<Rectangle.Fill>

</LinearGradientBrush>

</Rectangle.Fill>

</Rectangle>

<ControlTemplate.Triggers>

<Setter.Value>

</LinearGradientBrush>

</Setter.Value>

</Setter>

</Trigger>

</ControlTemplate.Triggers>

</ControlTemplate>

</Button.Template>

</Button>

But if you have a more complicated object, such as a Drawing, you might not want to have to duplicate the entire thing. The difficulty is that the Setter of a Trigger can only target a FrameworkElement or FrameworkContentElement, not (in this case) a GradientStop. For example, if you tried make the second stop green with a trigger:

<Button.Template>

<Rectangle.Fill>

</LinearGradientBrush>

</Rectangle.Fill>

</Rectangle>

<ControlTemplate.Triggers>

</Trigger>

</ControlTemplate.Triggers>

</ControlTemplate>

</Button.Template>

</Button>

… you’ll get the error “Cannot find the Trigger target 'SecondStop'. (The target must appear before any Setters, Triggers, or Conditions that use it.)”.

There is a way to solve this, though, by using data binding. With a Binding, you can find your way out of the brush to an element, and then the Setter can target that element. For example, the following markup gives us the desired LinearGradientBrush of red-to-blue when the mouse is away from the button, and red-to-green when the mouse is over it:

<Button.Template>

<Rectangle.Tag>

</Rectangle.Tag>

<Rectangle.Fill>

<GradientStop

Color='{Binding ElementName=MyRectangle, Path=Tag}'

Offset='1' x:Name='SecondStop' />

</LinearGradientBrush>

</Rectangle.Fill>

</Rectangle>

<ControlTemplate.Triggers>

</Trigger>

</ControlTemplate.Triggers>

</ControlTemplate>

</Button.Template>

</Button>

Tip: Using XamlWriter and XamlReader to clone an object

Mike Hillberg [MSFT] — Tue, 01 May 2007 23:35:00 GMT

There are multiple ways to clone objects, and multiple definitions of what “clone” should even mean.

The main issue is usually about cloning “deep” vs. “shallow”. For example, if you have a Customer object that points to an Address object, and you clone the Customer object, does the Address object get cloned too (this would be a deep clone)? Or does the new Customer reference the original Address object (a shallow clone)?

Another issue is whether you clone just the properties of the object, or the fields instead. And do you clone event listeners?

Yet another issue is that you might clone a property value on the object that is supposed to be unique, such as the Name property on WPF elements.

Anyway, here’s a trick to clone an object by writing it out to Xaml then reading it back in. As such, the cloning rules it follows matches the Xaml serialization rules (see here for some more info). That primarily means that it only clones public properties – no fields or events – and it clones deep.

So here’s what the code might look like:

private Object CloneUsingXaml(Object o)

{

string xaml = XamlWriter.Save(o);

return XamlReader.Load(new XmlTextReader(new StringReader(xaml)));

}

(XamlReader.Load doesn’t have an overload that takes a string, so you need to convert the string into an XmlReader.)

Here’s an example. Take this app:

<Window x:Class="WPFApplication1.Window1"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

Title="Window1" Height="300" Width="300" Loaded='OnLoaded'>

<Grid.ColumnDefinitions>

</Grid.ColumnDefinitions>

</Grid>

</Window>

… and clone the Rectangle during load with:

private void OnLoaded(object sender, RoutedEventArgs args)

{

Rectangle rectangle2 = CloneUsingXaml(_rectangle1) as Rectangle;

rectangle2.Name = null;

Grid.SetColumn(rectangle2, 1);

Grid1.Children.Add(rectangle2);

}

… and you get a Window with two rectangles.

Here’s what the intermediate Xaml looks like:

<Rectangle

Fill="#FFFF0000"

Name="_rectangle1"

Width="20" Height="20"

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" />

Note here that the Name property got cloned, which isn’t what we want, so I removed the Name after running the clone.

Parameterized templates; and how to create reusable, custom-looking buttons without code

Mike Hillberg [MSFT] — Thu, 01 Feb 2007 20:26:00 GMT

I’ve talked to a few people recently about parameterized templates, and so I wanted to write some of it down. Here’s the scenario … I want to create an application that has a main window with several buttons on it. Clicking one of the buttons navigates you to part of an app. For example, in this picture, you can click on either Flower, Trees, or Toucans:

The question is, how do you create those buttons? By the time we’re done, the markup for the buttons is going to look like this, and we won’t have to write any code:

(“Tag” is a funny name for a property. If you want to write a little code, you can give it a better name, but I’m just going to use Tag for simplicity, as it already exists.)

The key here is that we can now just plop down buttons with a reference to the image/text, and they automatically get the custom look. Also notice that these are real buttons, and are still getting the standard button behavior and chrome. For example, the Toucan button above is the current keyboard focus, so it has a slight dotted outline, like any other button.

But let’s back up. For starters, the easiest way to create one of those buttons is to simply make the text and image part of the button’s content, such as:

</StackPanel>

</Button>

To fill in more of the overall look of the button (with some colors, fonts, etc.) we expand this to:

<Button Margin="5" Width="200"

Foreground="White" >

<Button.Background>

<SolidColorBrush

Color="Orange"

Opacity="0.4" />

</Button.Background>

<TextBlock Text="Flowers"

FontWeight="Bold"

HorizontalAlignment="Center"

Margin="10" />

</StackPanel>

</Button>

(Note that the button’s background has some opacity, so it looks slightly different here than in the original picture, where it was composited on top of a panel that itself had a background color.)

But you don’t want to repeat all of that button content for every button; you’d like to define it once, and just change the text and image for each button instance. One way to share the common look of the buttons is to create a custom Button template, and put the StackPanel, Image, and TextBlock within. That might look like this:

<Setter.Value>

</Setter.Value>

</Setter>

<Setter.Value>

<TextBlock

Text="Flowers"

FontWeight="Bold"

HorizontalAlignment="Center" Margin="10" />

</StackPanel>

</ControlTemplate>

</Setter.Value>

</Setter>

</Style>

Note here that in addition to creating a ControlTemplate for the button, I put some of the common properties in a Style.

And since this is defined as the default button style, my “Flowers” button markup just looks like this:

That’s a good start, but this template isn’t really re-usable, because it has the image source and text content (“Flowers”) hard-coded. To solve that, we can parameterize the template using data binding. So update the ControlTemplate like this:

<Image Source="{Binding

Tag,

RelativeSource={RelativeSource

FindAncestor,

AncestorType=Button}}" />

<TextBlock Text="{Binding

Content,

RelativeSource={RelativeSource

FindAncestor,

AncestorType=Button}}"

FontWeight="Bold"

HorizontalAlignment="Center"

Margin="10" />

</StackPanel>

</ControlTemplate>

The bindings just bind a property on the Image and TextBlock to properties on the Button to which the template is applied. That is, the TextBlock.Text is bound to the Button.Content, and the Image.Source is bound to the Button.Tag. That “Tag” property is a funny name, but is just an un-used property that’s defined on all elements, and that isn’t used by WPF internally; it’s just there as a place for you to store random values, like the Tag property in VB.

(Note that these bindings find the Button to which the template is applied using a RelativeSource of FindAncestor. You could also using a RelativeSource of TemplatedParent, or you could use a TemplateBinding in this case. But the FindAncestor approach is simpler here because it will continue to work in the next step below when we make some changes to the template.)

And now we can do an easier button, passing arguments to customize the template instance:

So that’s more usable, but the button doesn’t look the same any more. For one thing, the orange background isn’t showing up. That’s because that is normally the button template’s responsibility, and we didn’t fill it in. That seems easy enough to fix by putting the background color on the template’s StackPanel. That gives us this:

<Setter Property="Width"

Value="200" />

<Setter Property="Margin"

Value="5" />

<Setter Property="Foreground"

Value="White" />

<Setter.Value>

<ControlTemplate

TargetType="Button">

<StackPanel.Background>

<SolidColorBrush

Color="Orange"

Opacity="0.4" />

</StackPanel.Background>

<Image

Source="Garden.jpg" />

<TextBlock Text="Flowers"

FontWeight="Bold"

HorizontalAlignment="Center"

Margin="10" />

</StackPanel>

</ControlTemplate>

</Setter.Value>

</Setter>

</Style>

...

<Button

Content="Flowers"

Tag="Garden.jpg" />

But that still doesn’t look quite right. For example, the corners and border don’t look the same. And if you look at this in a real app (or XamlPad), you’ll see that it doesn’t change its look correctly when the mouse is over it, when it is clicked, etc. That’s because we aren’t just filling in the content of the button with our StackPanel/Image/TextBlock, we’re replacing the whole Button template.

We could fix all of these problems, but there is a way we can let the Button keep it’s normal “chrome” template, and use our template just for the content of the Button – Instead of setting the Button’s Template property, we set the ContentTemplate property.

So make two changes. First, set the ContentTemplate property rather than the Template property. Second, use a DataTemplate rather than a ControlTemplate (otherwise you’ll get a compiler error). There are some differences between a DataTemplate and a ContentTemplate, but none that show up here, it’s just a rename.

That gives us; this:

<Setter Property="Width"

Value="200" />

<Setter Property="Foreground"

Value="White" />

<Setter.Value>

<SolidColorBrush

Color="Orange"

Opacity="0.4" />

</Setter.Value>

</Setter>

<Setter.Value>

<DataTemplate

DataType="Button">

<Image

Source="{Binding Tag,

RelativeSource=

{RelativeSource

FindAncestor,

AncestorType=Button}}" />

<TextBlock

Text="{Binding Content,

RelativeSource=

{RelativeSource

FindAncestor,

AncestorType=Button}}"

FontWeight="Bold"

HorizontalAlignment="Center"

Margin="10" />

</StackPanel>

</DataTemplate>

</Setter.Value>

</Setter>

</Style>

...

And now we’ve accomplished the original goal; defining the look for the buttons in one template, and parameterizing that template so that each instance of the button can set the image and text.

Here’s the end-result sample that runs in XamlPad (you might need to update the Image URIs to point to images you have, although on Windows Vista this will likely just work):

<Page.Resources>

<Setter.Value>

</Setter.Value>

</Setter>

<Setter.Value>

<Image Source="{Binding Tag,

RelativeSource={RelativeSource

FindAncestor,

AncestorType='Button'}}" />

<TextBlock Margin="10"

HorizontalAlignment="Center"

Text="{Binding Content,

RelativeSource={RelativeSource

FindAncestor,

AncestorType='Button'}}" />

</StackPanel>

</DataTemplate>

</Setter.Value>

</Setter>

</Style>

</Page.Resources>

<Page.Background>

</LinearGradientBrush>

</Page.Background>

Select an item:

</TextBlock>

</WrapPanel>

</StackPanel>

</Page>

How to control the mouse cursor

Mike Hillberg [MSFT] — Wed, 22 Nov 2006 20:48:00 GMT

How to set the mouse cursor in one easy step: set the Cursor property. For example, this markup:

… sets the cursor to be the “Help” cursor, when the mouse is over the button.

Note that since Cursor is scoped, it doesn’t apply to other parts of the page (anything other than that button). Also note that if a child and parent both set the Cursor, the child wins. For example, in this markup:

</Border>

… the cursor is a hand over the Border area, except over the Button it’s overridden to a Help cursor.

So by default the child wins over the parent, but the parent gets the final say with the ForceCursor property. When this property is set, the parent’s cursor overrides the child. So add this to the border in the above markup, and the cursor is a Hand, even over the button:

</Border>

Here’s an example of Cursor and ForceCursor that you can use to see everything (just paste this into XamlPad):

<Page

xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"

xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"

<Page.Resources>

</Style>

<!-- Tell all the borders to give some padding to their child,

and to have a light blue background. This will

make it easier to see which part is the border,

and which part is the child of the border -->

</Style>

</Page.Resources>

Just a text block. This will have a hand cursor over the text block,

coming from the style. But the light blue border area will have the

default arrow cursor.

</TextBlock>

</Border>

Same text block, but now the border has a cross cursor set. This will

still have a hand cursor over the text block, but the light blue border

will have a cross cursor (the text block's cursor is overriding the

border's cursor).

</TextBlock>

</Border>

Now the border also has ForceCursor set. So both the border

area and the text block will have a cross cursor (the border's cursor

is now winning over the text block's cursor.

</TextBlock>

</Border>

</StackPanel>

</Page>