异步回调函数如何等待它完成并在调用时获取结果

Question

我有两个功能

public void sendGETRequest()
        {
            System.Uri targetUri = new System.Uri(urlAdress);
            HttpWebRequest request = (HttpWebRequest)HttpWebRequest.Create(targetUri);
            request.Method = "GET";
            request.Headers["X_USERNAME"] = username;
            request.Headers["X_PASSWORD"] = password;
            request.BeginGetResponse(new AsyncCallback(ReadWebRequestCallback), request);
        }

        private void ReadWebRequestCallback(IAsyncResult callbackResult)
        {           
            HttpWebRequest myRequest = (HttpWebRequest)callbackResult.AsyncState;
            HttpWebResponse myResponse = (HttpWebResponse)myRequest.EndGetResponse(callbackResult);

            using (StreamReader httpwebStreamReader = new StreamReader(myResponse.GetResponseStream()))
            {
                result = httpwebStreamReader.ReadToEnd();                
            }
            myResponse.Close();
            if (Completed != null)
                Completed();
        }
    }

当我打电话时

 GET user;   
    public String GETUserList()
    {
      user  = new GET();
      user.urlAdress = url + "user";
       user.username = username;
       user.password = password;

       user.sendGETRequest();

       user.Completed += OutUser;

       if (Completed != null)
           Completed();


       return user.result;
    }


void OutUser()
            {
               Debug.WriteLine(user.result);
            }

如果我这样调用 1 个操作来获取用户列表

String json = GETUserList();
            Debug.WriteLine("Hllllllllllll" + json + "HHHHHHHHHHHHHH" + p.resultWords);

它返回

HllllllllllllHHHHHHHHHHHHHH
[{"id":"3","username":"a","password":"a"},{"id":"4","username":"bbbb\n","password":"aaa"},{,{"id":"13","username":"abcdef","password":"abcdef"},{"id{"id":"10","word":"AAAAddddA","definition":"aaa","cardset_id":"2"},{"id":"11","word":"AAAAddddA","definition":"aaa","cardset_id":"2"},{"id":"12","word":"asdas","definition":"asdasd","cardset_id":"1"},{"id":"13","word":"asdas","definition":"asdasd","cardset_id":"1"},{"id":"14","word":"AAAAddddA","definition":"aaa","cardset_id":"2"},{"id":"15","word":"AAAAddddA","definition":"aaa","cardset_id":"2"},{"id":"16","word":"asdas","definition":"asdasd","

我认为函数 ReadWebRequestCallback(IAsyncResult callbackResult) 是在后台运行的，当 I Call to get list of User 之后它会执行（我已经调试过。 如何让它在动作完成之前运行？ 感谢提前！

Answer 1

这是它的工作原理：

BeginGetResponse -> makes an asynchronous web request
asynchronous request completed -> makes a callback: ReadWebRequestCallback
ReadWebRequestCallback -> read the response

因此，您似乎希望在到达GETUserList() 中的return 语句之前等待ReadWebRequestCallback...然后不要发出异步请求。做任何其他事情都会有效地破坏发出异步请求的目的。

您可以使用手动重置事件：

public class GETUser
{
    public ManualResetEvent mrse = new ManualResetEvent(false);
    // other fields 
}

在

public String GETUserList()
{
    user  = new GET();
    user.urlAdress = url + "user";
    user.username = username;
    user.password = password;

    user.sendGETRequest();

    user.Completed += OutUser;

    if (Completed != null)
       Completed();

    user.mrse.WaitOne(); // <--- block until the event is raised
    return user.result;
}

在回调中：

private void ReadWebRequestCallback(IAsyncResult callbackResult)
{           
    HttpWebRequest myRequest = (HttpWebRequest)callbackResult.AsyncState;
    HttpWebResponse myResponse = (HttpWebResponse)myRequest.EndGetResponse(callbackResult);

    using (StreamReader httpwebStreamReader = new StreamReader(myResponse.GetResponseStream()))
    {
        result = httpwebStreamReader.ReadToEnd();                
    }
    myResponse.Close();
    if (Completed != null)
        Completed();

    mrse.Signal();// <-- signal that you have finished reading
}

同样，这违背了发出异步 Web 请求的目的。