如何在等待中取消任务？

Question

我正在玩这些Windows 8 WinRT任务,我正在尝试使用下面的方法取消任务,并且它在某种程度上起作用.CancelNotification方法被调用,这使您认为任务已被取消,但在后台任务继续运行,然后在完成后,任务的状态始终完成且永不取消.有没有办法在取消任务时完全停止？

private async void TryTask()
{
    CancellationTokenSource source = new CancellationTokenSource();
    source.Token.Register(CancelNotification);
    source.CancelAfter(TimeSpan.FromSeconds(1));
    var task = Task<int>.Factory.StartNew(() => slowFunc(1, 2), source.Token);

    await task;            

    if (task.IsCompleted)
    {
        MessageDialog md = new MessageDialog(task.Result.ToString());
        await md.ShowAsync();
    }
    else
    {
        MessageDialog md = new MessageDialog("Uncompleted");
        await md.ShowAsync();
    }
}

private int slowFunc(int a, int b)
{
    string someString = string.Empty;
    for (int i = 0; i < 200000; i++)
    {
        someString += "a";
    }

    return a + b;
}

private void CancelNotification()
{
}

Answer 1

阅读上取消(这是介绍在.NET 4.0中,是基本不变从那时起)和基于任务的异步模式,它提供了关于如何使用的指导方针CancellationToken与async方法.

总而言之,您将CancellationToken每个支持取消的方法传递给该方法,并且该方法必须定期检查它.

private async Task TryTask()
{
  CancellationTokenSource source = new CancellationTokenSource();
  source.CancelAfter(TimeSpan.FromSeconds(1));
  Task<int> task = Task.Run(() => slowFunc(1, 2, source.Token), source.Token);

  // (A canceled task will raise an exception when awaited).
  await task;
}

private int slowFunc(int a, int b, CancellationToken cancellationToken)
{
  string someString = string.Empty;
  for (int i = 0; i < 200000; i++)
  {
    someString += "a";
    if (i % 1000 == 0)
      cancellationToken.ThrowIfCancellationRequested();
  }

  return a + b;
}

Answer 2

或者,为了避免修改slowFunc(例如,您没有访问源代码):

var source = new CancellationTokenSource(); //original code
source.Token.Register(CancelNotification); //original code
source.CancelAfter(TimeSpan.FromSeconds(1)); //original code
var completionSource = new TaskCompletionSource<object>(); //New code
source.Token.Register(() => completionSource.TrySetCanceled()); //New code
var task = Task<int>.Factory.StartNew(() => slowFunc(1, 2), source.Token); //original code

//original code: await task;  
await Task.WhenAny(task, completionSource.Task); //New code

您还可以使用https://github.com/StephenCleary/AsyncEx中的良好扩展方法,并使其看起来如下所示:

await Task.WhenAny(task, source.Token.AsTask());

Answer 3

一个未涉及的案例是如何处理异步方法内部的取消.举一个简单的例子,你需要将一些数据上传到服务,让它计算一些东西,然后返回一些结果.

public async Task<Results> ProcessDataAsync(MyData data)
{
    var client = await GetClientAsync();
    await client.UploadDataAsync(data);
    await client.CalculateAsync();
    return await client.GetResultsAsync();
}

如果你想支持取消,那么最简单的方法是传入一个令牌并检查它是否在每次异步方法调用(或使用ContinueWith)之间被取消.如果他们是非常长时间的通话,你可能会等待一段时间取消.我创建了一个小帮手方法,一旦取消就失败了.

public static class TaskExtensions
{
    public static async Task<T> WaitOrCancel<T>(this Task<T> task, CancellationToken token)
    {
        token.ThrowIfCancellationRequested();
        await Task.WhenAny(task, token.WhenCanceled());
        token.ThrowIfCancellationRequested();

        return await task;
    }

    public static Task WhenCanceled(this CancellationToken cancellationToken)
    {
        var tcs = new TaskCompletionSource<bool>();
        cancellationToken.Register(s => ((TaskCompletionSource<bool>)s).SetResult(true), tcs);
        return tcs.Task;
    }
}

所以要使用它然后只需添加.WaitOrCancel(token)到任何异步调用:

public async Task<Results> ProcessDataAsync(MyData data, CancellationToken token)
{
    Client client;
    try
    {
        client = await GetClientAsync().WaitOrCancel(token);
        await client.UploadDataAsync(data).WaitOrCancel(token);
        await client.CalculateAsync().WaitOrCancel(token);
        return await client.GetResultsAsync().WaitOrCancel(token);
    }
    catch (OperationCanceledException)
    {
        if (client != null)
            await client.CancelAsync();
        throw;
    }
}

Answer 4

我只想补充已经接受的答案.我被困在这上面,但我在处理完整的事件时走的是另一条路.我没有运行await,而是为任务添加了一个完整的处理程序.

Comments.AsAsyncAction().Completed += new AsyncActionCompletedHandler(CommentLoadComplete);

事件处理程序看起来像这样

private void CommentLoadComplete(IAsyncAction sender, AsyncStatus status )
{
    if (status == AsyncStatus.Canceled)
    {
        return;
    }
    CommentsItemsControl.ItemsSource = Comments.Result;
    CommentScrollViewer.ScrollToVerticalOffset(0);
    CommentScrollViewer.Visibility = Visibility.Visible;
    CommentProgressRing.Visibility = Visibility.Collapsed;
}

使用此路由,所有处理都已为您完成,当任务被取消时,它只会触发事件处理程序,您可以看到它是否在那里被取消.