How to unit test Stream.listen() in Dart

Consider the following test code ...

main() {
  StreamController controller;
  setUp(() {
    controller = StreamController.broadcast(sync: false);
  });

  tearDown(() {
    controller.close();
  });

  test('Stream listen test', () {
    var stream = controller.stream;

    stream.listen((event) {
      // >>>>> How can I test if this was called?? <<<<<<
    });

    controller.add('Here is an event!');
  });
}

... what is the correct way to test that the stream.listen(...) call-back was invoked?

stream.listen(...) doesn't return a Future so traditional expectAsyncX style matchers will not work here.

Perhaps there is some way to use the StreamSubscription.asFuture() that is returned by stream.listen(...)?

You can use expectLater function to wait for async response from your Stream and compare your Stream to your emitted value using emits , like this:

  test('Stream listen test', () {
    var stream = controller.stream;

    final String value = 'Here is an event!';

    stream.listen((event) {
      // >>>>> How can I test if this was called?? <<<<<<
    });

    expectLater(stream, emits(value));

    controller.add(value);
  });

UPDATE

You can try expectAsync1 around your listen method. If no value is emitted, the timeout will be fired.

stream.listen(
      expectAsync1(
        (event) {
          expect(event, value);
        },
      ),
    );