自awaitables傻瓜 [英] Custom awaitables for dummies

问题描述

在异步/等待常见问题解答，斯蒂芬Toub说：

In Async/Await FAQ, Stephen Toub says:

这是 awaitable 是任何类型的暴露了一个 GetAwaiter 方法，该方法返回一个有效的 awaiter 的。照片   ...
  一个 awaiter 是任何的从的 awaitable 的的 GetAwaiter 方法和符合返回类型到特定的图案。

An awaitable is any type that exposes a GetAwaiter method which returns a valid awaiter.
...
An awaiter is any type returned from an awaitable’s GetAwaiter method and that conforms to a particular pattern.

因此​​，为了成为一个的 awaiter 的，一种类型的应：

So in order to be an awaiter, a type should:

• 实施<一个href="http://msdn.microsoft.com/en-us/library/system.runtime.compilerservices.inotifycompletion.aspx"><$c$c>INotifyCompletion接口。
• 提供称为布尔属性 IsCompleted 。
• 提供一个无参数调用getResult 方法，返回无效或 TResult 。

• Implement the INotifyCompletion interface.
• Provide a boolean property called IsCompleted.
• Provide a parameterless GetResult method that returns void or TResult.

（我忽略 ICriticalNotifyCompletion 现在的）

但什么是那些成员？ 我知道我上面提到的同一页面有一个示例，演示如何编译器将等待的操作，但我只是不明白这一点的。

But what are those members for? I know the same page I mentioned above has a sample that shows how the compiler translates await operations but I just don't get it.

谁能解释这些尽可能简单，像告诉它一个5岁？
当我的计谋的的的 awaitable 的

Could anyone explain these as simple as possible like telling it to a 5 years old?
When I await an awaitable,

• 在什么情况下，code检查 IsCompleted ？我应该在哪里设置？
• 当它调用 OnCompleted ？
• 是否叫 OnCompleted 并行还是应该放在 OnCompleted 的code是异步的？
• 在我看到这两个实例直接调用 OnCompleted 的延续参数，并使用 Task.Run（续）的不同的例子，这应该我去当？

• When does the code checks IsCompleted? Where should I set it?
• When does it call OnCompleted?
• Does it call OnCompleted in parallel or should the code inside OnCompleted be asynchronous?
• I saw examples of both directly invoking the continuation parameter of OnCompleted and using Task.Run(continuation) in different examples, which should I go for and when?

推荐答案

为什么要自定义awaiter？

Why would you want a custom awaiter?

您可以看到编译器的跨$ P $的计谋 <一ptation href="http://blogs.msdn.com/b/lucian/archive/2011/04/15/async-ctp-refresh-design-changes.aspx">here.从本质上讲：

You can see the compiler's interpretation of await here. Essentially:

var temp = e.GetAwaiter();
if (!temp.IsCompleted)
{
  SAVE_STATE()
  temp.OnCompleted(&cont);
  return;

cont:
  RESTORE_STATE()
}
var i = temp.GetResult();

的评论编辑： OnCompleted 应安排其作为一个持续的异步操作的参数

Edit from comments: OnCompleted should schedule its argument as a continuation of the asynchronous operation.

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