How to test navigation via Navigator in Flutter

Question

Let\'s say, I have a test for a screen in Flutter using WidgetTester. There is a button, which executes a navigation via Navigator. I would like to

Answer 1

Following solution is, let's say, a general approach and it's not specific to Flutter.

Navigation could be abstracted away from a screen or a widget. Test can mock and inject this abstraction. This approach should be sufficient for testing such behavior.

There are several ways how to achieve that. I will show one of those, for purpose of this response. Perhaps it's possible to simplify it a bit or to make it more "Darty".

Abstraction for navigation

class AppNavigatorFactory {
  AppNavigator get(BuildContext context) =>
      AppNavigator._forNavigator(Navigator.of(context));
}

class TestAppNavigatorFactory extends AppNavigatorFactory {
  final AppNavigator mockAppNavigator;

  TestAppNavigatorFactory(this.mockAppNavigator);

  @override
  AppNavigator get(BuildContext context) => mockAppNavigator;
}

class AppNavigator {
  NavigatorState _flutterNavigator;
  AppNavigator._forNavigator(this._flutterNavigator);

  void showNextscreen() {
    _flutterNavigator.pushNamed('/nextscreen');
  }
}

Injection into a widget

class MyScreen extends StatefulWidget {
  final _appNavigatorFactory;
  MyScreen(this._appNavigatorFactory, {Key key}) : super(key: key);

  @override
  _MyScreenState createState() => _MyScreenState(_appNavigatorFactory);
}

class _MyScreenState extends State<MyScreen> {
  final _appNavigatorFactory;

  _MyScreenState(this._appNavigatorFactory);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Center(
            child: RaisedButton(
                onPressed: () {
                    _appNavigatorFactory.get(context).showNextscreen();
                },
                child: Text(Strings.traktTvUrl)
            )
        )
    );
  }

}

Example of a test (Uses Mockito for Dart)

class MockAppNavigator extends Mock implements AppNavigator {}

void main() {
  final appNavigator = MockAppNavigator();

  setUp(() {
    reset(appNavigator);
  });


  testWidgets('Button is present and triggers navigation after tapped',
      (WidgetTester tester) async {

    await tester.pumpWidget(MaterialApp(home: MyScreen(TestAppNavigatorFactory())));

    expect(find.byType(RaisedButton), findsOneWidget);
    await tester.tap(find.byType(RaisedButton));

    verify(appNavigator.showNextscreen());
  });
}

Answer 2

In the navigator tests in the flutter repo they use the NavigatorObserver class to observe navigations:

class TestObserver extends NavigatorObserver {
  OnObservation onPushed;
  OnObservation onPopped;
  OnObservation onRemoved;
  OnObservation onReplaced;

  @override
  void didPush(Route<dynamic> route, Route<dynamic> previousRoute) {
    if (onPushed != null) {
      onPushed(route, previousRoute);
    }
  }

  @override
  void didPop(Route<dynamic> route, Route<dynamic> previousRoute) {
    if (onPopped != null) {
      onPopped(route, previousRoute);
    }
  }

  @override
  void didRemove(Route<dynamic> route, Route<dynamic> previousRoute) {
    if (onRemoved != null)
      onRemoved(route, previousRoute);
  }

  @override
  void didReplace({ Route<dynamic> oldRoute, Route<dynamic> newRoute }) {
    if (onReplaced != null)
      onReplaced(newRoute, oldRoute);
  }
}

This looks like it should do what you want, however it may only work form the top level (MaterialApp), I'm not sure if you can provide it to just a widget.

Answer 3

While what Danny said is correct and works, you can also create a mocked NavigatorObserver to avoid any extra boilerplate:

class MockNavigatorObserver extends Mock implements NavigatorObserver {}

That would translate to your test case as follows:

void main() {
  testWidgets('Button is present and triggers navigation after tapped',
      (WidgetTester tester) async {
    final mockObserver = MockNavigatorObserver();
    await tester.pumpWidget(
      MaterialApp(
        home: MyScreen(),
        navigatorObservers: [mockObserver],
      ),
    );

    expect(find.byType(RaisedButton), findsOneWidget);
    await tester.tap(find.byType(RaisedButton));
    await tester.pumpAndSettle();

    /// Verify that a push event happened
    verify(mockObserver.didPush(any, any));

    /// You'd also want to be sure that your page is now
    /// present in the screen.
    expect(find.byType(DetailsPage), findsOneWidget);
  });
}

I wrote an in-depth article about this on my blog, which you can find here.