異步使用ASP.NET MVC內的WebClient？

Question

我有一個ASP.NET MVC應用程序，當前使用WebClient類從控制器操作中對外部Web服務進行簡單調用。

目前我正在使用同步運行的DownloadString方法。我遇到了外部Web服務無響應的問題，這導致我的整個ASP.NET應用程序處於線程飢餓和無響應狀態。

解決此問題的最佳方法是什麼？有一個DownloadStringAsync方法，但我不確定如何從控制器調用它。我需要使用AsyncController類嗎？如果是這樣，AsyncController和DownloadStringAsync方法如何交互？

感謝您的幫助。

Answer 1

我認爲使用AsyncControllers將幫助你在這裏，因爲他們卸載請求線程的處理。

我會用這樣的事情（使用作爲this article描述的事件模式）：

public class MyAsyncController : AsyncController 
{ 
    // The async framework will call this first when it matches the route 
    public void MyAction() 
    { 
     // Set a default value for our result param 
     // (will be passed to the MyActionCompleted method below) 
     AsyncManager.Parameters["webClientResult"] = "error"; 
     // Indicate that we're performing an operation we want to offload 
     AsyncManager.OutstandingOperations.Increment(); 

     var client = new WebClient(); 
     client.DownloadStringCompleted += (s, e) => 
     { 
      if (!e.Cancelled && e.Error == null) 
      { 
       // We were successful, set the result 
       AsyncManager.Parameters["webClientResult"] = e.Result; 
      } 
      // Indicate that we've completed the offloaded operation 
      AsyncManager.OutstandingOperations.Decrement(); 
     }; 
     // Actually start the download 
     client.DownloadStringAsync(new Uri("http://www.apple.com")); 
    } 

    // This will be called when the outstanding operation(s) have completed 
    public ActionResult MyActionCompleted(string webClientResult) 
    { 
     ViewData["result"] = webClientResult; 
     return View(); 
    } 
} 

並確保你安裝你需要的任何途徑，例如（在Global.asax.cs中）：

public class MvcApplication : System.Web.HttpApplication 
{ 
    public static void RegisterRoutes(RouteCollection routes) 
    { 
     routes.IgnoreRoute("{resource}.axd/{*pathInfo}"); 

     routes.MapAsyncRoute(
      "Default", 
      "{controller}/{action}/{id}", 
      new { controller = "Home", action = "Index", id = "" } 
     ); 
    } 
} 

Answer 2

的DownloadStringAsync方法使用的事件模型，提高DownloadStringCompleted當它已經完成。您也可以通過撥打WebClient.CancelAsync()來停止請求。這將使您的主要請求線程和您的WebClient線程並行運行，並允許您確定您的主線程在返回之前等待多久。

在下面的例子中，我們開始下載並設置我們要在完成時調用的事件處理程序。 DownloadStringAsync立即返回，所以我們可以繼續處理剩餘的請求。

爲了證明在這個操作的更精細的控制，當我們達到我們的控制器動作結束後，我們可以檢查，看是否下載完成尚未;如果沒有，再給它3秒鐘然後中止。

string downloadString = null; 

ActionResult MyAction() 
{ 
    //get the download location 
    WebClient client = StartDownload(uri); 
    //do other stuff 
    CheckAndFinalizeDownload(client); 
    client.Dispose(); 
} 

WebClient StartDownload(Uri uri) 
{ 
    WebClient client = new WebClient(); 
    client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(Download_Completed); 
    client.DownloadStringAsync(uri); 
    return client; 
} 

void CheckAndFinalizeDownload(WebClient client) 
{ 
    if(this.downloadString == null) 
    { 
     Thread.Sleep(3000); 
    } 
    if(this.downloadString == null) 
    { 
     client.CancelAsync(); 
     this.downloadString = string.Empty; 
    } 
} 

void Download_Completed(object sender, DownloadStringCompletedEventArgs e) 
{ 
    if(!e.Cancelled && e.Error == null) 
    { 
     this.downloadString = (string)e.Result; 
    } 
}