单次事件订阅 [英] Single-shot event subscription

问题描述

我相当确信，这是不可能的，但我要问不过。

为了使单次认购活动，我经常发现自己使用这个（自主发明）模式：

 事件处理程序处理程序= NULL;
处理器=（发件人，E）=＆GT;
{
    SomeEvent  -  =处理程序;
    初始化（）;
};
SomeEvent + =处理程序;
 

这是相当多的锅炉板，而这也使得关于修改关闭ReSharper的whinge。有没有把这个模式变成一个扩展方法或类似的方法吗？这样做的更好的办法？

在理想情况下，我想是这样的：

  SomeEvent.OneShot（句柄）
 

解决方案

这不是很容易重构为一个扩展方法，因为你可以参考在C＃中的事件的唯一途径是通过订阅（ + = ）或退订（ - 。= ）从它（除非它在当前类的声明）

您可以用同样的方法，因为在无扩展：的 Observable.FromEvent 需要两个代表从它订阅事件退订。所以，你可以做这样的事情：

 公共静态类EventHelper
{
    公共静态无效SubscribeOneShot（
        动作＆LT;＆事件处理GT;订阅，
        动作＆LT;＆事件处理GT;退订，
        事件处理程序处理程序）
    {
        事件处理actualHandler = NULL;
        actualHandler =（发件人，E）=＆GT;
        {
            退订（actualHandler）;
            处理器（发件人，E）;
        };
        订阅（actualHandler）;
    }
}...美孚F =新的Foo（）;
EventHelper.SubscribeOneShot（
    处理器= GT; f.Bar + =处理程序，
    处理器= GT; f.Bar  -  =处理程序，
    （发件人，E）=＆GT; { /* 随你 */ }）;
 

I'm fairly convinced that this isn't possible, but I'm going to ask nonetheless.

In order to make a single-shot subscription to events, I frequently find myself using this (self-invented) pattern:

EventHandler handler=null;
handler = (sender, e) =>
{
    SomeEvent -= handler;
    Initialize();
};
SomeEvent += handler;

It's quite a lot of boiler-plate, and it also makes Resharper whinge about modified closures. Is there a way of turning this pattern into an extension method or similar? A better way of doing it?

Ideally, I'd like something like:

SomeEvent.OneShot(handler)

解决方案

It's not very easy to refactor to an extension method, because the only way you can refer to an event in C# is by subscribing (+=) to or unsubscribing (-=) from it (unless it's declared in the current class).

You could use the same approach as in Reactive Extensions: Observable.FromEvent takes two delegates to subscribe to the event an unsubscribe from it. So you could do something like that:

public static class EventHelper
{
    public static void SubscribeOneShot(
        Action<EventHandler> subscribe,
        Action<EventHandler> unsubscribe,
        EventHandler handler)
    {
        EventHandler actualHandler = null;
        actualHandler = (sender, e) =>
        {
            unsubscribe(actualHandler);
            handler(sender, e);
        };
        subscribe(actualHandler);
    }
}

...

Foo f = new Foo();
EventHelper.SubscribeOneShot(
    handler => f.Bar += handler,
    handler => f.Bar -= handler,
    (sender, e) => { /* whatever */ });

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