fakeAsync vs async*

fake_async_test.dart

import 'dart:async';

import 'package:fake_async/fake_async.dart';
import 'package:test/test.dart';

Future<int> firstEven(Stream<int> numberStream) =>
    numberStream.firstWhere((element) => element.isEven);

void main() {
  void runTest(Stream<int> stream, StreamSink<int> sink) {
    fakeAsync((async) {
      final completer = Completer<int>();

      firstEven(stream).then(completer.complete);

      async.flushMicrotasks();
      expect(completer.isCompleted, false);

      sink.add(1);
      sink.add(3);
      sink.add(5);
      async.flushMicrotasks();
      expect(completer.isCompleted, false);

      sink.add(2);
      async.flushMicrotasks();
      expect(completer.isCompleted, true);
    });
  }

  test('test with StreamController', () {
    final controller = StreamController<int>();
    runTest(controller.stream, controller.sink);
  });

  test('test with StreamController.broadcast', () {
    final controller = StreamController<int>.broadcast();
    runTest(controller.stream, controller.sink);
  });

  test('test with async* generator', () {
    final controller = StreamController<int>();
    runTest(
      () async* {
        yield* controller.stream;
      }(),
      controller.sink,
    );
  });

  test('test with asyncExpand', () {
    final controller = StreamController<int>();
    runTest(
      Stream.fromFuture(Future.value()).asyncExpand((_) => controller.stream),
      controller.sink,
    );
  });

  test('test with populating another controller', () {
    final controller = StreamController<int>();
    runTest(
      () {
        final anotherController = StreamController<int>();
        late final StreamSubscription<int> subscription;
        subscription = controller.stream.listen(
          anotherController.add,
          onDone: () => subscription.cancel(),
        );
        return anotherController.stream;
      }(),
      controller.sink,
    );
  });
}