Question

I have developed a behavior that changes the Clip property of the AssociatedObject. When I run the application, all is well. But when I view the page in Blend it appears like the behavior does not affect its associated object.

I tried to “debug” blend by attaching visual studio 2010 debugger to it’s process and set a break point on the behavior’s OnAttached method, but the breakpoint is never reached. As if blend prevents that behavior from being attached at design time.

Is there a way around it?

Cheers,

Answer 1

I finally found a workable way around it, but there’s a big caveat which I put at the end of this answer. Here’s my CustomAttachManager:

public class CustomAttachManager : DependencyObject
{
    #region Object CustomAttachManager.Attached = null

    public static IAttachedObject GetAttached(DependencyObject obj) { return (IAttachedObject)obj.GetValue(AttachedProperty); }
    public static void SetAttached(DependencyObject obj, IAttachedObject value) { obj.SetValue(AttachedProperty, value); }

    public static readonly DependencyProperty AttachedProperty =
        DependencyProperty.RegisterAttached("Attached", typeof(IAttachedObject), typeof(CustomAttachManager), new PropertyMetadata(null, StaticHandleAttachedChanged));

    static void StaticHandleAttachedChanged(DependencyObject self, DependencyPropertyChangedEventArgs args)
    {
        var ov = (IAttachedObject)args.OldValue;
        var nv = (IAttachedObject)args.NewValue;

        if (ov != null) ov.Detach();
        if (nv != null) nv.Attach(self);
    }
    #endregion
}

You can then use it to attach behaviors like this:

<my:CustomAttachManager.Attached>
    <my:RedBackgroundBehavior></my:RedBackgroundBehavior>
</my:CustomAttachManager.Attached>

My example behavior changes the background of a panel to red, which is visible in the designer.

The remaining problem is the case of multiple behaviors. The best solution I could come up with is a proxy:

[ContentProperty("Children")]
public class MultipleBehavior : Behavior<DependencyObject>
{
    DependencyObjectCollection<IAttachedObject> children = new DependencyObjectCollection<IAttachedObject>();
    public DependencyObjectCollection<IAttachedObject> Children { get { return children; } }

    protected override void OnAttached()
    {
        foreach (var child in Children) child.Attach(AssociatedObject);
    }

    protected override void OnDetaching()
    {
        foreach (var child in Children) child.Detach();
    }
}

which can be used this way:

    <my:CustomAttachManager.Attached>
        <my:MultipleBehavior>
            <my:RedBackgroundBehavior />
            <my:SupriseBehavior />
        </my:MultipleBehavior>
    </my:CustomAttachManager.Attached>

The proxy has a defect in that it won’t properly handle behaviors added after something already got attached, but that is not going to happen in the classic use case.

The caveat is that I don’t know a way to make Blend display the behaviors in the object tree. The “AlternateContentProperty” can’t be used because it doesn’t work for attached properties, which is what CustomAttachManager uses.

I will update this answer should I find an answer to this problem.