c# - 使用 CancellationToken 中断嵌套任务

Question

这是我的场景:用户单击 WPF 按钮，该按钮启动 map 上点集合的开放时间段。当用户单击“完成收集”按钮时，我希望 CollectPoints() 任务完成。

这是我的 SegmentRecorder 类的片段:

    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 View 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) 行上放置一个断点并单击取消按钮时，我永远不会到达那个点。

我在这里使用取消 token 的方式是否正确？

Answer 1

CollectPoints 方法将在您调用 后第一次遇到 while 条件 !token.IsCancellationRequested 时返回CancellationTokenSource 的 Cancel() 方法。

while 循环中的代码仍在执行时，任务不会被取消。

正如@JSteward 在他的评论中建议的那样，您应该在 StopSegment() 方法中取消或完成 TaskCompletionSource。

像这样:

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();
}