我實際上如何從方法外部獲取異步Web請求的響應？

Question

我有點困惑。 我正在嘗試以異步方式發布到我的Web服務，理想情況下，我想啟動請求，在UI上顯示一個加載微調器，然后在異步請求完成后處理響應，如果有一個，則顯示錯誤，或對結果進行其他操作。

這是我的代碼，我在這里調用請求並傳遞一些數據。

private void SignInExecute()
{

        if (Username == null || Password == null)
        {
            LoginOutput = "Please provide a username or password.";
        }
        else
        {
            this.webService.SendLoginRequest("http://localhost:3000/client_sessions", "username=" + Username + "&password=" + Password);

        }

}

這是實際的Web請求代碼：

public void SendLoginRequest(string url, string postdata)
{
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);

        request.Method = "POST";

        request.ContentType = "application/x-www-form-urlencoded";

        request.Accept = "application/json";

        byte[] byteArray = Encoding.UTF8.GetBytes(postdata);

        request.CookieContainer = new CookieContainer();

        request.ContentLength = byteArray.Length;

        Stream dataStream = request.GetRequestStream();

        dataStream.Write(byteArray, 0, byteArray.Length);

        dataStream.Close();

        ((HttpWebRequest)request).KeepAlive = false;

        request.BeginGetResponse(new AsyncCallback(GetLoginResponseCallback), request);


    }

    private static void GetLoginResponseCallback(IAsyncResult asynchronousResult)
    {
        HttpWebRequest request = (HttpWebRequest)asynchronousResult.AsyncState;

        // End the operation
        HttpWebResponse response =  (HttpWebResponse)request.EndGetResponse(asynchronousResult);
        Stream streamResponse = response.GetResponseStream();
        StreamReader streamRead = new StreamReader(streamResponse);
        string responseString = streamRead.ReadToEnd();

        Console.WriteLine(responseString);

        // Close the stream object
        streamResponse.Close();
        streamRead.Close();
        response.Close();
    }

綜上所述。 我希望能夠將響應返回到最初發出調用以啟動Web請求的對象。 有什么幫助嗎？

Answer 1

您需要告訴BeginGetResponse返回到通過SynchronizationContext.Current調用的上下文。 這樣的事情（代碼沒有正確的錯誤檢查，因此您應該考慮正確）（此外，Platinum Azure是正確的，您應該使用using來讓您的流正確關閉（並保證）：

在您的SendLoginRequest中：

//Box your object state with the current thread context
object[] boxedItems = new []{request, SynchronizationContext.Current};
request.BeginGetResponse(new AsyncCallback(GetLoginResponseCallback), 
    boxedItems);

getresponse代碼：

private static void GetLoginResponseCallback(IAsyncResult asynchronousResult)
{
//MY UPDATE 
    //Unbox your object state with the current thread context
    object[] boxedItems = asynchronousResult.AsyncState as object[];
    HttpWebRequest request = boxedItems[0] as HttpWebRequest;
    SynchronizationContext context = boxedItems[1] as SynchronizationContext;

    // End the operation
    using(HttpWebResponse response =  
        (HttpWebResponse)request.EndGetResponse(asynchronousResult))
    {
        using(Stream streamResponse = response.GetResponseStream())
        {
            using(StreamReader streamRead = new StreamReader(streamResponse))
            {
                string responseString = streamRead.ReadToEnd();

                Console.WriteLine(responseString);
//MY UPDATE
                //Make an asynchronous call back onto the main UI thread 
                //(context.Send for a synchronous call)
                //Pass responseString as your method parameter 
                //If you have more than one object, you will have to box again
                context.Post(UIMethodToCall, responseString);
            }
        }
    }
}

實施UI處理

public static void UIMethodCall(object ObjectState)
{
    String response = ObjectState as String;
    label1.Text = String.Format("Output: {0}", response);
    //Or whatever you need to do in the UI...
}

現在，我將首先進行測試。 我對Microsoft實現的事件驅動異步的理解是，響應是上下文感知的，並且知道要返回到哪個上下文。 因此，在跳到您不在同一個上下文的假設之前，請嘗試通過嘗試更新UI進行測試（如果您不在調用（UI）線程上，這將導致線程上下文異常）