Example implementation of weak events using .NET's WeakEventManager

Question

Is there an example implementation of weak events using .NET's WeakEventManager?

I'm trying to implement it by following the "Notes to Inheritors" in the documentation, but it is vague. For example, I can't figure out how to call ProtectedAddListener from my static AddListener function in my custom manager.

Kidnap

Answer 1

I figured it out on my own following the guidelines in the "Notes for Inheritors" section of the WeakEventManager documentation. Here's a basic implementation of WeakEventManager. The class sourcing the event is named PropertyValue and the event is named Changed.

public class PropertyValueChangedEventManager : WeakEventManager
{
    public static PropertyValueChangedEventManager CurrentManager
    {
        get
        {
            var manager_type = typeof(PropertyValueChangedEventManager);
            var manager = WeakEventManager.GetCurrentManager(manager_type) as PropertyValueChangedEventManager;

            if (manager == null)
            {
                manager = new PropertyValueChangedEventManager();
                WeakEventManager.SetCurrentManager(manager_type, manager);
            }

            return manager;
        }
    }

    public static void AddListener(PropertyValue source, IWeakEventListener listener)
    {
        CurrentManager.ProtectedAddListener(source, listener);
    }

    public static void RemoveListener(PropertyValue source, IWeakEventListener listener)
    {
        CurrentManager.ProtectedRemoveListener(source, listener);
    }

    protected override void StartListening(object source)
    {
        ((PropertyValue)source).Changed += DeliverEvent;
    }

    protected override void StopListening(object source)
    {
        ((PropertyValue)source).Changed -= DeliverEvent;
    }
}

Kidnap

Microsoft provides PropertyChangedEventManager (msdn.microsoft.com/en-us/library/…) and CollectionChangedEventManager (msdn.microsoft.com/en-us/library/…) built-in to .NET. – Kidnap

Thanks for your example. Athough the shorter implementation will be ((PropertyValue)source).Changed += DeliverEvent; – Significs

@alpha-mouse: Where exactly is the "weak" in your "shorter implementation"? I cannot find it. – Pitchdark

@Paul Groke: my comment was relevant to the previous unedited version of this answer. Now I have no objections at all. – Significs

Answer 2

Sharp Observation is an open source project that has an easy to use implementation. You might want to take a look at their code or just use it as-is.

Usage

The MakeWeak() method returns a new delegate which invokes the same target as the original delegate, but holds a weak reference to the target so that the listener is not kept alive by the delegate:

var handler= new PropertyChangingEventHandler(listener.HandleChange);
observable.PropertyChanging += handler.MakeWeak<PropertyChangingEventHandler>();

Limitations

The current implementation has the following restrictions on delegates:

• Return values are not supported.
• Arguments of Out and Ref are not supported.

Harbin