通過Live SDK使用異步/等待

Question

從我所看到的有關使用具有事件異步模式的Async CTP中看到的代碼我應該可以正常工作，var result1 = await tcs1.Task會阻止，直到clientGetFileList.GetCompleted被觸發。然而，最終發生的是我得到彈回到GetRestoreStream，return GetRestoreStreamAwait().Result永遠不會返回 - 相反，我的應用程序幾乎鎖定了我。

有人可以請向我解釋我做錯了什麼？

protected override Stream GetRestoreStream() 
{ 
    if (SkyDriveFolderId != null) 
     return GetRestoreStreamAwait().Result; 

    return Stream.Null; 
} 

private async Task<Stream> GetRestoreStreamAwait() 
{ 
    LiveConnectClient clientGetFileList = new LiveConnectClient(_session); 
    TaskCompletionSource<LiveOperationCompletedEventArgs> tcs1 = new TaskCompletionSource<LiveOperationCompletedEventArgs>(); 
    EventHandler<LiveOperationCompletedEventArgs> d1 = (o, e) => { tcs1.TrySetResult(e); }; 

    clientGetFileList.GetCompleted += d1; 
    clientGetFileList.GetAsync(SkyDriveFolderId + "/files"); 
    var result1 = await tcs1.Task; 
    clientGetFileList.GetCompleted -= d1; 

    // ... method continues for a while 
} 

更新：此代碼位似乎看透一路移動，但task.Start()扔關的InvalidOperationException所以我從來沒有真正得到末尾的流。將它封裝在try/catch中並不會改變任何內容 - 如果沒有try/catch，則InvalidOperationException將被進一步捕獲到堆棧中，而操作愉快地忽略了它的結果永遠不會被使用;有了它，task.Result凍結的東西就像上面的代碼一樣肯定。

protected override Stream GetRestoreStream() 
{ 
    if (SkyDriveFolderId != null) 
    { 
     var task = GetRestoreStreamImpl(); 
     task.Start(); 
     return task.Result; 
    } 

    return Stream.Null; 
} 

private async Task<Stream> GetRestoreStreamImpl() 
{ 
    var getResult = await GetTaskAsync(SkyDriveFolderId + "/files"); 

    List<object> data = (List<object>)getResult["data"]; 
    foreach (IDictionary<string, object> dictionary in data) 
    { 
     if (dictionary.ContainsKey("name") && (string)dictionary["name"] == BackupFileName) 
     { 
      if (dictionary.ContainsKey("id")) 
      { 
       SkyDriveFileId = (string)dictionary["id"]; 
       break; 
      } 
     } 
    } 

    if (String.IsNullOrEmpty(SkyDriveFileId)) 
    { 
     MessageBox.Show("Restore failed: could not find backup file", "Backup", MessageBoxButton.OK); 
     return Stream.Null; 
    } 

    return await DownloadTaskAsync(SkyDriveFileId + "/content"); 
} 

private Task<IDictionary<string,object>> GetTaskAsync(string path) 
{ 
    var client = new LiveConnectClient(_session); 
    var tcs = new TaskCompletionSource<IDictionary<string, object>>(); 

    client.GetCompleted += (o, e) => 
     { 
      if (e.Error != null) 
       tcs.TrySetException(e.Error); 
      else if (e.Cancelled) 
       tcs.TrySetCanceled(); 
      else 
       tcs.TrySetResult(e.Result); 
     }; 
    client.GetAsync(path); 
    return tcs.Task; 
} 

private Task<Stream> DownloadTaskAsync(string path) 
{ 
    var client = new LiveConnectClient(_session); 
    var tcs = new TaskCompletionSource<Stream>(); 

    client.DownloadCompleted += (o, e) => 
     { 
      if (e.Error != null) 
       tcs.TrySetException(e.Error); 
      else if (e.Cancelled) 
       tcs.TrySetCanceled(); 
      else 
       tcs.TrySetResult(e.Result); 
     }; 
    client.DownloadAsync(path); 
    return tcs.Task; 
} 

Answer 1

您誤解了async/await的工作方式。基本上，你的代碼在var result1和下面被阻塞。但是，等待允許的是，調用異步方法（本例中爲GetRestoreStream）的代碼在調用前面的await長時間運行的任務*後立即返回。如果你不依賴於.Result，那麼你的GetRestoreStream方法就完成了。但是，由於您需要結果，因此您的GetRestoreStream方法在等待GetRestoreStreamAwait完成時變爲同步。我會盡快添加一些視覺效果。

下面是一些示例代碼流：

-GetRestoreStream calls GetRestoreStreamAwait 
---GetRestoreStreamAwait calls an async task 
-GetRestoreStreamAwait returns to GetRestoreStream with a pending result 
-GetRestoreStream can do anything it wants, but if it calls for the pending result, it will block 
---GetRestoreStreamAwait finally finishes its async task and continues through its code, returning a result 
-Any code in GetRestoreStream that was waiting for the result receives the Result 

這是不是最好的圖形表示，希望這有助於解釋它雖然有點。需要注意的是，由於異步的本質，代碼流並不是你所習慣的

所以，我的猜測是，你的應用程序只能鎖定，因爲你試圖訪問尚不可用的Result ，你所需要做的就是等待tcs1.Task完成。如果你想避免鎖定，那麼你需要嵌套這個調用，這樣GetRestoreStream也是一個異步方法。但是，如果結果是你最終尋找的，那麼你將需要等待回報，或者簡單地設置一個回調，就像你通常會爲異步模式設置回調

*請注意，我說長時間運行的任務，因爲編譯器不會浪費時間重寫一個已經完成

UPDATE（如果它確實是由AWAIT被調用的時候完成）代碼...試試這個

protected override Stream GetRestoreStream() 
{ 
    if (SkyDriveFolderId != null) 
     return GetRestoreStreamAwait().Result; 

    return Stream.Null; 
} 

private async Task<Stream> GetRestoreStreamAwait() 
{ 

    try 
    { 
    LiveConnectClient clientGetFileList = new LiveConnectClient(_session); 
    TaskCompletionSource<LiveOperationCompletedEventArgs> tcs1 = new TaskCompletionSource<LiveOperationCompletedEventArgs>(); 
    EventHandler<LiveOperationCompletedEventArgs> d1 = 
     (o, e) => 
      { 
       try 
       { 
        tcs1.TrySetResult(e); 
       } 
       catch(Exception ex) 
       { 
        tcs1.TrySetResult(null); 
       } 
      }; 

    clientGetFileList.GetCompleted += d1; 
    clientGetFileList.GetAsync(SkyDriveFolderId + "/files"); 
    var result1 = await tcs1.Task; 
    clientGetFileList.GetCompleted -= d1; 

    // ... method continues for a while 
    } 
    catch(Exception ex) 
    { 
     return null; 
    } 
}