Example implementation of weak events using .NET's WeakEventManager

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时光取名叫无心 2021-02-04 00:24

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 documentati

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  • 2021-02-04 00:45

    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;
        }
    }
    
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  • 2021-02-04 00:53

    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.
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