c# – WeakEventManager保存对订阅者的引用

发布时间：2020-12-15 05:38:20 所属栏目：百科来源：网络整理

导读：我一直在使用WeakEventManager来避免内存泄漏,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.AddHandl

我一直在使用WeakEventManager来避免内存泄漏,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,这不起作用.实际上,您无法真正使用匿名方法进行弱事件处理. (如果我理解正确,那么编译器为它创建一个’闭包类'(用于保存引用的值),它具有处理程序,但由于你的闭包类没有在任何地方被引用,因此GC将清除它,并且事件处理程序不会被叫)

问题1：这是正确的吗？我的意思是它是正确的,然后当使用匿名方法(或lambda)作为弱事件处理程序时,只有当GC没有同时运行时才会调用处理程序(例如,它是不确定的)？

嗯,我非常感谢,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,它似乎还可以：

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：任何人都可以解释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),我只是有一个对象(闭包),用WeakEventManager订阅一个事件,然后我删除它的引用( closure = null;).

我期待2个GC.Collect()调用,以清理我的旧闭包类,因此WeakEventManager将删除订阅者,而不是运行处理程序,但测试失败.有任何想法吗？

编辑：对不起,通用参数不可见,现在它们是

解决方法

你是正确的,如果没有对它的引用,GC将收集在你的lambda周围创建的闭包.

在单元测试中,您将TestClosure的本地实例清空,但是您已将处理程序的硬引用传递给WeakEventManager,而不是TestClosure的实例.所以处理程序依然存在……

我相信这些例子证明了你关闭的问题：

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,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);
}

至于你的原始问题,你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,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);
}

（编辑：李大同）

【声明】本站内容均来自网络，其相关言论仅代表作者个人观点，不代表本站立场。若无意侵犯到您的权利，请及时与联系站长删除相关内容!