Broadcast StreamGroup doesn't listen to later added streams after _onCancelBroadcast is called

[anhtuan23]

Hi. I'm getting a problem with broadcast StreamGroup.

Specifically, when adding a new stream after all previous subscriptions to a broadcast StreamGroup have been canceled, the StreamGroup won't listen to the newly added stream.

Here's how to reproduce this problem:

import 'package:async/async.dart';

void main() async {
  final streamGroup = StreamGroup<String>.broadcast();

  Stream<String> getFirstStream() async* {
    var i = 0;
    while (true) {
      await Future.delayed(Duration(seconds: 1));
      yield '1st stream: ${i++}';
    }
  }

  // Add the first single subscription stream
  await streamGroup.add(getFirstStream());

  final firstSubs = streamGroup.stream.listen((event) {
    print(event);
  });

  await Future.delayed(Duration(seconds: 3));
  // * Cancel the only subscription to the StreamGroup, putting it into dormant
  await firstSubs.cancel();

  Stream<String> getSecondStream() async* {
    var i = 0;
    while (true) {
      await Future.delayed(Duration(seconds: 1));
      yield '2nd stream: ${i++}';
    }
  }

  // Add another single subscription stream
  await streamGroup.add(getSecondStream());

  // * Start to listen to the StreamGroup again, which should emits events from both the 1st stream and the 2nd stream
  streamGroup.stream.listen((event) {
    print(event);
  });
}

Expected behavior:

Events from both the 1st stream and the 2nd stream are emitted by the StreamGroup.

Actual behavior:

Only Events from the 1st stream are emitted. The 2nd stream is ignored.

Proposal:

I believe the problem is in this line
https://github.com/dart-lang/async/blob/f1c8882c927af68df6786124c94e857de0fa22c1/lib/src/stream_group.dart#L195
Here, since the 1st stream is already being listened and have value != null, the function returns without listening to subsequent streams.

Therefore, it should be continue instead of return.

Thank you guys for this very helpful package.