在WebService内按时间调用IHttpAsyncHandler

Question

我试图在我的IHttpAsyncHandler中调用一个IHttpAsyncHandler，我发现有一个类似于使用IHttpAsyncHandler异步调用WebService的答案

我对答案有疑问。如果有人能帮我我很感激。

它有

Task webClientDownloadTask = webClientDownloadCompletionSource.Task;

我的问题是

1. webClientDownloadCompletionSource与Webclient (client)对象无关，那么这样做有什么意义： //获取TCS的任务，以便我们可以附加一些延续任务webClientDownloadTask = webClientDownloadCompletionSource.Task；
2. 这里的"taskCompletionSource“是什么： //向已完成的TCS发送信号(我们实际上不查看bool结果，但它是必需的)taskCompletionSource.SetResult(真)；
3. 为什么我在ContinueWith()回调中释放()ContinueWith()，为什么不在我们设置taskCompletionSource.SetResult(True)之后直接释放() WebClient呢？ //总是在工作完成后释放客户机(_ TaskContinuationOptions.ExecuteSynchronously)；{ client.Dispose()；} )

以下是完整的代码：

    public class MyAsyncHandler : IHttpAsyncHandler
{
    public IAsyncResult BeginProcessRequest(HttpContext context, AsyncCallback cb, object extraData)
    {
        // NOTE: the result of this operation is void, but TCS requires some data type so we just use bool
        TaskCompletionSource<bool> webClientDownloadCompletionSource = new TaskCompletionSource<bool>();

        WebClient webClient = new WebClient())
        HttpContext currentHttpContext = HttpContext.Current;

        // Setup the download completed event handler
        client.DownloadDataCompleted += (o, e) =>
        {
            if(e.Cancelled)
            {
                // If it was canceled, signal the TCS is cacnceled
                // NOTE: probably don't need this since you have nothing canceling the operation anyway
                webClientDownloadCompletionSource.SetCanceled();
            }
            else if(e.Error != null)
            {
                // If there was an exception, signal the TCS with the exception
                webClientDownloadCompletionSource.SetException(e.Error);
            }
            else
            {
                // Success, write the response
                currentHttpContext.Response.ContentType = "text/xml";
                currentHttpContext.Response.OutputStream.Write(e.Result, 0, e.Result.Length);

                // Signal the TCS that were done (we don't actually look at the bool result, but it's needed)
                taskCompletionSource.SetResult(true);
            }
        };

        string url = "url_web_service_url";

        // Kick off the download immediately
        client.DownloadDataAsync(new Uri(url));

        // Get the TCS's task so that we can append some continuations
        Task webClientDownloadTask = webClientDownloadCompletionSource.Task;

        // Always dispose of the client once the work is completed
        webClientDownloadTask.ContinueWith(
            _ =>
            {
                client.Dispose();
            },
            TaskContinuationOptions.ExecuteSynchronously);

        // If there was a callback passed in, we need to invoke it after the download work has completed
        if(cb != null)
        {
            webClientDownloadTask.ContinueWith(
               webClientDownloadAntecedent =>
               {
                   cb(webClientDownloadAntecedent);
               },
               TaskContinuationOptions.ExecuteSynchronously);
         }

        // Return the TCS's Task as the IAsyncResult
        return webClientDownloadTask;
    }

    public void EndProcessRequest(IAsyncResult result)
    {
        // Unwrap the task and wait on it which will propagate any exceptions that might have occurred
        ((Task)result).Wait();
    }

    public bool IsReusable
    {
        get 
        { 
            return true; // why not return true here? you have no state, it's easily reusable!
        }
    }

    public void ProcessRequest(HttpContext context)
    {
    }
}

Answer 1

看看这个，看看它是用来干什么的。最后还有一个例子。

TaskCompletionSource

在许多情况下，启用任务来表示外部异步操作非常有用。TaskCompletionSource{TResult}就是为此目的而提供的。它允许创建一个可以分发给消费者的任务，这些使用者可以像其他人一样使用任务的成员。但是，与大多数任务不同，TaskCompletionSource创建的任务的状态由TaskCompletionSource上的方法显式控制。这使得外部异步操作的完成能够传播到基础任务。这种分离还确保了在不访问相应的TaskCompletionSource的情况下，使用者无法转换状态。