基于任务异步编程模式（TAP）

前言

TAP是基于Task的异步编程设计模式,

异步编程

async : async关键字修饰的方法都会被视为异步方法，编译器会为方法生成一个对应的状态机
await : await关键字后面代表可等待方法，编译器会转化成TaskAwait去处理异步

async修饰符代表了这个方法是异步方法，编译器会生成状态机。

//
public async Task MethodAsync()
{
    //do some thing

    //可等待方法，clr会挂起，等异步方法执行完才会执行剩下的逻辑
    await Method1Async();

    //do some thing
}

异步取消

采用CancellationToken来管理整个取消逻辑

CancellationTokenSource source = new CancellationTokenSource();
CancellationToken token = source.Token;

var result await MethodAsync(token);

//外部取消
source.Cancel();

public async Task MethodAsync(CancellationToken token)
{
    //内部取消
    source.Cancel();

}

TAP转换APM

采用TaskCompletionSource

public static Task<int> ReadTask(this Stream stream, byte[] buffer, int offset, int count, object state)
{
    var tcs = new TaskCompletionSource<int>();
    stream.BeginRead(buffer, offset, count, ar =>
    {
        try 
        { 
            tcs.SetResult(stream.EndRead(ar)); 
        }
        catch (Exception exc) 
        { 
            tcs.SetException(exc); 
        }
    }, state);
    return tcs.Task;
}

官方封装了工具类TaskToApm

//Task转APM密封类
internal sealed class TaskAsyncResult:IAsyncResult{...}

//Task转成APM对应的Begin方法
public static IAsyncResult Begin(Task task, AsyncCallback? callback, object? state) =>
            new TaskAsyncResult(task, state, callback);

public static TResult End<TResult>(IAsyncResult asyncResult)
{
    if (GetTask(asyncResult) is Task<TResult> task)
      {
          return task.GetAwaiter().GetResult();
      }

      ThrowArgumentException(asyncResult);
      return default!; // unreachable
}

//转换后的Begin方法使用
public void BeginMethod(AsyncCallback callback,object? )
{
    TaskToApm.Begin(MethodAsync(),callback,null);
}