如何將結果返回到Windows Phone中的UI線程？

Question

有誰知道好主意如何將結果返回給UI線程？ 我寫了這段代碼，但它會編譯錯誤，因爲它不能在異步中返回「img」。

public byte[] DownloadAsync2(Uri address) 
{ 
    byte[] img; 
    byte[] buffer = new byte[4096]; 

    var wc = new WebClient(); 

    wc.OpenReadCompleted += ((sender, e) => 
    { 
     using (MemoryStream memoryStream = new MemoryStream()) 
     { 
      int count = 0; 
      do 
      { 
       count = e.Result.Read(buffer, 0, buffer.Length); 
       memoryStream.Write(buffer, 0, count); 
      } while (count != 0); 

      Deployment.Current.Dispatcher.BeginInvoke(() => 
       { 
        if (e.Error == null) img = memoryStream.ToArray(); 
       }); 
     } 
    } 
    ); 
    wc.OpenReadAsync(address); 

    return img; //error : Use of unassigned local variable 'img' 
} 

Answer 1

你的方法更改爲：

public void DownloadAsync2(Uri address, Action<byte[]> callback, Action<Exception> exception) 
{ 
    var wc = new WebClient(); 

    wc.OpenReadCompleted += ((sender, e) => 
    { 
     using (MemoryStream memoryStream = new MemoryStream()) 
     { 
      int count = 0; 
      do 
      { 
       count = e.Result.Read(buffer, 0, buffer.Length); 
       memoryStream.Write(buffer, 0, count); 
      } while (count != 0); 

      Deployment.Current.Dispatcher.BeginInvoke(() => 
      { 
       if (e.Error == null) callback(memoryStream.ToArray()); 
       else exception(e.Error); 
      }); 
     } 
    } 
    ); 
    wc.OpenReadAsync(address); 
} 

用法：

DownloadAsync2(SomeUri, (img) => 
{ 
    // this line will be executed when image is downloaded, 
    // img - returned byte array 
}, 
(exception) => 
{ 
    // handle exception here 
}); 

或（不lambda表達式舊式代碼）：

DownloadAsync2(SomeUri, LoadCompleted, LoadFailed); 

// And define two methods for handling completed and failed events 

private void LoadCompleted(byte[] img) 
{ 
    // this line will be executed when image is downloaded, 
    // img - returned byte array 
} 

private void LoadFailed(Exception exception) 
{ 
    // handle exception here 
}