通过Live SDK使用异步/等待

Question

从我所看到的有关使用具有事件异步模式的Async CTP中看到的代码我应该可以正常工作，var result1 = await tcs1.Task会阻止，直到clientGetFileList.GetCompleted被触发。然而，最终发生的是我得到弹回到GetRestoreStream，return GetRestoreStreamAwait().Result永远不会返回 - 相反，我的应用程序几乎锁定了我。

有人可以请向我解释我做错了什么？

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：此代码位似乎看透一路移动，但task.Start()扔关的InvalidOperationException所以我从来没有真正得到末尾的流。将它封装在try/catch中并不会改变任何内容 - 如果没有try/catch，则InvalidOperationException将被进一步捕获到堆栈中，而操作愉快地忽略了它的结果永远不会被使用;有了它，task.Result冻结的东西就像上面的代码一样肯定。

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

您误解了async/await的工作方式。基本上，你的代码在var result1和下面被阻塞。但是，等待允许的是，调用异步方法（本例中为GetRestoreStream）的代码在调用前面的await长时间运行的任务*后立即返回。如果你不依赖于.Result，那么你的GetRestoreStream方法就完成了。但是，由于您需要结果，因此您的GetRestoreStream方法在等待GetRestoreStreamAwait完成时变为同步。我会尽快添加一些视觉效果。

下面是一些示例代码流：

-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 

这是不是最好的图形表示，希望这有助于解释它虽然有点。需要注意的是，由于异步的本质，代码流并不是你所习惯的

所以，我的猜测是，你的应用程序只能锁定，因为你试图访问尚不可用的Result ，你所需要做的就是等待tcs1.Task完成。如果你想避免锁定，那么你需要嵌套这个调用，这样GetRestoreStream也是一个异步方法。但是，如果结果是你最终寻找的，那么你将需要等待回报，或者简单地设置一个回调，就像你通常会为异步模式设置回调

*请注意，我说长时间运行的任务，因为编译器不会浪费时间重写一个已经完成

UPDATE（如果它确实是由AWAIT被调用的时候完成）代码...试试这个

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; 
    } 
}