Weak Event模型

事件可能会引起一个内存泄露问题，而该模型就是为了解决该问题。

为什么会出现memory Leak

事件的使用语法如：source.Event+=Listener.Func，这样Source即建立了一个Listener的强引用，从而Listener的生命周期将受到Source的影响，生命周期延长，从而产生内存泄露。

如何完成弱事件模型

1. 从WeakEventManager派生出一个Manager。
2. 在Listerner对象上实现IWeakEventListener接口。
3. 当对Source添加Listener时，不要使用事件的Add或Remove操作，应该使用Manager中的AddListener及RemoveListener。

自定义WeakEventManager

• 重写StartListening及StopListening
• 提供两个方法来操作Listeners List，常用命名为‘Addlistener’及‘RemoveListener’
• 实现一个CurrentManage属性

代码示例

    class Source
    {
        public void Time()
        {
            var timeHandler = TimeChanged;
            if (null != timeHandler)
                timeHandler(this, new TimeEventArgs { TimeNow = DateTime.Now });
        }

        public event Action<object, TimeEventArgs> TimeChanged;
    }

    class TimeChangeEventManager : WeakEventManager
    {
        private static TimeChangeEventManager CurrentManager
        {
            get
            {
                var manager = (TimeChangeEventManager)GetCurrentManager(typeof(TimeChangeEventManager));

                if (manager == null)
                {
                    manager = new TimeChangeEventManager();
                    SetCurrentManager(typeof(TimeChangeEventManager), manager);
                }

                return manager;
            }
        }

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

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

        protected override void StartListening(object source)
        {
            ((Source)source).TimeChanged += DeliverEvent;
        }

        protected override void StopListening(object source)
        {
            ((Source)source).TimeChanged -= DeliverEvent;
        }
    }

    class ListenerTwo : IWeakEventListener
    {
        private static int S_ListenCount = 0;

        private int _currentIndex = 0;

        public ListenerTwo()
        {
            _currentIndex = S_ListenCount++;
        }

        public void Show(object sender, TimeEventArgs e)
        {
            Console.WriteLine("Index: {0} Time: {1}", _currentIndex, e.TimeNow);
        }

        public bool ReceiveWeakEvent(Type managerType, object sender, EventArgs e)
        {
            if (managerType == typeof (TimeChangeEventManager))
                Show(sender, (TimeEventArgs) e);
            else
            {
                return false;
            }
            return true;
        }
    }

    class TimeEventArgs : EventArgs
    {
        public DateTime TimeNow { get; set; }
    }
    
    static void Main(string[] args)
    {
        Source ss = new Source();
        AddListener(ss);
        ss.Time();
        Console.Read();
    }

    private static void AddListener(Source ss)
    {
        var lis1 = new ListenerTwo();
        var lis2 = new ListenerTwo();
        //可以释放
        TimeChangeEventManager.AddListener(ss,lis1);
        TimeChangeEventManager.AddListener(ss,lis2);
        //不会释放
        ss.TimeChanged += lis1.Show;
        ss.TimeChanged += lis2.Show;
        lis1 = null;
        lis2 = null;
        GC.Collect();
    }

.Net 4.5的处理方式

4.5提供了WeakEventManager的一个泛型版本，即WeakEventManager< TEventSource,TEventArgs >。使用泛型版本则不需要自定义一个新的EventManager类。
用法相当的简单，如下代码所示：
WeakEventManager<Source, TimeEventArgs>.AddHandler(ss, "TimeChanged", lis1.Show);

引用

http://www.codeproject.com/Articles/738109/The-NET-weak-event-pattern-in-Csharp
https://msdn.microsoft.com/en-us/library/aa970850(v=vs.100).aspx
https://msdn.microsoft.com/en-us/library/system.windows.iweakeventlistener.receiveweakevent(v=vs.100).aspx
https://msdn.microsoft.com/en-us/library/system.windows.weakeventmanager(v=vs.100).aspx