dart - 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 test behavior of that button.

Widget/Screen

class MyScreen extends StatefulWidget {

  MyScreen({Key key}) : super(key: key);

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

class _MyScreenState extends State<MyScreen> {

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: Center(
            child: RaisedButton(
                onPressed: () {
                    Navigator.of(context).pushNamed("/nextscreen");
                },
                child: Text(Strings.traktTvUrl)
            )
        )
    );
  }

}

Test

void main() {

  testWidgets('Button is present and triggers navigation after tapped',
      (WidgetTester tester) async {
    await tester.pumpWidget(MaterialApp(home: MyScreen()));
    expect(find.byType(RaisedButton), findsOneWidget);
    await tester.tap(find.byType(RaisedButton));
    //how to test navigator?    
  });
}

I there a proper way how to check, that Navigator was called? Or is there a way to mock and replace navigator?

Pleas note, that code above will actually fail with an exception, because there is no named route '/nextscreen' declared in application. That's simple to solve and you don't need to point it out.

My main concern is how to correctly approach this test scenario in Flutter.

Answer 1

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

import 'package:mockito/mockito.dart';

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.