与 lambda 一起使用的弱事件处理程序模型

Question

好的，所以这更像是一个答案而不是一个问题，但是在询问这个问题，并将 达斯汀·坎贝尔，Egor，还有来自“IObservable/Rx/Reactive 框架'，我想我已经为这个特定问题找到了一个可行的解决方案。它可能会被 IObservable/Rx/Reactive 框架完全取代，但只有经验才能证明这一点。

我故意创建了一个新问题，以便给我空间来解释我如何得到这个解决方案，因为它可能不会立即显而易见。

有许多相关的问题，大多数告诉您如果您希望稍后能够分离它们，则不能使用内联 lambda：

• .Net 中的弱事件？
• 在 C# 中使用 lambda 取消挂钩事件< /a>
• 使用 lambda 作为事件处理程序会导致内存泄漏泄漏？
• 如何取消订阅使用 lambda 表达式的事件？
• 取消订阅 C# 中的匿名方法

以及确实，如果您希望稍后能够分离它们，则需要保留对 lambda 的引用。但是，如果您只是希望事件处理程序在您的订阅者超出范围时自行分离，那么这个答案适合您。

Answer 1

'The' 答案

（如果您想了解我如何得到这个解决方案，请阅读下面的更多内容）

用法，给定一个带有普通 MouseDown 事件的控件和一个特定的 EventHandler; ValueEvent 事件：

// for 'vanilla' events
SetAnyHandler<Subscriber, MouseEventHandler, MouseEventArgs>(
    h => (o,e) => h(o,e), //don't ask me, but it works*.
    h => control.MouseDown += h,
    h => control.MouseDown -= h,
    subscriber,
    (s, e) => s.DoSomething(e));  //**See note below

// for generic events
SetAnyHandler<Subscriber, ValueEventArgs>(
    h => control.ValueEvent += h,
    h => control.ValueEvent -= h,
    subscriber,
    (s, e) => s.DoSomething(e));  //**See note below

（*这是来自 Rx)

(** 避免直接在此处调用订阅者对象非常重要（例如放置订阅者.DoSomething(e)，或调用 DoSomething( e) 如果我们在 Subscriber 类内部，则直接这样做会有效地创建对订阅者的引用，这完全破坏了该对象...）

注意：在某些情况下，这可以留下对包装类的引用。为内存中的 lambda 创建，但它们只重字节，所以我不太担心

实现：

//This overload handles any type of EventHandler
public static void SetAnyHandler<S, TDelegate, TArgs>(
    Func<EventHandler<TArgs>, TDelegate> converter, 
    Action<TDelegate> add, Action<TDelegate> remove,
    S subscriber, Action<S, TArgs> action)
    where TArgs : EventArgs
    where TDelegate : class
    where S : class
{
    var subs_weak_ref = new WeakReference(subscriber);
    TDelegate handler = null;
    handler = converter(new EventHandler<TArgs>(
        (s, e) =>
        {
            var subs_strong_ref = subs_weak_ref.Target as S;
            if(subs_strong_ref != null)
            {
                action(subs_strong_ref, e);
            }
            else
            {
                remove(handler);
                handler = null;
            }
        }));
    add(handler);
}

// this overload is simplified for generic EventHandlers
public static void SetAnyHandler<S, TArgs>(
    Action<EventHandler<TArgs>> add, Action<EventHandler<TArgs>> remove,
    S subscriber, Action<S, TArgs> action)
    where TArgs : EventArgs
    where S : class
{
    SetAnyHandler<S, EventHandler<TArgs>, TArgs>(
        h => h, add, remove, subscriber, action);
}

细节

我的出发点是 Egor 的出色回答（请参阅带评论的版本链接）：

public static void Link(Publisher publisher, Control subscriber) {
    var subscriber_weak_ref = new WeakReference(subscriber);
    EventHandler<ValueEventArgs<bool>> handler = null;
    handler = delegate(object sender, ValueEventArgs<bool> e) {
            var subscriber_strong_ref = subscriber_weak_ref.Target as Control;
            if (subscriber_strong_ref != null) subscriber_strong_ref.Enabled = e.Value;
            else {
                    ((Publisher)sender).EnabledChanged -= handler;
                    handler = null; 
            }
    };

    publisher.EnabledChanged += handler;
}

令我困扰的是该事件被硬编码到方法中这意味着对于每个新事件，都有一个新的方法可以编写。

我摆弄并设法想出了这个通用的解决方案：

private static void SetAnyGenericHandler<S, T>(
     Action<EventHandler<T>> add,     //to add event listener to publisher
     Action<EventHandler<T>> remove,  //to remove event listener from publisher
     S subscriber,                    //ref to subscriber (to pass to action)
     Action<S, T> action)             //called when event is raised
    where T : EventArgs
    where S : class
{
    var subscriber_weak_ref = new WeakReference(subscriber);
    EventHandler<T> handler = null;
    handler = delegate(object sender, T e)
    {
        var subscriber_strong_ref = subscriber_weak_ref.Target as S;
        if(subscriber_strong_ref != null)
        {
            Console.WriteLine("New event received by subscriber");
            action(subscriber_strong_ref, e);
        }
        else
        {
            remove(handler);
            handler = null;
        }
    };
    add(handler);
}

但是该解决方案的问题是它只是通用的，它无法处理标准的 winforms MouseUp、MouseDown 等...

所以我试图使它甚至 <强>更通用：

private static void SetAnyHandler<T, R>(
    Action<T> add,      //to add event listener to publisher
    Action<T> remove,   //to remove event listener from publisher
    Subscriber subscriber,  //ref to subscriber (to pass to action)
    Action<Subscriber, R> action) 
    where T : class
{
    var subscriber_weak_ref = new WeakReference(subscriber);
    T handler = null;
    handler = delegate(object sender, R e) //<-compiler doesn't like this line
    {
        var subscriber_strong_ref = subscriber_weak_ref.Target as Subscriber;
        if(subscriber_strong_ref != null)
        {
            action(subscriber_strong_ref, e);
        }
        else
        {
            remove(handler);
            handler = null;
        }
    };
    remove(handler);
}

但是，正如我暗示的这里，这不会编译，因为没有办法限制 T 成为委托。

那一刻，我几乎放弃了。尝试与 C# 规范作斗争是没有意义的。

然而，昨天，我发现了Reactive框架中的Observable.FromEvent方法，我没有实现，但用法似乎有点熟悉，而且很有趣：

var mousedown = Observable.FromEvent<MouseEventHandler, MouseDownEventArgs>(
      h => new MouseEventHandler(h),
      h => control.MouseDown += h,
      h => control.MouseDown -= h);

这是引起我注意的第一个参数。这是缺少委托类型约束的解决方法。我们通过传入创建委托的函数来获取它。

将所有这些放在一起，我们就得到了本答案顶部所示的解决方案。

事后

我彻底建议花时间了解反应式框架（或者无论它最终被称为什么）。这非常有趣，而且有点令人兴奋。我怀疑它也会使这样的问题变得完全多余。

到目前为止，我见过的最有趣的内容是 Channel9 上的视频。

Answer 2

如果您访问 CodePlex，有一个名为 Sharp Observation 的项目，其中作者构建了一个良好的弱委托提供程序，并在 MSIL 中实现。快速、灵活、易于使用：
例如

Action<int,int> myDelegate = new Action<int,int>( aMethodOnMyClass );
myDelegate.MakeWeak();

，就这么简单！

Answer 3

我长期以来一直在寻找解决方案，大多数人都使用令人讨厌的反射，但 Benjohl 的答案很棒。我对其进行了调整，添加了对非通用 EventHandler、DependencyPropertyChangedEventArgs 的支持，它不继承自 EventArgs，并允许您自己手动取消注册事件。我对人们的想法非常感兴趣，尤其是 Benjohl。

/// <summary>
/// Weakly registers for events using <see cref="WeakReference"/>.
/// </summary>
public sealed class WeakEvent
{
    private Action removeEventHandler;

    /// <summary>
    /// Initializes a new instance of the <see cref="WeakEvent"/> class.
    /// </summary>
    /// <param name="removeEventHandler">The remove event handler function.</param>
    private WeakEvent(Action removeEventHandler)
    {
        this.removeEventHandler = removeEventHandler;
    }

    /// <summary>
    /// Weakly registers the specified subscriber to the the given event of type 
    /// <see cref="EventHandler"/>.
    /// </summary>
    /// <example>
    /// Application application;
    /// WeakEvent.Register{TextBox, TextChangedEventArgs>(
    ///     this,
    ///     eventHandler => textBox.TextChanged += eventHandler,
    ///     eventHandler => textBox.TextChanged -= eventHandler,
    ///     (sender, e) => this.OnTextChanged(sender, e));
    /// </example>
    /// <typeparam name="S">The type of the subscriber.</typeparam>
    /// <param name="subscriber">The subscriber.</param>
    /// <param name="addEventhandler">The add eventhandler.</param>
    /// <param name="removeEventHandler">The remove event handler function.</param>
    /// <param name="action">The event execution function.</param>
    public static WeakEvent Register<S>(
        S subscriber,
        Action<EventHandler> addEventhandler,
        Action<EventHandler> removeEventHandler,
        Action<S, EventArgs> action)
        where S : class
    {
        return Register<S, EventHandler, EventArgs>(
            subscriber,
            eventHandler => (sender, e) => eventHandler(sender, e),
            addEventhandler,
            removeEventHandler,
            action);
    }

    /// <summary>
    /// Weakly registers the specified subscriber to the the given event of type 
    /// <see cref="EventHandler{T}"/>.
    /// </summary>
    /// <example>
    /// Application application;
    /// WeakEvent.Register{TextBox, TextChangedEventArgs>(
    ///     this,
    ///     eventHandler => textBox.TextChanged += eventHandler,
    ///     eventHandler => textBox.TextChanged -= eventHandler,
    ///     (sender, e) => this.OnTextChanged(sender, e));
    /// </example>
    /// <typeparam name="S">The type of the subscriber.</typeparam>
    /// <typeparam name="TEventArgs">The type of the event arguments.</typeparam>
    /// <param name="subscriber">The subscriber.</param>
    /// <param name="addEventhandler">The add eventhandler.</param>
    /// <param name="removeEventHandler">The remove event handler function.</param>
    /// <param name="action">The event execution function.</param>
    public static WeakEvent Register<S, TEventArgs>(
        S subscriber, 
        Action<EventHandler<TEventArgs>> addEventhandler, 
        Action<EventHandler<TEventArgs>> removeEventHandler,
        Action<S, TEventArgs> action)
        where S : class
        where TEventArgs : EventArgs
    {
        return Register<S, EventHandler<TEventArgs>, TEventArgs>(
            subscriber,
            eventHandler => eventHandler, 
            addEventhandler, 
            removeEventHandler, 
            action);
    }

    /// <summary>
    /// Weakly registers the specified subscriber to the the given event.
    /// </summary>
    /// <example>
    /// TextBox textbox;
    /// WeakEvent.Register{TextBox, TextChangedEventHandler, TextChangedEventArgs>(
    ///     this,
    ///     eventHandler => (sender, e) => eventHandler(sender, e),
    ///     eventHandler => textBox.TextChanged += eventHandler,
    ///     eventHandler => textBox.TextChanged -= eventHandler,
    ///     (sender, e) => this.OnTextChanged(sender, e));
    /// </example>
    /// <typeparam name="S">The type of the subscriber.</typeparam>
    /// <typeparam name="TEventHandler">The type of the event handler.</typeparam>
    /// <typeparam name="TEventArgs">The type of the event arguments.</typeparam>
    /// <param name="subscriber">The subscriber.</param>
    /// <param name="getEventHandler">The get event handler function.</param>
    /// <param name="addEventHandler">The add event handler function.</param>
    /// <param name="removeEventHandler">The remove event handler function.</param>
    /// <param name="action">The event execution function.</param>
    public static WeakEvent Register<S, TEventHandler, TEventArgs>(
        S subscriber, 
        Func<EventHandler<TEventArgs>, TEventHandler> getEventHandler,
        Action<TEventHandler> addEventHandler, 
        Action<TEventHandler> removeEventHandler,
        Action<S, TEventArgs> action)
        where S : class
        where TEventHandler : class
        where TEventArgs : EventArgs

    {
        WeakReference weakReference = new WeakReference(subscriber);

        TEventHandler eventHandler = null;
        eventHandler = getEventHandler(
            new EventHandler<TEventArgs>(
                (sender, e) =>
                {
                    S subscriberStrongRef = weakReference.Target as S;

                    if (subscriberStrongRef != null)
                    {
                        action(subscriberStrongRef, e);
                    }
                    else
                    {
                        removeEventHandler(eventHandler);
                        eventHandler = null;
                    }
                }));

        addEventHandler(eventHandler);

        return new WeakEvent(() => removeEventHandler(eventHandler));
    }

    public static WeakEvent Register<S>(
        S subscriber,
        Action<DependencyPropertyChangedEventHandler> addEventHandler,
        Action<DependencyPropertyChangedEventHandler> removeEventHandler,
        Action<S, DependencyPropertyChangedEventArgs> action)
        where S : class
    {
        WeakReference weakReference = new WeakReference(subscriber);

        DependencyPropertyChangedEventHandler eventHandler = null;
        eventHandler = new DependencyPropertyChangedEventHandler(
            (sender, e) =>
            {
                S subscriberStrongRef = weakReference.Target as S;

                if (subscriberStrongRef != null)
                {
                    action(subscriberStrongRef, e);
                }
                else
                {
                    removeEventHandler(eventHandler);
                    eventHandler = null;
                }
            });

        addEventHandler(eventHandler);

        return new WeakEvent(() => removeEventHandler(eventHandler));
    }

    /// <summary>
    /// Manually unregisters this instance from the event.
    /// </summary>
    public void Unregister()
    {
        if (this.removeEventHandler != null)
        {
            this.removeEventHandler();
            this.removeEventHandler = null;
        }
    }
}

Answer 4

达斯汀·坎贝尔的方法已经很出色了。剩下的唯一的事情，保存集成到 .NET 中的解决方案，是创建真正通用的弱事件处理程序的非常简单的方法：

http://puremsil.wordpress.com/2010/05/03/generic-weak-event-handlers/