A pattern for self-cancelling and restarting task

Question

Is there a recommended established pattern for self-cancelling and restarting tasks?

E.g., I\'m working on the API for background spellchecker. The spellcheck sessio

Answer 1

Here's the most recent version of the cancel-and-restart pattern that I use:

class AsyncWorker
{
    Task _pendingTask;
    CancellationTokenSource _pendingTaskCts;

    // the actual worker task
    async Task DoWorkAsync(CancellationToken token)
    {
        token.ThrowIfCancellationRequested();
        Debug.WriteLine("Start.");
        await Task.Delay(100, token);
        Debug.WriteLine("Done.");
    }

    // start/restart
    public void Start(CancellationToken token)
    {
        var previousTask = _pendingTask;
        var previousTaskCts = _pendingTaskCts;

        var thisTaskCts = CancellationTokenSource.CreateLinkedTokenSource(token);

        _pendingTask = null;
        _pendingTaskCts = thisTaskCts;

        // cancel the previous task
        if (previousTask != null && !previousTask.IsCompleted)
            previousTaskCts.Cancel();

        Func runAsync = async () =>
        {
            // await the previous task (cancellation requested)
            if (previousTask != null)
                await previousTask.WaitObservingCancellationAsync();

            // if there's a newer task started with Start, this one should be cancelled
            thisTaskCts.Token.ThrowIfCancellationRequested();

            await DoWorkAsync(thisTaskCts.Token).WaitObservingCancellationAsync();
        };

        _pendingTask = Task.Factory.StartNew(
            runAsync,
            CancellationToken.None,
            TaskCreationOptions.None,
            TaskScheduler.FromCurrentSynchronizationContext()).Unwrap();
    }

    // stop
    public void Stop()
    {
        if (_pendingTask == null)
            return;

        if (_pendingTask.IsCanceled)
            return;

        if (_pendingTask.IsFaulted)
            _pendingTask.Wait(); // instantly throw an exception

        if (!_pendingTask.IsCompleted)
        {
            // still running, request cancellation 
            if (!_pendingTaskCts.IsCancellationRequested)
                _pendingTaskCts.Cancel();

            // wait for completion
            if (System.Threading.Thread.CurrentThread.GetApartmentState() == ApartmentState.MTA)
            {
                // MTA, blocking wait
                _pendingTask.WaitObservingCancellation();
            }
            else
            {
                // TODO: STA, async to sync wait bridge with DoEvents,
                // similarly to Thread.Join
            }
        }
    }
}

// useful extensions
public static class Extras
{
    // check if exception is OperationCanceledException
    public static bool IsOperationCanceledException(this Exception ex)
    {
        if (ex is OperationCanceledException)
            return true;

        var aggEx = ex as AggregateException;
        return aggEx != null && aggEx.InnerException is OperationCanceledException;
    }

    // wait asynchrnously for the task to complete and observe exceptions
    public static async Task WaitObservingCancellationAsync(this Task task)
    {
        try
        {
            await task;
        }
        catch (Exception ex)
        {
            // rethrow if anything but OperationCanceledException
            if (!ex.IsOperationCanceledException())
                throw;
        }
    }

    // wait for the task to complete and observe exceptions
    public static void WaitObservingCancellation(this Task task)
    {
        try
        {
            task.Wait();
        }
        catch (Exception ex)
        {
            // rethrow if anything but OperationCanceledException
            if (!ex.IsOperationCanceledException())
                throw;
        }
    }
}

Test use (producing only a single "Start/Done" output for DoWorkAsync):

private void MainForm_Load(object sender, EventArgs e)
{
    var worker = new AsyncWorker();
    for (var i = 0; i < 10; i++)
        worker.Start(CancellationToken.None);
}