Asyncallback功能如何等待它完成，并得到结果时调用

Question

我有2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

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); 
      } 

如果我叫1个ACTION像这样得到的用户列表

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"," 

我觉得功能ReadWebRequestCallback（IAsyncResult的callbackResult）在后台运行，当我打电话让用户的列表（我已经调试后执行。 如何在动作完成前运行它？ 感谢您的提前！

Answer 1

这里是它如何工作的：

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

如此看来，要等待ReadWebRequestCallback你GETUserList()到达return语句之前......那么不发出异步请求。做其他事情有效地破坏了发布异步请求的目的。

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; 
} 

而且回调：

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，这违背了使异步Web的目的请求。