A utility to wait for notifyListeners to be called

change_notifier_listener_example.dart

// When there are external events triggering notifyListeners() in a ChangeNotifier,
// how can we easily test them? Not sure if something like this `ChangeNotifierListener` below
// is a valid approach, or if there are easy ways to test `ChangeNotifier` in a unit test, without a widget test. 

// Just for sake of the example
class MyViewModel with ChangeNotifier {

  MyViewModel(Stream<Object> eventStream) {
    eventStream.listen((event) {
      _counter++;
      notifyListeners();
    });
  }

  int _counter = 0;
  int get count => _counter;
}

// A utility class for unit testing ChangeNotifier
class ChangeNotifierListener {

  ChangeNotifierListener(this._changeNotifier) {
    _changeNotifier.addListener(_incrementTimesNotified);
  }

  final ChangeNotifier _changeNotifier;
  int _timesNotified = 0;
  int get timesNotified => _timesNotified;
  Completer<void>? _completer;

  void _incrementTimesNotified() {
    _timesNotified++;
    if (_completer != null) {
      _completer!.complete();
    }
  }

  Future<void> untilNotified() async {
    if (_completer != null) {
      throw StateError('Cannot call untilNotified while waiting for a previous call to complete');
    }
    _completer = Completer<void>();
    await _completer?.future;
  }

  void dispose() {
    _changeNotifier.removeListener(_incrementTimesNotified);
  }
}

extension ChangeNotifierListenerExtension on ChangeNotifier {
  ChangeNotifierListener listen() => ChangeNotifierListener(this);
}

// Usage
test('MyViewModel should notify listeners when an event comes in', () async {
  final streamController = StreamController<Object>();
  final viewModel = MyViewModel(streamController.stream);
  final listener = viewModel.listen();

  expect(viewModel.count, 0);

  streamController.add(Object());

  // Without waiting here, the viewModel.count would still be at 0
  await listener.untilNotified();

  expect(viewModel.count, 1);
  expect(listener.timesNotified, 1);
});