使用异步任务取消任务

Question

我正在尝试使用 this FAQ 中所述的取消令牌。这是我最初的想法：

private async void OnLoginButtonClicked(object sender, EventArgs e)
{
    if (this.cancelToken == null)
    {
        this.cancelToken = new CancellationTokenSource();
    }

    try
    {
        bool loginSuccess = await AsyncLoginTask(this.cancelToken.Token);

        if (loginSuccess)
        {
            // Show main page
        }
    }
    catch (OperationCanceledException ex)
    {
        System.Diagnostics.Debug.WriteLine(ex.Message);
    }
    catch (Exception ex)
    {
        System.Diagnostics.Debug.WriteLine(ex.Message);
    }
    finally
    {
        this.cancelToken = null;
    }
}

private async Task<bool> AsyncLoginTask(CancellationToken cancellationToken = default(CancellationToken))
{
    // Pass the token to HttpClient()
}

现在我修改了它，结果是这样的：

private async void OnLoginButtonClicked(object sender, EventArgs e)
{
    this.cancelToken?.Dispose();
    this.cancelToken = new CancellationTokenSource();

    try
    {
        var ui = TaskScheduler.FromCurrentSynchronizationContext();
        var loginTask = Task.Factory.StartNew(async () =>
        {
            bool loginSuccess = await AsyncLoginTask(this.cancelToken.Token);
        }, this.cancelToken.Token);

        var displayResults = loginTask.ContinueWith(resultTask =>
                            {
                                // How do I know if the login was successful?
                                // Because AsyncLoginTask() returns bool.
                                System.Diagnostics.Debug.WriteLine("done");
                            },
                             CancellationToken.None,
                             TaskContinuationOptions.OnlyOnRanToCompletion,
                             ui);

        var displayCancelledTasks = loginTask.ContinueWith(resultTask =>
                                    {
                                        System.Diagnostics.Debug.WriteLine("canceled");
                                    },
                                   CancellationToken.None,
                                   TaskContinuationOptions.OnlyOnCanceled, ui);
    }
    catch (Exception ex)
    {
        System.Diagnostics.Debug.WriteLine(ex.Message);
    }
}

问题：

如何知道登录是否成功？因为AsyncLoginTask()returnsbool.
如何正确创建和销毁令牌以允许多次启动和取消操作？
任务中的任务怎么处理？ "done" 显示在控制台中，而任务 (AsyncLoginTask) 尚未完成。

Answer 1

I'm trying to make use of cancellation tokens as described in this FAQ.

该博客 post 使用动态任务并行（StartNew 和 ContinueWith）。动态任务并行是指当您有很多 CPU 绑定操作要做，并且在您已经处理它们之前不知道您有多少（即，您处理的每个操作都可以添加零个或多个额外的任务到同一进程）。

在您的例子中，您只有一个异步操作。因此，该文章中的方法对于您的用例来说是完全错误的。你原来的想法更正确。

你更想这样做：

private async void OnLoginButtonClicked(object sender, EventArgs e)
{
  // Cancel the previous attempt (if any) and start a new one.
  this.cts?.Cancel();
  this.cts = new CancellationTokenSource();

  try
  {
    bool loginSuccess = await AsyncLoginTask(this.cts.Token);
    // Resolve race condition where user cancels just as it completed.
    this.cts.Token.ThrowIfCancellationRequested();
    if (loginSuccess)
    {
      // Show main page
    }
  }
  catch (OperationCanceledException ex)
  {
    System.Diagnostics.Debug.WriteLine(ex.Message);
  }
  catch (Exception ex)
  {
    System.Diagnostics.Debug.WriteLine(ex.Message);
  }
}

private async Task<bool> AsyncLoginTask(CancellationToken cancellationToken = default(CancellationToken))
{
  // Pass the token to HttpClient()
}

使用异步任务取消任务

Task cancellation with async task

c#

asynchronous

task

cancellation

async-await