How to add a Blend Behavior in a Style Setter

I had the same problem and I’ve come up with a solution. I found this question after I solved it and I see that my solution bears a lot in common with Mark’s. However, this approach is a little different.

The main problem is that behaviors and triggers associate with a specific object and so you cannot use the same instance of a behavior for multiple different associated objects. When you define your behavior inline XAML enforces this one-to-one relationship. However, when you try to set a behavior in a style, the style can be re-used for all the objects it applies to and this will throw exceptions in the base behavior classes. In fact the authors went to considerable effort to prevent us from even trying to do this, knowing that it wouldn’t work.

The first problem is that we cannot even construct a behavior setter value because the constructor is internal. So we need our own behavior and trigger collection classes.

The next problem is that the behavior and trigger attached properties don’t have setters and so they can only be added to with in-line XAML. This problem we solve with our own attached properties that manipulate the primary behavior and trigger properties.

The third problem is that our behavior collection is only good for a single style target. This we solve by utilizing a little-used XAML feature x:Shared="False" which creates a new copy of the resource each time it is referenced.

The final problem is that behaviors and triggers are not like other style setters; we don’t want to replace the old behaviors with the new behaviors because they could do wildly different things. So if we accept that once you add a behavior you cannot take it away (and that’s the way behaviors currently work), we can conclude that behaviors and triggers should be additive and this can be handled by our attached properties.

Here is a sample using this approach:

<Grid>
    <Grid.Resources>
        <sys:String x:Key="stringResource1">stringResource1</sys:String>
        <local:Triggers x:Key="debugTriggers" x:Shared="False">
            <i:EventTrigger EventName="MouseLeftButtonDown">
                <local:DebugAction Message="DataContext: {0}" MessageParameter="{Binding}"/>
                <local:DebugAction Message="ElementName: {0}" MessageParameter="{Binding Text, ElementName=textBlock2}"/>
                <local:DebugAction Message="Mentor: {0}" MessageParameter="{Binding Text, RelativeSource={RelativeSource AncestorType={x:Type FrameworkElement}}}"/>
            </i:EventTrigger>
        </local:Triggers>
        <Style x:Key="debugBehavior" TargetType="FrameworkElement">
            <Setter Property="local:SupplementaryInteraction.Triggers" Value="{StaticResource debugTriggers}"/>
        </Style>
    </Grid.Resources>
    <StackPanel DataContext="{StaticResource stringResource1}">
        <TextBlock Name="textBlock1" Text="textBlock1" Style="{StaticResource debugBehavior}"/>
        <TextBlock Name="textBlock2" Text="textBlock2" Style="{StaticResource debugBehavior}"/>
        <TextBlock Name="textBlock3" Text="textBlock3" Style="{StaticResource debugBehavior}"/>
    </StackPanel>
</Grid>

The example uses triggers but behaviors work the same way. In the example, we show:

• the style can be applied to multiple text blocks
• several types of data binding all work correctly
• a debug action that generates text in the output window

Here’s an example behavior, our DebugAction. More properly it is an action but through the abuse of language we call behaviors, triggers and actions “behaviors”.

public class DebugAction : TriggerAction<DependencyObject>
{
    public string Message
    {
        get { return (string)GetValue(MessageProperty); }
        set { SetValue(MessageProperty, value); }
    }

    public static readonly DependencyProperty MessageProperty =
        DependencyProperty.Register("Message", typeof(string), typeof(DebugAction), new UIPropertyMetadata(""));

    public object MessageParameter
    {
        get { return (object)GetValue(MessageParameterProperty); }
        set { SetValue(MessageParameterProperty, value); }
    }

    public static readonly DependencyProperty MessageParameterProperty =
        DependencyProperty.Register("MessageParameter", typeof(object), typeof(DebugAction), new UIPropertyMetadata(null));

    protected override void Invoke(object parameter)
    {
        Debug.WriteLine(Message, MessageParameter, AssociatedObject, parameter);
    }
}

Finally, our collections and attached properties to make this all work. By analogy with Interaction.Behaviors, the property you target is called SupplementaryInteraction.Behaviors because by setting this property, you will add behaviors to Interaction.Behaviors and likewise for triggers.

public class Behaviors : List<Behavior>
{
}

public class Triggers : List<TriggerBase>
{
}

public static class SupplementaryInteraction
{
    public static Behaviors GetBehaviors(DependencyObject obj)
    {
        return (Behaviors)obj.GetValue(BehaviorsProperty);
    }

    public static void SetBehaviors(DependencyObject obj, Behaviors value)
    {
        obj.SetValue(BehaviorsProperty, value);
    }

    public static readonly DependencyProperty BehaviorsProperty =
        DependencyProperty.RegisterAttached("Behaviors", typeof(Behaviors), typeof(SupplementaryInteraction), new UIPropertyMetadata(null, OnPropertyBehaviorsChanged));

    private static void OnPropertyBehaviorsChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var behaviors = Interaction.GetBehaviors(d);
        foreach (var behavior in e.NewValue as Behaviors) behaviors.Add(behavior);
    }

    public static Triggers GetTriggers(DependencyObject obj)
    {
        return (Triggers)obj.GetValue(TriggersProperty);
    }

    public static void SetTriggers(DependencyObject obj, Triggers value)
    {
        obj.SetValue(TriggersProperty, value);
    }

    public static readonly DependencyProperty TriggersProperty =
        DependencyProperty.RegisterAttached("Triggers", typeof(Triggers), typeof(SupplementaryInteraction), new UIPropertyMetadata(null, OnPropertyTriggersChanged));

    private static void OnPropertyTriggersChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var triggers = Interaction.GetTriggers(d);
        foreach (var trigger in e.NewValue as Triggers) triggers.Add(trigger);
    }
}

and there you have it, fully-functional behaviors and triggers applied through styles.