In Flutter I have a unit test where I am trying to test whether my code is correctly navigating to another page. I use mocktail for testing.

To start I created a mock router as follows:

class MockGoRouter extends Mock implements GoRouter {}

Then I created a router provider to provide the go router to my context:

class MockGoRouterProvider extends StatelessWidget {
  const MockGoRouterProvider({
    required this.goRouter,
    required this.child,
    super.key,
  });

  final MockGoRouter goRouter;

  final Widget child;

  @override
  Widget build(BuildContext context) => InheritedGoRouter(
        goRouter: goRouter,
        child: child,
      );
}

And when pump my widget I do the following:

 MaterialApp(
  home: MockGoRouterProvider(
  goRouter: router ?? MockGoRouter(),
  child: widget,
),

Now finally to use this in my tests where I’m trying to test code that calls goRouter.replace I do:

final mockRouter = MockGoRouter();
when(() => mockRouter.replace(any())).thenAnswer((_) async {});

However, when this code runs I get the following error:

══╡ EXCEPTION CAUGHT BY FLUTTER TEST FRAMEWORK ╞════════════════════════════════════════════════════
The following _TypeError was thrown running a test:
type 'Null' is not a subtype of type 'Future<Object?>'

When the exception was thrown, this was the stack:
#0      MockGoRouter.replace (file:///...mock_router.dart:5:7)
#1      GoRouterHelper.replace (package:go_router/src/misc/extensions.dart:116:25)
#2      GetCodeViewController._updateValidationStatus (package:.....)
#3      GetCodeViewController.validateCode (package:....)
<asynchronous suspension>

Unfortunately I can’t find a lot of resources on mocking go_router so any help is appreciated.