SL4/MVVM: Handle MouseDragElementBehavior.Dragging event with void Foo() in VM

I am trying to handle the MouseDragElementBehavior.Dragging event on a control I have. S开发者_JAVA百科ee here for background on why I want to do this.

I am having trouble wiring up this event. From the XAML you can see I have added a behavior to the user control. Then I attempted to add a handler to the Dragging event on the behavior via the CallMethodAction EventTrigger.

<i:Interaction.Behaviors>
    <ei:MouseDragElementBehavior ConstrainToParentBounds="True">
        <i:Interaction.Triggers>
            <i:EventTrigger EventName="Dragging">
                <ei:CallMethodAction MethodName="NotifyChildrenYouAreDragging" TargetObject="{Binding}"/>
            </i:EventTrigger>
        </i:Interaction.Triggers>
    </ei:MouseDragElementBehavior>
</i:Interaction.Behaviors>

I have tried the following method signatures with no luck:

void NotifyChildrenYouAreDragging(){}
void NotifyChildrenYouAreDragging(object sender, EventArgs e){}
void NotifyChildrenYouAreDragging(object sender, MouseEventArgs e){}

Anyone have experience using triggers to handle events in attached behaviors?

The problem is that the EventTrigger doesn't hook up to the Behavior's events. Instead it is hooking up to the Behavior's AssociatedObject's events. Here is the relevant source code:

 protected override void OnAttached()
    {
        base.OnAttached();
        DependencyObject associatedObject = base.AssociatedObject;
        Behavior behavior = associatedObject as Behavior;
        FrameworkElement element = associatedObject as FrameworkElement;
        this.RegisterSourceChanged();
        if (behavior != null)
        {
            associatedObject = ((IAttachedObject) behavior).AssociatedObject;
            behavior.AssociatedObjectChanged += new EventHandler(this.OnBehaviorHostChanged);
        }
        ....
  }

So you can see that if the associated object of the trigger is a behavior, then it sets the associated object to the Behavior's associated object which is your items collection. The items collection doesn't have a dragging event so nothing is ever fired.

You could probably get the results you want by creating another behavior that checks to see if the associated object has a drag behavior and if so have your behavior attach to the dragging event. Then call the method on the object from there.