虚拟人的自定义等待

问题描述

在异步/等待常见问题解答中，斯蒂芬·图布 说:

In Async/Await FAQ, Stephen Toub says:

awaitable 是任何公开 GetAwaiter 方法的类型，该方法返回一个有效的 awaiter.
...
awaiter 是从 awaitable 的 GetAwaiter 方法返回的任何类型，并且符合特定模式.

所以为了成为等待者，类型应该:

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

• 实施 INotifyCompletion 界面.
• 提供一个名为 IsCompleted 的布尔属性.
• 提供一个无参数的 GetResult 方法，返回 void 或 TResult.

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

(我暂时忽略了 ICriticalNotifyCompletion.)

我知道我提到的页面有一个示例，展示了编译器如何翻译 await 操作，但我仍然很难理解.

I know the page I mentioned has a sample that shows how the compiler translates await operations but I'm stil having a hard time understanding.

当我等待一个等待时，

• 什么时候检查IsCompleted?我应该在哪里设置它?
• 什么时候调用 OnCompleted?
• 哪个线程调用OnCompleted?
• 我在不同的示例中看到了直接调用 OnCompleted 的 continuation 参数和使用 Task.Run(continuation) 的示例，我应该选择哪个，为什么?

• When is IsCompleted checked? Where should I set it?
• When is OnCompleted called?
• Which thread calls OnCompleted?
• 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 why?

推荐答案

为什么需要自定义等待器?

Why would you want a custom awaiter?

可以看到编译器对await的解释此处.本质上:

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();

从评论中 OnCompleted 应该将其参数安排为异步操作的延续.

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

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