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 session is wrapped as Task. Every new session should cancel the previous one and wait for its termination (to properly re-use the resources like spellcheck service provider, etc).

I've come up with something like this:

class Spellchecker
{
    Task pendingTask = null; // pending session
    CancellationTokenSource cts = null; // CTS for pending session

    // SpellcheckAsync is called by the client app
    public async Task<bool> SpellcheckAsync(CancellationToken token)
    {
        // SpellcheckAsync can be re-entered
        var previousCts = this.cts;
        var newCts = CancellationTokenSource.CreateLinkedTokenSource(token);
        this.cts = newCts;

        if (IsPendingSession())
        {
            // cancel the previous session and wait for its termination
            if (!previousCts.IsCancellationRequested)
                previousCts.Cancel();
            // this is not expected to throw
            // as the task is wrapped with ContinueWith
            await this.pendingTask; 
        }

        newCts.Token.ThrowIfCancellationRequested();
        var newTask = SpellcheckAsyncHelper(newCts.Token);

        this.pendingTask = newTask.ContinueWith((t) => {
            this.pendingTask = null;
            // we don't need to know the result here, just log the status
            Debug.Print(((object)t.Exception ?? (object)t.Status).ToString());
        }, TaskContinuationOptions.ExecuteSynchronously);

        return await newTask;
    }

    // the actual task logic
    async Task<bool> SpellcheckAsyncHelper(CancellationToken token)
    {
        // do not start a new session if the the previous one still pending
        if (IsPendingSession())
            throw new ApplicationException("Cancel the previous session first.");

        // do the work (pretty much IO-bound)
        try
        {
            bool doMore = true;
            while (doMore)
            {
                token.ThrowIfCancellationRequested();
                await Task.Delay(500); // placeholder to call the provider
            }
            return doMore;
        }
        finally
        {
            // clean-up the resources
        }
    }

    public bool IsPendingSession()
    {
        return this.pendingTask != null &&
            !this.pendingTask.IsCompleted &&
            !this.pendingTask.IsCanceled &&
            !this.pendingTask.IsFaulted;
    }
}

The client app (the UI) should just be able to call SpellcheckAsync as many times as desired, without worrying about cancelling a pending session. The main doMore loop runs on the UI thread (as it involves the UI, while all spellcheck service provider calls are IO-bound).

I feel a bit uncomfortable about the fact that I had to split the API into two peices, SpellcheckAsync and SpellcheckAsyncHelper, but I can't think of a better way of doing this, and it's yet to be tested.

Answer 1

I think the general concept is pretty good, though I recommend you not use ContinueWith.

I'd just write it using regular await, and a lot of the "am I already running" logic is not necessary:

Task pendingTask = null; // pending session
CancellationTokenSource cts = null; // CTS for pending session

// SpellcheckAsync is called by the client app on the UI thread
public async Task<bool> SpellcheckAsync(CancellationToken token)
{
    // SpellcheckAsync can be re-entered
    var previousCts = this.cts;
    var newCts = CancellationTokenSource.CreateLinkedTokenSource(token);
    this.cts = newCts;

    if (previousCts != null)
    {
        // cancel the previous session and wait for its termination
        previousCts.Cancel();
        try { await this.pendingTask; } catch { }
    }

    newCts.Token.ThrowIfCancellationRequested();
    this.pendingTask = SpellcheckAsyncHelper(newCts.Token);
    return await this.pendingTask;
}

// the actual task logic
async Task<bool> SpellcheckAsyncHelper(CancellationToken token)
{
    // do the work (pretty much IO-bound)
    using (...)
    {
        bool doMore = true;
        while (doMore)
        {
            token.ThrowIfCancellationRequested();
            await Task.Delay(500); // placeholder to call the provider
        }
        return doMore;
    }
}

Answer 2

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<Task> 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);
}