c# - WeakEventManager 持有对订阅者的引用

Question

我一直在使用 Wea​​kEventManager 来避免内存泄漏，但我开始过度使用它们。 我为 INotifyPropertyChanged 创建了扩展方法，例如:

public static void AddWeakPropertyChanged(this INotifyPropertyChanged item, Action handler)
{
    PropertyChangedEventManager.AddHandler(item, (s, e) => handler(e.PropertyName), string.Empty);
}

现在我很快意识到，这是行不通的。事实上，你不能真正使用匿名方法来进行弱事件处理。 (如果我理解正确，那么编译器会为它创建一个“闭包类”(以保存引用的值)，它有处理程序，但由于你的闭包类没有在任何地方被引用，GC 将清除它，事件处理程序不会被调用)

问题 #1:这是正确的吗？我的意思是这样正确吗，然后当对弱事件处理程序使用匿名方法(或 lambda)时，仅当 GC 未同时运行时才调用处理程序(例如，它是不确定的)？

好吧，我是这么认为的，所以我做了一些单元测试以确保我做对了。在我进行以下单元测试之前，一切似乎都还不错:

        class DidRun
        {
            public bool Value { get; set; }
        }
        class TestEventPublisher
        {
            public event EventHandler<EventArgs> MyEvent;
            public void RaiseMyEvent()
            {
                if (MyEvent != null)
                    MyEvent(this, EventArgs.Empty);

            }
        }
        class TestClosure
        {
            public DidRun didRun { get; set; }
            public EventHandler<EventArgs> Handler { get; private set; }
            public TestClosure()
            {
                this.Handler = new EventHandler<EventArgs>((s, e) => didRun.Value = true);
            }
        }
        [TestMethod]
        public void TestWeakReference()
        {
            var raiser = new TestEventPublisher();
            var didrun = new DidRun();
            var closure = new TestClosure { didRun = didrun };
            WeakEventManager<TestEventPublisher, EventArgs>.AddHandler(raiser, "MyEvent", closure.Handler);
            closure = null;

            GC.Collect();
            GC.Collect();
            raiser.RaiseMyEvent();
            Assert.AreEqual(false, didrun.Value);
        }

问题 #2:谁能解释我为什么这个测试失败了？

期望:这里我没有任何闭包(我把它们拿出来，以确保发生了什么)，我只有一个对象(闭包)，它使用 Wea​​kEventManager 订阅一个事件，然后我删除引用到它(closure = null;)。

我期待 2 次 GC.Collect() 调用，以清理我的旧闭包类，因此 WeakEventManager 将删除订阅者，而不运行处理程序，但测试失败。有什么想法吗？

编辑:抱歉，通用参数不可见，现在可见

Answer 1

您是正确的，如果没有对它的引用，GC 将收集围绕您的 lambda 创建的闭包。

在您的单元测试中，您取消了 TestClosure 的本地实例，但是您将处理程序的硬引用传递给了 WeakEventManager，而不是 TestClosure 的实例。所以处理程序继续存在...

我相信这些例子证明了你在闭包方面的问题:

class DidRun
{
    public bool Value { get; set; }
}

class TestEventPublisher
{
    public event EventHandler<EventArgs> MyEvent;
    public void RaiseMyEvent()
    {
        if (MyEvent != null)
            MyEvent(this, EventArgs.Empty);
    }
}

class TestClosure
{
    static public EventHandler<EventArgs> Register(TestEventPublisher raiser, DidRun didrun)
    {
        EventHandler<EventArgs> handler = (s, e) => didrun.Value = true;
        WeakEventManager<TestEventPublisher, EventArgs>.AddHandler(raiser, "MyEvent", handler);
        return handler;
    }
}

[TestMethod]
public void Test1()
{
    var raiser = new TestEventPublisher();
    var didrun = new DidRun();

    TestClosure.Register(raiser, didrun);

    // The reference to the closure 'handler' is not being held,
    //  it may or may not be GC'd (indeterminate result)

    raiser.RaiseMyEvent();
    Assert.IsTrue(didrun.Value);
}

[TestMethod]
public void Test2()
{
    var raiser = new TestEventPublisher();
    var didrun = new DidRun();

    // The reference to the closure 'handler' is not being held, it's GC'd
    TestClosure.Register(raiser, didrun);

    GC.Collect();
    GC.Collect();

    raiser.RaiseMyEvent();
    Assert.IsFalse(didrun.Value);
}

[TestMethod]
public void Test3()
{
    var raiser = new TestEventPublisher();
    var didrun = new DidRun();

    // Keep local copy of handler to prevent it from being GC'd
    var handler = TestClosure.Register(raiser, didrun);

    GC.Collect();
    GC.Collect();

    raiser.RaiseMyEvent();
    Assert.IsTrue(didrun.Value);
}

至于您的原始问题，您可以尝试保存处理程序(闭​​包)以防止它被 GC。 ConditionalWeakTable 应该适用于此:

// ConditionalWeakTable will hold the 'value' as long as the 'key' is not marked for GC
static private ConditionalWeakTable<INotifyPropertyChanged, EventHandler<PropertyChangedEventArgs>> _eventMapping =
  new ConditionalWeakTable<INotifyPropertyChanged, EventHandler<PropertyChangedEventArgs>>();

public static void AddWeakPropertyChanged(this INotifyPropertyChanged item, Action<string> handlerAction)
{
    EventHandler<PropertyChangedEventArgs> handler;

    // Remove any existing handler for this item in case it's registered more than once
    if (_eventMapping.TryGetValue(item, out handler))
    {   
        _eventMapping.Remove(item);
        PropertyChangedEventManager.RemoveHandler(item, handler, string.Empty);
    }   

    handler = (s, e) => handlerAction(e.PropertyName);

    // Save handler (closure) to prevent GC
    _eventMapping.Add(item, handler);

    PropertyChangedEventManager.AddHandler(item, handler, string.Empty);
}

class DidRun
{
    static public string Value { get; private set; }
    public void SetValue(string value) { Value = value; }
}

[TestMethod]
public void Test4()
{
    var property = new ObservableObject<string>();

    var didrun = new DidRun();
    property.AddWeakPropertyChanged(
        (x) => 
        {
            didrun.SetValue("Property Name = " + x);
        });

    GC.Collect();
    GC.Collect();

    property.Value = "Hello World";

    Assert.IsTrue(DidRun.Value != null);
}