How to plug a cancellation token into a ReactiveX stream (IObservable) before publishing it?

Question

How to plug a cancellation token into an existing IObservable pipeline before calling Publish on it (i.e., before it becomes an IConnectableObservable)?

This must be a part of a cold observable pipeline, before subscribing to it (otherwise, I could pass a CancellationToken token to IObservable's Subscribe, RunAsync, ToTask etc).

Is there a recommended pattern for this?

I can think of using TakeUntil to achieve that, as suggested by Theodor Zoulias here. For example:

using System.Reactive.Linq;
using System.Reactive.Threading.Tasks;

async Task Test(CancellationToken token)
{
    var publishedSequence = Observable
        .Interval(TimeSpan.FromMilliseconds(100))
        .Do(n => Console.WriteLine($"Emitting: {n}"))
        .Skip(3)
        .TakeUntil(
            Observable.Create<long>(
                observer => token.Register(
                    (_, token) => observer.OnError(new OperationCanceledException(token)),
                    null)))
        .Finally(() => Console.WriteLine($"Finally"))
        .Publish();

    using var subscription = publishedSequence.Subscribe(
            onNext: n => Console.WriteLine($"OnNext: {n}"),
            onError: e => Console.WriteLine($"OnError: {e}"),
            onCompleted: () => Console.WriteLine("OnCompleted"));

    using var connection = publishedSequence.Connect();
    await publishedSequence.ToTask();
}

var cts = new CancellationTokenSource(1000);
await Test(cts.Token);

Ouput:

Emitting: 0
Emitting: 1
Emitting: 2
Emitting: 3
OnNext: 3
Emitting: 4
OnNext: 4
Emitting: 5
OnNext: 5
Emitting: 6
OnNext: 6
Emitting: 7
OnNext: 7
Emitting: 8
OnNext: 8
OnError: System.OperationCanceledException: The operation was canceled.
Finally

I also have a prototype of a custom operator, WithCancellation, which is basically a pass-through IObservable also listening for a cancellation signal. I'd rather stick with a standard approach though.

Updated, I think I've found a race condition in how TakeUntil works (maybe a bug or just a behavior I can't explain), fiddle. I can't repro it with my homegrown WithCancellation implementation (commented out in the fiddle).

Updated, neither can I repro it if I use .TakeUntil(Task.Delay(Timeout.Infinite, token).ToObservable()).

Psychotherapy

Answer 1

Indeed, there is no library method for this. I would create an observable from the CancellationToken and then use the TakeUntil operator.

public static IObservable<Unit> ToObservable(this CancellationToken cancellationToken) {
  // if the token can't be cancelled, use Never which will not complete
  if (!cancellationToken.CanBeCanceled) {
    return Observable.Never<Unit>();
  }

  // if the token is already cancelled, use Return which publishes
  // Unit and completes immediately.
  // This may save you from ObjectDisposedException later
  if (cancellationToken.IsCancellationRequested) {
    return Observable.Return<Unit>();
  }

  // use Create so that each .Subscribe is handled independently
  return Observable.Create<Unit>(observer => {

    // Observable.Create does not handle errors on its own
    try {
      // return the registration because Dispose will unregister it
      return cancellationToken.Register(() => {
        // When the token is cancelled, publish and complete
        observer.OnNext(Unit.Default);
        observer.OnCompleted();
      });
    }
    catch (ObjectDisposedException e) {
      // todo: consider handling this as if it were cancellation
      observer.OnError(e);
    }
  });
}

public static IObservable<T> TakeUntil<T>(this IObservable<T> source, CancellationToken cancellationToken)
  => source.TakeUntil(cancellationToken.ToObservable());

Upheave