在异步无效中取消任务

Question

嗨，我有一个列表框 每当用户选择一个项目时，就会向网络发送一个请求 现在我想在用户选择该项目时取消之前的操作，然后开始新的操作。

我使用以下代码来做到这一点，我想知道这些代码是否工作正常。还是我应该尝试另一种方式？

private CancellationTokenSource ts = new CancellationTokenSource();
private async void Subf2mCore(CancellationToken ct)
{
  HtmlDocument doc = await web.LoadFromWebAsync(url);
   ...
  foreach (var node in doc)
  {
    if (!ct.IsCancellationRequested)
    {
      ....
    }
  }
}

我以这种方式运行 func

ts?.Cancel();
ts = new CancellationTokenSource();
Subf2mCore(ts.Token);

Answer 1

从技术上讲，您可以这样说，但是请注意：您触发并忘记了，让Task调用者知道是否Subf2mCore已完成，失败或取消：

private async Task Subf2mCore(CancellationToken ct)
{
  HtmlDocument doc = await web.LoadFromWebAsync(url);
   ...
  foreach (var node in doc)
  {
    // Often we cancel by throwing exception: 
    // it's easy to detect that the task is cancelled by catching this exception
    // ct.ThrowIfCancellationRequested();

    // You prefer to cancel manually: 
    // your cancellation can be silent (no exceptions) but it'll be 
    // difficult for caller to detect if task completed or not 
    // (partially completed and cancelled)
    if (!ct.IsCancellationRequested)
    {
      ....
    }
  }
}

// If we don't want to cancel 
private async Task Subf2mCore() => Subf2mCore(CancellationToken.None);

用法：不要忘记Dispose CancellationTokenSource实例：

using (CancellationTokenSource ts = new CancellationTokenSource()) {
  ...
  await Subf2mCore(ts.Token);
  ...
}

编辑：如果你想从外面取消：

private CancellationTokenSource ts = null;

...

using (CancellationTokenSource _ts = new CancellationTokenSource()) {
  // previous task (if any) cancellation
  if (null != ts)
    ts.Cancel();

  // let cancel from outside
  ts = _ts;

  try {
    ...
    await Subf2mCore(_ts.Token);
    ...
  }
  finally {
    // task completed, we can't cancel it any more
    ts = null;
  }
}