Regression when switchMap is used together with asynchronous generator

[ky1vstar]

Here is minimal reproducible example for regression which I found during migration from rxdart 0.27.7 to 0.28.0.

Dart SDK version: 3.5.1

Source code

import 'package:rxdart/rxdart.dart';

void main(List<String> arguments) async {
  getStream().listen(print);
}

Stream<String> getStream() {
  final stream1 = Future.delayed(Duration(seconds: 1), () => "1").asStream();
  final stream2 = stream1.startWith("2");

  final stream3 = stream2.switchMap((value) async* {
    await for (final innerValue in BehaviorSubject.seeded(value)) {
      yield innerValue;
    }
  });

  return stream3;
}

Expected result (rxdart 0.27.7):

2
1

Actual result (rxdart 0.28.0):

2

[hoc081098]

possibly related to dart-lang/sdk#42717 (comment), dart-lang/sdk#56619

[drstranges]

The issue is not only with the asynchronous generator, but also with someFuture.asStream too.
Here is the test that will fail on 0.28.0 but succeed on 0.27.7

  testWidgets('switchMap regression test', (tester) async {

    const delay = Duration(seconds: 1);

    Future<String> someFuture(String value) async {
      await Future<void>.delayed(delay);
      return value;
    }

    final subject = PublishSubject<String>();
    final beforeSwitchMap = <String>[];
    final afterSwitchMap = <String>[];

    subject
        .doOnData(beforeSwitchMap.add)
        .switchMap(
          (value) => someFuture(value).asStream(),
        )
        .doOnData(afterSwitchMap.add)
        .listen((event) {
      debugPrint('event: $event');
    });

    await tester.pump(const Duration(seconds: 2));
    subject.add('1');
    await tester.pump(const Duration(seconds: 2));
    subject.add('2');
    // Delay before next value < [delay] will fail the test in 0.28.0
    await tester.pump(delay - const Duration(microseconds: 1));
    subject.add('3');
    await tester.pump(const Duration(seconds: 2));
    subject.add('4');
    await tester.pump(const Duration(seconds: 2));

    expect(beforeSwitchMap, ['1', '2', '3', '4']);

    // Next line will succeed in 0.27.7 but fail in 0.28.0
    // with the message:
    // Expected: ['1', '3', '4']
    // Actual: ['1']
    expect(afterSwitchMap, ['1', '3', '4']);
  });

I can't reproduce the same behavior in runtime. Only widget tests seem to be affected (26 of our tests are red now ).

[duduBaiao]

After upgrading to 0.28.0, all code that uses BehaviorSubject.switchMap started to fail when I run the tests.
The production code works fine.

It only emits data for the first stream.

When I add a new value to the BehaviorSubject, I can see it's reprocessing the new stream, but it doesn't emit the value on time to be detected by the test. I'm using fake_async in these tests.

It seems to be a side effect of this change:

Flutter 3.24.5 • channel [user-branch] • unknown source
Framework • revision dec2ee5c1f (8 weeks ago) • 2024-11-13 11:13:06 -0800
Engine • revision a18df97ca5
Tools • Dart 3.5.4 • DevTools 2.37.3

[hoc081098]

After upgrading to 0.28.0, all code that uses BehaviorSubject.switchMap started to fail when I run the tests. The production code works fine.

It only emits data for the first stream.

When I add a new value to the BehaviorSubject, I can see it's reprocessing the new stream, but it doesn't emit the value on time to be detected by the test. I'm using fake_async in these tests.

It seems to be a side effect of this change:

Flutter 3.24.5 • channel [user-branch] • unknown source
Framework • revision dec2ee5c1f (8 weeks ago) • 2024-11-13 11:13:06 -0800
Engine • revision a18df97ca5
Tools • Dart 3.5.4 • DevTools 2.37.3

Maybe related: #395 (comment)