与lambdas一起使用的弱事件处理程序模型

Question

好吧，所以这更像是一个答案而不是一个问题，但在提出这个问题之后 ，将Dustin Campbell ， Egor的各个部分以及“ IObservable / Rx / Reactive框架 ”的最后一个提示拉到一起，我想我为这个特殊问题制定了可行的解决方案。 它可能会被IObservable / Rx / Reactive框架完全取代，但只有经验才会显示出来。

我故意创造了一个新问题，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。

有很多相关的问题，大多数人告诉你，如果你想以后能够分离它们，你就不能使用内联lambda：

• .Net中的弱事件？
• 在C＃中使用lambda取消事件＃
• 使用lambdas作为事件处理程序会导致内存泄漏吗？
• 如何取消订阅使用lambda表达式的事件？
• 在C＃中取消订阅匿名方法

确实，如果您希望以后能够分离它们，您需要保留对lambda的引用。 但是，如果您只是希望事件处理程序在订阅者超出范围时自行分离，则此答案适合您。

Answer 1

'答案

（如果您想了解我如何获得此解决方案，请阅读以下内容）

用法，给定带有vanilla MouseDown事件的控件，以及特定的EventHandler<ValueEventArgs> 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Rx的解决方法）

（**重要的是避免直接在这里调用订阅者对象（例如，如果我们在Subscriber类中，则直接调用subscriber.DoSomething（e）或直接调用DoSomething（e）。这样做有效地创建了对订阅者的引用，完全击败了对象...）

注意 ：在某些情况下，这个CAN会在内存中引用为lambdas创建的包装类，但它们只会权衡字节，所以我不会太烦恼。

执行：

//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的优秀答案（请参阅带注释的版本链接）：

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;
}

困扰我的是事件被硬编码到方法中。 这意味着对于每个新事件，都有一种新的写入方法。

我摆弄着并设法提出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);
}

然而，该解决方案的问题在于它只是通用的，它无法处理标准winforms MouseUp，MouseDown等...

所以我试着让它更通用：

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，正如我在这里暗示的那样，这将无法编译，因为没有办法将T限制为委托。

那时，我几乎放弃了。 尝试与C＃规范作斗争是没有意义的。

但是，昨天，我从Reactive框架中发现了Observable.FromEvent方法，我没有实现，但是用法似乎有点熟悉，而且非常有趣：

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论点。 这是缺少委托类型约束的解决方法。 我们通过传递将创建委托的函数来接受它。

将所有这些放在一起为我们提供了本答案顶部显示的解决方案。

事后

我完全建议花时间了解反应框架（或最终被调用的内容）。 它非常有趣，而且有点令人费解。 我怀疑它也会使这样的问题变得多余。

到目前为止，我见过的最有趣的东西是Channel9上的视频。

Answer 2

如果您前往CodePlex，那么有一个名为Sharp Observation的项目，其中作者已经构建了一个良好的弱委托提供程序，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

我一直在寻找一个解决方案很长一段时间，大多数都使用讨厌的反思，但Benjohl的答案很棒。 我已经调整它以添加对非泛型EventHandler，DependencyPropertyChangedEventArgs的支持，它不会从EventArgs继承并允许您自己手动取消注册事件。 我会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非常出色。 除了集成到.NET中的解决方案之外，唯一剩下的就是创建真正通用的弱事件处理程序的一种非常简单的方法：

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