Automatically calling OnDetaching() for Silverlight Behaviors

Question

I am using several Blend behaviors and triggers on a silverlight control. I am wondering if there is any mechanism for automatically detaching or ensuring that OnDetaching() is called for a behavior or trigger when the control is no longer being used (i.e. removed from the visual tree).

My problem is that there is a managed memory leak with the control because of one of the behaviors. The behavior subscribes to an event on some long-lived object in the OnAttached() override and should be unsubscribing from that event in the OnDetaching() override so that it can become a candidate for garbage collection. However, OnDetaching() never seems to be getting called when I remove the control from the visual tree... the only way I can get it to happen is by explicitly detaching the problematic behaviors BEFORE removing the control and then it is properly garbage collected.

Right now my only solution was to create a public method in the code-behind for the control that can go through and detach any known behaviors that would cause garbage collection problems. It would be up to the client code to know to call this before removing the control from the panel. I don't really like this approach, so I am looking for some automatic way of doing this that I am overlooking or a better suggestion.

public void DetachBehaviors()
{
     foreach (var behavior in Interaction.GetBehaviors(this.LayoutRoot))
     {
          behavior.Detach();
     }

     //continue detaching all known problematic behaviors on the control....
}

Answer 1

What you really need in this case is not some way to automatically detach but to ensure that the reference held by the long lived object does not keep the behaviour (and therefore everything else it has a reference to) from being garbage collected.

This is acheived by implementing a Mediator pattern. The concept is that you don't give the long-lived object a delegate with a reference to your Behaviour, instead you create a Mediator class as a go-between. The mediator attaches to the long-lived objects event and holds a WeakReference to the behaviour. When the long-lived object fires the event the mediator checks that the WeakReference is still alive, if so calls a method on it to pass on the event. If when the event occurs the mediator finds that the WeakReference is no longer alive it detaches its event handler from the long lived object.

Hence there is nothing stopping the behaviour and everything else involved from being garbage collected all thats left is a very small mediator instance with a dead reference still attached to the long-lived object. Since these mediators are tiny they don't represent a real problem and even those will disappear the next time the event fires.

Fortunately you don't have to build this stuff yourself others have already done it. It is called the WeakEventListener. This blog: Highlighting a "weak" contribution; Enhancements make preventing memory leaks with WeakEventListener even easier! has an excellent set of links on the subject.

Answer 2

Joost van Schaik offers an alternative way to clean up references from attached behaviors, while avoiding the problem with the memory leak. It depends on doing the cleanup work using delegates of the Loaded and Unloaded events of the AssociatedObject.

He also offers a code-snippet for generating stubs for attached behaviors.