Question

我正在尝试在IHttpAsyncHandler中调用WebService，我看到有这样的答案 Using an IHttpAsyncHandler to call a WebService Asynchronously

我对答案有疑问。如果有人能帮助我，我感激不尽。

它有

Task webClientDownloadTask = webClientDownloadCompletionSource.Task;

我的问题是

webClientDownloadCompletionSource与Webclient（客户端）对象无关，因此有什么意义：

//获取TCS的任务，以便我们可以添加一些延续任务webClientDownloadTask = webClientDownloadCompletionSource.Task;

什么是＆＃34; taskCompletionSource＆＃34;在这里：

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

为什么我在KeepWith（）回调中释放（）WebClient，为什么不在设置taskCompletionSource.SetResult（true）之后只处理WEbClient;？

//完成工作后，务必处理客户端 webClientDownloadTask.ContinueWith（ _ =＆gt; { client.Dispose（）; }， TaskContinuationOptions.ExecuteSynchronously）;

以下是完整代码：

    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

检查一下，了解它的用途。最后还有一个例子

TaskCompletionSource

在许多情况下，启用任务来表示非常有用外部异步操作。 TaskCompletionSource {TResult} 是为此目的而提供。它可以创建可以执行的任务分发给消费者，消费者可以使用任务与其他任务一样。但是，与大多数任务不同，由TaskCompletionSource创建的任务的状态受到控制通过TaskCompletionSource上的方法显式地显示。这使得完成要传播的外部异步操作基础任务。分离也确保消费者没有访问相应的权限就无法转换状态 TaskCompletionSource。

在IHttpAsyncHandler中调用WebService Aysnchronously

1 个答案: