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Weak Event模型

Weak Event模型

作者: ArtisCoder | 来源:发表于2016-05-14 14:15 被阅读119次

    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. 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AddListenerRemoveListener
    自定义WeakEventManager
    • 重写StartListeningStopListening
    • 提供两个方法来操作Listeners List,常用命名为AddlistenerRemoveListener
    • 实现一个CurrentManager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&lt;TEventSource,TEventArgs&gt;。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

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