C# 控件开发之-Weak Event模式

C#的事件挂载语法是：
source.SomeEvent += new SomeEventHandler(MyEventHandler)
这在事件源上创建了一个监听器的强引用，使得事件监听器的生命周期收到事件源的限制，会造成内存泄露问题。

控件的作者应该避免给使用者引入内存泄漏问题，控件应该采用Weak Event模式。

实现Weak Event模式

Before .Net 4.5 the .Net framework came with a class and an interface that allowed implementation of the weak event pattern:

• WeakEventManager which is where all the pattern plumbing is encapsulated
• IWeakEventListener which is the pipe that allows a component to connect to the WeakEventManager
  plumbing

(Both are located in the WindowsBase assembly that you’ll need to reference yourself if you’re not developing a WPF project which should already correctly reference it.)
First the custom weak event manager:

public class EventManager : WeakEventManager
{
    private static EventManager CurrentManager
    {
        get
        {
            EventManager manager = (EventManager)GetCurrentManager(typeof(EventManager));

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

            return manager;
        }
    }


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

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

    protected override void StartListening(object source)
    {
        ((EventSource)source).Event += DeliverEvent;
    }

    protected override void StopListening(object source)
    {
        ((EventSource)source).Event -= DeliverEvent;
    }
}

Then our event listener:

public class LegacyWeakEventListener : IWeakEventListener
{
    private void OnEvent(object source, EventArgs args)
    {
        Console.WriteLine("LegacyWeakEventListener received event.");
    }

    public LegacyWeakEventListener(EventSource source)
    {
        EventManager.AddListener(source, this);
    }

    public bool ReceiveWeakEvent(Type managerType, object sender, EventArgs e)
    {
        OnEvent(sender, e);

        return true;
    }

    ~LegacyWeakEventListener()
    {
        Console.WriteLine("LegacyWeakEventListener finalized.");
    }
}

.Net 4.5 has introduced a new generic version of the legacy WeakEventManager: class MSDN documentation" href="http://msdn.microsoft.com/en-us/library/hh199438(v=vs.110).aspx" style="margin: 0px; padding: 0px; border: 0px; text-decoration: none; color: rgb(128, 0, 128);">WeakEventManager.
(This class is located in the WindowsBase assembly too.)

public class WeakEventListener
{
    private void OnEvent(object source, EventArgs args)
    {
        Console.WriteLine("WeakEventListener received event.");
    }

    public WeakEventListener(EventSource source)
    {
        WeakEventManager.AddHandler(source, "Event", OnEvent);
    }

    ~WeakEventListener()
    {
        Console.WriteLine("WeakEventListener finalized.");
    }
}

There is only a single line of code to write, really clean.

对于客户端透明的实现方式

上述两种方式都需要客户端在订阅事件的时候修改原有的代码。

参考资料[3] 提供了对于客户端透明的实现方式，但其中使用的DynamicInvoke会造成性能问题。参考资料[4]是更可行、实际可用的方式。
作者Thomas Levesque在MAY 6, 2016 AT 20:06说，相比于.Net 4.5提供的WeakEventManager，他的方案更加方便实用：

Well, IMO WeakEventManager isn’t very natural to use; there’s a lot of boilerplate code to write, and it doesn’t look anything like a normal event. Other than that, I don’t think there are fundamental differences between the two.

参考

[1] The .NET weak event pattern in C#
[2] [C#] A SIMPLE IMPLEMENTATION OF THE WEAKEVENT PATTERN, 2010.
[3] thomaslevesque/WeakEvent
[4] WEAK EVENTS IN C#, TAKE TWO, 2015.08。