PCL WeakEventManager from Reactive extensions disposes event in 3 - 7 minutes

Question

I'm trying to implement WeakEventManager in PCL using the Reactive library.

So the point is that it keeps a weak reference for the subscriber and each time event fires - it gets the delegate of the subscriber and fires that, but if he couldn't get an object from weak reference, then it disposes the link to the delegate.

The problem is that after a short amount of time, the weak reference returns null (but the subscriber is still alive) and after that disposing of the link is being performed. So my question is why this is happening and how to fix that?

Here how it looks like: (Look at the note in the code)

    private static IDisposable InternalSubscribeWeakly<TEventPattern, TEvent>(this IObservable<TEventPattern> observable, TEvent Weak_onNext, Action<TEvent, TEventPattern> onNext)
where TEvent : class
    {
        if (onNext.Target != null)
            throw new ArgumentException("onNext must refer to a static method, or else the subscription will still hold a strong reference to target");

        // Is the delegate alive?
        var Weak_onNextReferance = new WeakReference(Weak_onNext);

        //This is a link for that event, so if you want to unsubscribe from event you have to dispose this object
        IDisposable subscription = null;
        subscription = observable.Subscribe(item =>
        {
            //So the library keeps weak reference for this object and each time event fired it tries to get that object
            var current_onNext = Weak_onNextReferance.Target as TEvent;
            if (current_onNext != null)
            {
                //If the object was found, it uses the delegate that subscriber provided and fires the event
                onNext(current_onNext, item);
            }
            else
            {
                //If the object is not found it disposes the link
                //NOTE: For some reasons after a short amount of time it can't get a reference from the WeakReference, however the subscriber is still alive
                subscription.Dispose();
            }
        });
        return subscription;
    }

And then here is how I'm subscribing using that manager:

private void NoLeakWindow_Loaded(object sender, RoutedEventArgs e)
{
    Loaded -= NoLeakWindow_Loaded;

    this.ObserveOn<Window, ElapsedEventHandler, ElapsedEventArgs>(h => (o, s) => h(o, s),
        r => MainWindow.EPublisher.EventTimer.Elapsed += r,
        r => MainWindow.EPublisher.EventTimer.Elapsed -= r)
        .SubscribeWeakly(EventTimer_Elapsed);

    this.ObserveOn<Window, ElapsedEventHandler, ElapsedEventArgs>(
        h => (o, s) => h(o, s),
        r => MainWindow.EPublisher.EventTimer.Elapsed += r,
        r => MainWindow.EPublisher.EventTimer.Elapsed -= r)
        .SubscribeWeakly(EventTimer_Elapsed2);
}

private void EventTimer_Elapsed(EventPattern<ElapsedEventArgs> e)
{
    MessageBox.Show("EventTimer_Elapsed By Timer");
}

private void EventTimer_Elapsed2(EventPattern<ElapsedEventArgs> e)
{
    MessageBox.Show("EventTimer2_Elapsed2 By Timer2");
}

And my event publisher:

public class EventPublisher
{
    public Timer EventTimer = new Timer(3000);
    public Timer EventTimer2 = new Timer(2700);

    public event EventHandler<EventArgs> TimeElapsed;

    public EventPublisher()
    {
        EventTimer.Start();
        EventTimer2.Start();
    }
}

And finally the WeakEventManager class full code:

/// <summary>
    /// Static Class that holds the extension methods to handle events using weak references.
    /// This way we do not need to worry about unregistered the event handler.
    /// </summary>
    public static class WeakEventManager
    {
        /// <summary>
        /// Creates Observable for subscribing to it's event
        /// </summary>
        /// <typeparam name="T">The type of the T.</typeparam>
        /// <typeparam name="TDelegate">The type of the T delegate.</typeparam>
        /// <typeparam name="TArgs">The type of the T args.</typeparam>
        /// <param name="subscriber">The subscriber</param>
        /// <param name="converter">The converter.</param>
        /// <param name="add">The add</param>
        /// <param name="remove">The remove</param>
        /// <returns>IObservable</returns>
        public static IObservable<EventPattern<TArgs>> ObserveOn<T, TDelegate, TArgs>(this T subscriber, Func<EventHandler<TArgs>, TDelegate> converter, Action<TDelegate> add, Action<TDelegate> remove)
            where T : class
        {
            return Observable.FromEventPattern<TDelegate, TArgs>(
                converter,
                add,
                remove);
        }
        /// <summary>
        /// Subscribe's action to event
        /// </summary>
        /// <typeparam name="T">The type of the T.</typeparam>
        /// <param name="observable">The observable</param>
        /// <param name="onNext">The action</param>
        /// <returns></returns>
        public static IDisposable SubscribeWeakly<T>(this IObservable<T> observable, Action<T> onNext) where T : class
        {
            IDisposable Result = null;
            WeakSubscriberHelper<T> SubscriptionHelper = new WeakSubscriberHelper<T>(observable, ref Result, onNext);
            return Result;
        }

        private class WeakSubscriberHelper<T> where T : class
        {
            public WeakSubscriberHelper(IObservable<T> observable, ref IDisposable Result, Action<T> eventAction)
            {
                Result = observable.InternalSubscribeWeakly(eventAction, WeakSubscriberHelper<T>.StaticEventHandler);
            }

            public static void StaticEventHandler(Action<T> subscriber, T item)
            {
                subscriber(item);
            }
        }

        private static IDisposable InternalSubscribeWeakly<TEventPattern, TEvent>(this IObservable<TEventPattern> observable, TEvent Weak_onNext, Action<TEvent, TEventPattern> onNext)
where TEvent : class
        {
            if (onNext.Target != null)
                throw new ArgumentException("onNext must refer to a static method, or else the subscription will still hold a strong reference to target");

            // Is the delegate alive?
            var Weak_onNextReferance = new WeakReference(Weak_onNext);

            //This is a link for that event, so if you want to unsubscribe from event you have to dispose this object
            IDisposable subscription = null;
            subscription = observable.Subscribe(item =>
            {
                //So the library keeps weak reference for this object and each time event fired it tries to get that object
                var current_onNext = Weak_onNextReferance.Target as TEvent;
                if (current_onNext != null)
                {
                    //If the object was found, it uses the delegate that subscriber provided and fires the event
                    onNext(current_onNext, item);
                }
                else
                {
                    //If the object is not found it disposes the link
                    //NOTE: For some reasons after a short amount of time it can't get a reference from the WeakReference, however the subscriber is still alive
                    subscription.Dispose();
                }
            });
            return subscription;
        }

        public static IDisposable SubscribeWeakly<T, TWeakClass>(this IObservable<T> observable, TWeakClass WeakClass, Action<T> onNext) where T : class where TWeakClass : class
        {
            IDisposable Result = null;
            WeakClassSubscriberHelper<T> SubscriptionHelper = new WeakClassSubscriberHelper<T>(observable, WeakClass, ref Result, onNext);
            return Result;
        }

        private class WeakClassSubscriberHelper<T> where T : class
        {
            public WeakClassSubscriberHelper(IObservable<T> observable, object WeakClass, ref IDisposable Result, Action<T> eventAction)
            {
                Result = observable.InternalSubscribeWeaklyToClass(eventAction, WeakClass, WeakClassSubscriberHelper<T>.StaticEventHandler);
            }

            public static void StaticEventHandler(Action<T> subscriber, T item)
            {
                subscriber(item);
            }
        }

        private static IDisposable InternalSubscribeWeaklyToClass<TEventPattern, TEvent, TClass>(this IObservable<TEventPattern> observable, TEvent Weak_onNext, TClass WeakClass, Action<TEvent, TEventPattern> onNext)
    where TEvent : class where TClass : class
        {
            if (onNext.Target != null)
                throw new ArgumentException("onNext must refer to a static method, or else the subscription will still hold a strong reference to target");

            // The class instance could live in a differnt 
            // place than the eventhandler. If either one is null,
            // terminate the subscribtion.
            var WeakClassReference = new WeakReference(WeakClass);
            var Weak_onNextReferance = new WeakReference(Weak_onNext);

            IDisposable subscription = null;
            subscription = observable.Subscribe(item =>
            {
                var currentWeakClass = WeakClassReference.Target as TClass;
                var current_onNext = Weak_onNextReferance.Target as TEvent;
                if (currentWeakClass != null && current_onNext != null)
                {
                    onNext(current_onNext, item);
                }
                else
                {
                    subscription.Dispose();
                }
            });
            return subscription;
        }
    }

Answer 1

I came up with my own solution. So basically I merged both of those implementations into one. So my solution is pretty straightforward and I think there are many ways to improve this.

So here is the solution:

/// <summary>
/// PclWeakEventManager base class
/// </summary>
/// <typeparam name="TEventSource">The type of the event source.</typeparam>
/// <typeparam name="TEventHandler">The type of the event handler.</typeparam>
/// <typeparam name="TEventArgs">The type of the event arguments.</typeparam>
public class PclWeakEventManager<TEventSource, TEventHandler, TEventArgs>
{
    static readonly object StaticSource = new object();

    /// <summary>
    /// Mapping between the target of the delegate (for example a Button) and the handler (EventHandler).
    /// Windows Phone needs this, otherwise the event handler gets garbage collected.
    /// </summary>
    ConditionalWeakTable<object, List<Delegate>> targetToEventHandler = new ConditionalWeakTable<object, List<Delegate>>();

    /// <summary>
    /// Mapping from the source of the event to the list of handlers. This is a CWT to ensure it does not leak the source of the event.
    /// </summary>
    ConditionalWeakTable<object, WeakHandlerList> sourceToWeakHandlers = new ConditionalWeakTable<object, WeakHandlerList>();

    /// <summary>
    /// Singleton instance
    /// </summary>
    static Lazy<PclWeakEventManager<TEventSource, TEventHandler, TEventArgs>> current =
        new Lazy<PclWeakEventManager<TEventSource, TEventHandler, TEventArgs>>(() => new PclWeakEventManager<TEventSource, TEventHandler, TEventArgs>());

    /// <summary>
    /// Get the singleton instance
    /// </summary>
    static PclWeakEventManager<TEventSource, TEventHandler, TEventArgs> Current
    {
        get { return current.Value; }
    }

    /// <summary>
    /// Initializes a new instance of the <see cref="PclWeakEventManager{TEventSource, TEventHandler, TEventArgs}"/> class.
    /// Protected to disallow instances of this class and force a subclass.
    /// </summary>
    protected PclWeakEventManager()
    {
    }

    #region Public static methods

    /// <summary>
    /// Adds a weak reference to the handler and associates it with the source.
    /// </summary>
    /// <param name="source">The source.</param>
    /// <param name="handler">The handler.</param>
    public static void AddHandler(TEventSource source, TEventHandler handler, Func<EventHandler<TEventArgs>, TEventHandler> converter, Action<TEventHandler> add, Action<TEventHandler> remove)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (handler == null) throw new ArgumentNullException("handler");

        if (!typeof(TEventHandler).GetTypeInfo().IsSubclassOf(typeof(Delegate)))
        {
            throw new ArgumentException("Handler must be Delegate type");
        }
        var observable = Observable.FromEventPattern(converter, add, remove);
        Current.PrivateAddHandler(source, observable, handler);
    }

    /// <summary>
    /// Removes the association between the source and the handler.
    /// </summary>
    /// <param name="source">The source.</param>
    /// <param name="handler">The handler.</param>
    public static void RemoveHandler(TEventSource source, TEventHandler handler)
    {
        if (source == null) throw new ArgumentNullException("source");
        if (handler == null) throw new ArgumentNullException("handler");

        if (!typeof(TEventHandler).GetTypeInfo().IsSubclassOf(typeof(Delegate)))
        {
            throw new ArgumentException("handler must be Delegate type");
        }

        Current.PrivateRemoveHandler(source, handler);
    }

    #endregion

    #region Event delivering

    /// <summary>
    /// Delivers the event to the handlers registered for the source. 
    /// </summary>
    /// <param name="sender">The sender.</param>
    /// <param name="args">The <see cref="TEventArgs"/> instance containing the event data.</param>
    public static void DeliverEvent(TEventSource sender, TEventArgs args)
    {
        Current.PrivateDeliverEvent(sender, args);
    }

    /// <summary>
    /// Override this method to attach to an event.
    /// </summary>
    /// <param name="source">The source.</param>
    protected virtual void StartListening(TEventSource source, IObservable<EventPattern<TEventArgs>> observable, TEventHandler handler)
    {
        //The handler - proxy should be static, otherwise it will create a strong reference
        InternalSubscribeWeakly(observable, source, handler, DeliverEvent);
    }

    /// <summary>
    /// Override this method to detach from an event.
    /// </summary>
    /// <param name="source">The source.</param>
    protected virtual void StopListening(object source)
    {
        //This method is for future usage
    }

    /// <summary>
    /// Fire the event handler
    /// </summary>
    /// <param name="sender">Event publisher</param>
    /// <param name="args">Event arguments</param>
    void PrivateDeliverEvent(object sender, TEventArgs args)
    {
        object source = sender != null ? sender : StaticSource;
        var weakHandlers = default(WeakHandlerList);

        bool hasStaleEntries = false;

        if (this.sourceToWeakHandlers.TryGetValue(source, out weakHandlers))
        {
            using (weakHandlers.DeliverActive())
            {
                hasStaleEntries = weakHandlers.DeliverEvent(source, args);
            }
        }

        if (hasStaleEntries)
        {
            this.Purge(source);
        }
    }

    #endregion

    #region Add weak handler methods

    /// <summary>
    /// Adds the event handler to WeakTables
    /// </summary>
    /// <param name="source">The event publisher source</param>
    /// <param name="observable">Observable object</param>
    /// <param name="handler">The event handler. This is used to create a weak reference</param>
    void PrivateAddHandler(TEventSource source, IObservable<EventPattern<TEventArgs>> observable, TEventHandler handler)
    {
        this.AddWeakHandler(source, observable, handler);
        this.AddTargetHandler(handler);
    }

    /// <summary>
    /// Add a weak handler
    /// </summary>
    /// <param name="source">The event publisher source</param>
    /// <param name="observable">Observable object</param>
    /// <param name="handler">The event handler. This is used to create a weak reference</param>
    void AddWeakHandler(TEventSource source, IObservable<EventPattern<TEventArgs>> observable, TEventHandler handler)
    {
        WeakHandlerList weakHandlers;

        //If for the event source table wasn't created, then it creates a new
        if (this.sourceToWeakHandlers.TryGetValue(source, out weakHandlers))
        {
            // clone list if we are currently delivering an event
            if (weakHandlers.IsDeliverActive)
            {
                weakHandlers = weakHandlers.Clone();
                this.sourceToWeakHandlers.Remove(source);
                this.sourceToWeakHandlers.Add(source, weakHandlers);
            }
            weakHandlers.AddWeakHandler(source, handler);
        }
        else
        {
            weakHandlers = new WeakHandlerList();
            weakHandlers.AddWeakHandler(source, handler);

            this.sourceToWeakHandlers.Add(source, weakHandlers);
            this.StartListening(source, observable, handler);
        }

        this.Purge(source);
    }

    /// <summary>
    /// Subscribe to the event
    /// </summary>
    /// <param name="observable">Observable object</param>
    /// <param name="source">The event publisher source</param>
    /// <param name="handler">The event handler. This is used to create a weak reference</param>
    /// <param name="onNext">Event handler delegate</param>
    private static void InternalSubscribeWeakly(IObservable<EventPattern<TEventArgs>> observable, TEventSource source, TEventHandler handler, Action<TEventSource, TEventArgs> onNext)
    {
        if (onNext.Target != null)
            throw new ArgumentException("onNext must refer to a static method, or else the subscription will still hold a strong reference to target");

        // Is the delegate alive?
        var Weak_onNextReferance = new WeakReference(handler);

        //This is a link for that event, so if you want to unsubscribe from event you have to dispose this object
        IDisposable subscription = null;
        subscription = observable.Subscribe(item =>
        {
            //Purge handler if the subscriber is not alive
            Current.Purge(source);
            //So the library keeps weak reference for this object and each time event fired it tries to get that object
            var current_onNext = Weak_onNextReferance.Target;
            if (current_onNext != null)
            {
                //If the object was found, it uses the delegate that subscriber provided and fires the event
                onNext((TEventSource)item.Sender, item.EventArgs);
            }
            else
            {
                //If the object is not found it disposes the link
                subscription.Dispose();
                Current.sourceToWeakHandlers.Remove(source);
            }
        });
    }

    /// <summary>
    /// Adds the event handler to the weak event handlers list
    /// </summary>
    /// <param name="handler">The event handler. This is used to create a weak reference</param>
    void AddTargetHandler(TEventHandler handler)
    {
        var @delegate = handler as Delegate;
        object key = @delegate.Target ?? StaticSource;
        List<Delegate> delegates;

        if (this.targetToEventHandler.TryGetValue(key, out delegates))
        {
            delegates.Add(@delegate);
        }
        else
        {
            delegates = new List<Delegate>();
            delegates.Add(@delegate);

            this.targetToEventHandler.Add(key, delegates);
        }
    }

    #endregion

    #region Remove weak handler methods

    /// <summary>
    /// Remove the event handler
    /// </summary>
    /// <param name="source">Event source object</param>
    /// <param name="handler">The event handler</param>
    void PrivateRemoveHandler(TEventSource source, TEventHandler handler)
    {
        this.RemoveWeakHandler(source, handler);
        this.RemoveTargetHandler(handler);
    }

    /// <summary>
    /// Remove the event handler
    /// </summary>
    /// <param name="source">Event source object</param>
    /// <param name="handler">The event handler</param>
    void RemoveWeakHandler(TEventSource source, TEventHandler handler)
    {
        var weakHandlers = default(WeakHandlerList);

        if (this.sourceToWeakHandlers.TryGetValue(source, out weakHandlers))
        {
            // clone list if we are currently delivering an event
            if (weakHandlers.IsDeliverActive)
            {
                weakHandlers = weakHandlers.Clone();
                this.sourceToWeakHandlers.Remove(source);
                this.sourceToWeakHandlers.Add(source, weakHandlers);
            }

            if (weakHandlers.RemoveWeakHandler(source, handler) && weakHandlers.Count == 0)
            {
                this.sourceToWeakHandlers.Remove(source);
                this.StopListening(source);
            }
        }
    }

    /// <summary>
    /// Remove the handler from weaktable
    /// </summary>
    /// <param name="handler">The event handler</param>
    void RemoveTargetHandler(TEventHandler handler)
    {
        var @delegate = handler as Delegate;
        object key = @delegate.Target ?? StaticSource;

        var delegates = default(List<Delegate>);
        if (this.targetToEventHandler.TryGetValue(key, out delegates))
        {
            delegates.Remove(@delegate);

            if (delegates.Count == 0)
            {
                this.targetToEventHandler.Remove(key);
            }
        }
    }

    /// <summary>
    /// Remove dead handlers
    /// </summary>
    /// <param name="source">Source object</param>
    void Purge(object source)
    {
        var weakHandlers = default(WeakHandlerList);

        if (this.sourceToWeakHandlers.TryGetValue(source, out weakHandlers))
        {
            if (weakHandlers.IsDeliverActive)
            {
                weakHandlers = weakHandlers.Clone();
                this.sourceToWeakHandlers.Remove(source);
                this.sourceToWeakHandlers.Add(source, weakHandlers);
            }
            else
            {
                weakHandlers.Purge();
            }
        }
    }

    #endregion

    #region WeakHandler table helper classes

    /// <summary>
    /// Weak handler helper class
    /// </summary>
    internal class WeakHandler
    {
        WeakReference source;
        WeakReference originalHandler;

        public bool IsActive
        {
            get { return this.source != null && this.source.IsAlive && this.originalHandler != null && this.originalHandler.IsAlive; }
        }

        public TEventHandler Handler
        {
            get
            {
                if (this.originalHandler == null)
                {
                    return default(TEventHandler);
                }
                else
                {
                    return (TEventHandler)this.originalHandler.Target;
                }
            }
        }

        public WeakHandler(object source, TEventHandler originalHandler)
        {
            this.source = new WeakReference(source);
            this.originalHandler = new WeakReference(originalHandler);
        }

        /// <summary>
        /// Checks if provided handler is the same
        /// </summary>
        /// <param name="source"></param>
        /// <param name="handler"></param>
        /// <returns>True if source.Target is equals to source, otherwise false</returns>
        public bool Matches(object source, TEventHandler handler)
        {
            return this.source != null &&
                object.ReferenceEquals(this.source.Target, source) &&
                this.originalHandler != null;
        }
    }

    /// <summary>
    /// Weak event handler manager
    /// </summary>
    internal class WeakHandlerList
    {
        int deliveries = 0;
        List<WeakHandler> handlers;

        public WeakHandlerList()
        {
            handlers = new List<WeakHandler>();
        }

        /// <summary>
        /// Adds new weak event handler to the list
        /// </summary>
        /// <param name="source">The event source</param>
        /// <param name="handler">The event handler</param>
        public void AddWeakHandler(TEventSource source, TEventHandler handler)
        {
            WeakHandler handlerSink = new WeakHandler(source, handler);
            handlers.Add(handlerSink);
        }

        /// <summary>
        /// Remove weak handler from the list
        /// </summary>
        /// <param name="source">The event source</param>
        /// <param name="handler">The event handler</param>
        /// <returns>True if the handler was removed, otherwise false</returns>
        public bool RemoveWeakHandler(TEventSource source, TEventHandler handler)
        {
            foreach (var weakHandler in handlers)
            {
                if (weakHandler.Matches(source, handler))
                {
                    return handlers.Remove(weakHandler);
                }
            }

            return false;
        }

        /// <summary>
        /// Clones the list
        /// </summary>
        /// <returns></returns>
        public WeakHandlerList Clone()
        {
            WeakHandlerList newList = new WeakHandlerList();
            newList.handlers.AddRange(this.handlers.Where(h => h.IsActive));

            return newList;
        }

        /// <summary>
        /// Items count
        /// </summary>
        public int Count
        {
            get { return this.handlers.Count; }
        }

        /// <summary>
        /// True if any of the events are still in delivering process
        /// </summary>
        public bool IsDeliverActive
        {
            get { return this.deliveries > 0; }
        }


        public IDisposable DeliverActive()
        {
            Interlocked.Increment(ref this.deliveries);

            return Disposable.Create(() => Interlocked.Decrement(ref this.deliveries));
        }

        /// <summary>
        /// Fire the handler
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="args"></param>
        /// <returns></returns>
        public virtual bool DeliverEvent(object sender, TEventArgs args)
        {
            bool hasStaleEntries = false;

            foreach (var handler in handlers)
            {
                if (handler.IsActive)
                {
                    var @delegate = handler.Handler as Delegate;
                    @delegate.DynamicInvoke(sender, args);
                }
                else
                {
                    hasStaleEntries = true;
                }
            }

            return hasStaleEntries;
        }

        /// <summary>
        /// Removes dead handlers
        /// </summary>
        public void Purge()
        {
            for (int i = handlers.Count - 1; i >= 0; i--)
            {
                if (!handlers[i].IsActive)
                {
                    handlers.RemoveAt(i);
                }
            }
        }
    }

    #endregion
}