Add FutureGroup.addCancelable()

[nex3]

No description provided.

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[mosuem]

@nex3 is it still your intention to land this PR? Just a friendly ping.

[nex3]

I'd be happy to land it. I'm still waiting on a review.

[mosuem]

Then redirecting the friendly ping to @lrhn or @natebosch :)

[natebosch]

I worry about the performance penalty here. By wrapping in a CancelableOperation we'd be creating 3 extra Completer instances along with a few book-keeping objects and function literals and tear-offs, for every Future added to the group.

Would it make sense to build a CancelableOperationGroup instead which has this overhead for the use cases that need it? It may be feasible to build as an adapter on FutureGroup to keep the implementation small.

Would it need to be much more complex than this?

class CancelableOperationGroup<T> {
  final FutureGroup<Object?> _group = FutureGroup();
  Future<List<T>> get future async =>
    (await _group.future).where((e) => e != _sentinel).whereType<T>().toList();
  void add(CancelableOperation<T> task) {
    final normalizedToSentinel =
      task.then((r) => r, onCancel: () => _sentinel);
    _group.add(normalizedToSentinel.valueOrCancellation);
  }
  // other members of FutureGroup may be useful to forward as well
  // bool get isClosed => _group.isClosed;
  // etc
}

[nex3]

Now that you mention it, we can actually use the same technique in this class to avoid adding overhead for normal futures.

[natebosch]

This looks good, I don't think I have concerns about the new conditional and cast per value in the result. I also think this behavior for cancelled operations makes sense - it's easy enough to understand how "without adding a value" from addCancelable docs interacts with "in the order they were added" from the class doc.

cc @lrhn in case he does have concern about the behavior of cancelled operations.

[mosuem]

@nex3 This is ready to merge I believe :)

[nex3]

I don't have the power to merge it.