How to unit test Stream.listen() in Dart

Question

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(...)?

Answer 1

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);
        },
      ),
    );