使用 CancellationToken 中断嵌套任务

Question

这是我的场景：用户单击一个 WPF 按钮，该按钮启动一个开放式时间段，用于在地图上收集点。当用户点击“收集完毕”按钮时，我希望CollectPoints()任务完成。

这是我的 SegmentRecorder 课程的片段：

    private CancellationTokenSource _cancellationToken;     

    public virtual async void RecordSegment(IRoadSegment segment)
    {
        _cancellationToken = new CancellationTokenSource();
        var token = _cancellationToken.Token;

        // await user clicking points on map
        await CollectPoints(token);

        // update the current segment with the collected shape.
        CurrentSegment.Shape = CurrentGeometry as Polyline;
    }

    // collect an arbitrary number of points and build a polyline.
    private async Task CollectPoints(CancellationToken token)
    {
        var points = new List<MapPoint>();
        while (!token.IsCancellationRequested)
        {
            // wait for a new point.
            var point = await CollectPoint();
            points.Add(point);

            // add point to current polyline
            var polylineBuilder = new PolylineBuilder(points, SpatialReferences.Wgs84);
            CurrentGeometry = polylineBuilder.ToGeometry();

            // draw points
            MapService.DrawPoints(CurrentGeometry);
        }
    }

    // collect a point from map click.
    protected override Task<MapPoint> CollectPoint()
    {
        var tcs = new TaskCompletionSource<MapPoint>();
        EventHandler<IMapClickedEventArgs> handler = null;
        handler = (s, e) =>
        {
            var mapPoint = e.Geometry as MapPoint;
            if (mapPoint != null)
            {
                tcs.SetResult(new MapPoint(mapPoint.X, mapPoint.Y, SpatialReferences.Wgs84));
            }
            MapService.OnMapClicked -= handler;
        };
        MapService.OnMapClicked += handler;

        return tcs.Task;
    }

    public void StopSegment(){
        // interrupt the CollectPoints task.
        _cancellationToken.Cancel();
    }

以下是我的视图模型的相关部分：

public SegmentRecorder SegmentRecorder { get; }
public RelayCommand StopSegment { get; }

public ViewModel(){
    StopSegment = new RelayCommand(ExecuteStopSegment);
    SegmentRecorder = new SegmentRecorder();
}

// execute on cancel button click.
public void ExecuteStopSegment(){
    SegmentRecorder.StopSegment();
}

当我在while (!token.IsCancellationRequested) 行上设置断点并单击取消按钮时，我永远不会到达那个点。

我在这里以正确的方式使用取消令牌吗？

Answer 1

在您调用CancellationTokenSource 的Cancel() 方法后，只要它第一次满足while 条件!token.IsCancellationRequested，CollectPoints 方法就会返回。

当while 循环内的代码仍在执行时，任务不会被取消。

正如@JSteward 在他的评论中所建议的，您应该在您的StopSegment() 方法中取消或完成TaskCompletionSource。

类似这样的：

public virtual async void RecordSegment(IRoadSegment segment)
{
    _cancellationToken = new CancellationTokenSource();
    var token = _cancellationToken.Token;

    // await user clicking points on map
    await CollectPoints(token);

    // update the current segment with the collected shape.
    CurrentSegment.Shape = CurrentGeometry as Polyline;
}

// collect an arbitrary number of points and build a polyline.
private async Task CollectPoints(CancellationToken token)
{
    var points = new List<MapPoint>();
    while (!token.IsCancellationRequested)
    {
        try
        {
            // wait for a new point.
            var point = await CollectPoint(token);

            //...
        }
        catch (Exception) { }
    }
}

private TaskCompletionSource<MapPoint> tcs;
protected override Task<MapPoint> CollectPoint()
{
    tcs = new TaskCompletionSource<MapPoint>();
    //...
    return tcs.Task;
}

public void StopSegment()
{
    // interrupt the CollectPoints task.
    _cancellationToken.Cancel();
    tcs.SetCanceled();
}