我试图理解这4个部分是如何组合在一起的：
IAwaiter
IAwaitable
IAsyncMethodBuilder
IAsyncStateMachine
我不理解IAsyncMethodBuilder等待者和状态机之间的关系。
如果我的方法构建者收到让我们说2个等待者为什么状态机在它的腹部只有一个等待它使用获取/设置结果？
我在谈论方法构建者的等待者：
var a= await MyMethod();
var b=await MyMethod();
MyMethod定义为：
async Task<T> MyMethod(){
await f1() ->don't care about thsese
await f2() -----/-----
await f3() -----/------
........
}
我将发布Dixin博客https://weblogs.asp.net/dixin/functional-csharp-asynchronous-function的两段代码：
用户编写的代码：
internal static async Task<T> Async<T>(T value)
{
T value1 = Start(value);
T result1 = await Async1(value1);
T value2 = Continuation1(result1);
T result2 = await Async2(value2);
T value3 = Continuation2(result2);
T result3 = await Async3(value3);
T result = Continuation3(result3);
return result;
}
internal static T Start<T>(T value) => value;
internal static Task<T> Async1<T>(T value) => Task.Run(() => value);
internal static T Continuation1<T>(T value) => value;
internal static Task<T> Async2<T>(T value) => Task.FromResult(value);
internal static T Continuation2<T>(T value) => value;
internal static Task<T> Async3<T>(T value) => Task.Run(() => value);
internal static T Continuation3<T>(T value) => value;
编译器生成的内容：
[CompilerGenerated]
[StructLayout(LayoutKind.Auto)]
private struct AsyncStateMachine<TResult> : IAsyncStateMachine
{
public int State;
public AsyncTaskMethodBuilder<TResult> Builder;
public TResult Value;
private TaskAwaiter<TResult> awaiter; //Why only one?
void IAsyncStateMachine.MoveNext()
{
TResult result;
try
{
switch (this.State)
{
case -1: // Start code from the beginning to the 1st await.
// Workflow begins.
TResult value1 = Start(this.Value);
this.awaiter = Async1(value1).GetAwaiter();
if (this.awaiter.IsCompleted)
{
// If the task returned by Async1 is already completed, immediately execute the continuation.
goto case 0;
}
else
{
this.State = 0;
// If the task returned by Async1 is not completed, specify the continuation as its callback.
this.Builder.AwaitUnsafeOnCompleted(ref this.awaiter, ref this);
// Later when the task returned by Async1 is completed, it calls back MoveNext, where State is 0.
return;
}
case 0: // Continuation code from after the 1st await to the 2nd await.
// The task returned by Async1 is completed. The result is available immediately through GetResult.
TResult result1 = this.awaiter.GetResult();
TResult value2 = Continuation1(result1);
this.awaiter = Async2(value2).GetAwaiter();
if (this.awaiter.IsCompleted)
{
// If the task returned by Async2 is already completed, immediately execute the continuation.
goto case 1;
}
else
{
this.State = 1;
// If the task returned by Async2 is not completed, specify the continuation as its callback.
this.Builder.AwaitUnsafeOnCompleted(ref this.awaiter, ref this);
// Later when the task returned by Async2 is completed, it calls back MoveNext, where State is 1.
return;
}
case 1: // Continuation code from after the 2nd await to the 3rd await.
// The task returned by Async2 is completed. The result is available immediately through GetResult.
TResult result2 = this.awaiter.GetResult();
TResult value3 = Continuation2(result2);
this.awaiter = Async3(value3).GetAwaiter();
if (this.awaiter.IsCompleted)
{
// If the task returned by Async3 is already completed, immediately execute the continuation.
goto case 2;
}
else
{
this.State = 2;
// If the task returned by Async3 is not completed, specify the continuation as its callback.
this.Builder.AwaitUnsafeOnCompleted(ref this.awaiter, ref this);
// Later when the task returned by Async3 is completed, it calls back MoveNext, where State is 1.
return;
}
case 2: // Continuation code from after the 3rd await to the end.
// The task returned by Async3 is completed. The result is available immediately through GetResult.
TResult result3 = this.awaiter.GetResult();
result = Continuation3(result3);
this.State = -2; // -2 means end.
this.Builder.SetResult(result);
// Workflow ends.
return;
}
}
catch (Exception exception)
{
this.State = -2; // -2 means end.
this.Builder.SetException(exception);
}
}
[DebuggerHidden]
void IAsyncStateMachine.SetStateMachine(IAsyncStateMachine asyncStateMachine) =>
this.Builder.SetStateMachine(asyncStateMachine);
}
考虑到我在开头写的例子，AsyncStateMachine不应该有一个等待者列表吗？如果我在方法构建器上有N个等待者，那么机器如何将SetResult传播给它所有的等待者？ / p>
答案 0 :(得分：3)
状态机一次只能等待一次等待操作。可能涉及多个操作，但它一次只能在一个等待点。因此，如果那些等待者属于同一类型，则只需要一个字段来等待它。
如果在同一种方法中有不同类型的等待者，我相信你会看到每个等待类型的一个字段。 （编译器可能会为所有等待者使用单个object字段，并在连续触发时适当地将其强制转换，但这会带来不同的问题，特别是如果awaiter是值类型。）
以下是一个例子：
using System;
using System.Threading.Tasks;
class Test
{
static async Task FooAsync()
{
await Bar<int>();
await Bar<string>();
await Task.Delay(1000);
await Bar<string>();
await Task.Yield();
}
static Task<T> Bar<T>() => Task.FromResult(default(T));
}
这里我们最终得到状态机中的awaiter字段：
TaskAwaiter<int> <>u__1; // From the Bar<int> call
TaskAwaiter<string> <>u__2; // From both Bar<string> calls
TaskAwaiter <>u__3; // From Task.Delay
YieldAwaitable.YieldAwaiter <>u__4; // From Task.Yield