'Null' is not a subtype of type 'Stream<int>' in type cast cubit(Bloc) flutter

Question

I have created a cubit testing project in flutter which is working fine, but when I am writing a UI test case with mockito for the same it is throwing the following error. 'Null' is not a subtype of type 'Stream' in typecast. If the real object is being passed then the unit test is working fine.

MyCubit myCubit = MyCubit(); //real object working fine with UT

MyCubit myCubit = MockMyCubit(); //mocked object not working fine with UT.

Previously the same code was working with mockito when I have not upgraded my flutter. I have also tried to mock Stream using mockito but it also did not work.

My code is as follows

flutter dependencies

flutter_bloc: ^8.0.1
mockito: ^5.1.0

my_cubit.dart

class MyCubit extends Cubit<int> {
  MyCubit() : super(0);

  void increment() {
    emit(state + 1);
  }

  void decrement() {
    emit(state - 1);
  }
}

main.dart

void main() {
  MyCubit myCubit = MyCubit();
  runApp(MyAppParent(myCubit));
}

class MyAppParent extends StatelessWidget {
  MyAppParent(this.myCubit);

  MyCubit myCubit;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('test'),
        ),
        body: BlocProvider<MyCubit>(
          create: (_) => myCubit,
          child: MyApp(),
        ),
      ),
    );
  }
}

class MyApp extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    MyCubit myCubit = context.read<MyCubit>();
    return Column(
      children: [
        BlocBuilder<MyCubit, int>(bloc: myCubit, builder: (BuildContext context, int count) {
            return Text('$count');
        }),
        TextButton(
          onPressed: () {
            myCubit.increment();
          },
          child: const Text('Increment'),
        ),
        TextButton(
          onPressed: () {
            myCubit.decrement();
          },
          child: const Text('Decrement'),
        )
      ],
    );
  }
}

widget_test.dart

class MockedMyCubit extends Mock implements MyCubit {}

void main() {
  testWidgets('Testing', (WidgetTester tester) async {
    MyCubit myCubit = MockMyCubit(); //fake object is not working, throwing exception
    // when(myCubit.stream).thenAnswer((_)  => StreamController<int>.broadcast().stream);

    // MyCubit myCubit = MyCubit(); //real object working fine
    await tester.pumpWidget(MyAppParent(myCubit));

    Finder finderCount = find.text('0');
    expect(finderCount, findsOneWidget);
    Finder finderIncrement = find.text('Increment');
    Finder finderDecrement = find.text('Decrement');

    await tester.tap(finderIncrement);
    await tester.pump();
    Finder finderCount1 = find.text('1');
    expect(finderCount1, findsOneWidget);

    await tester.tap(finderDecrement);
    await tester.pump();
    Finder finderCount0 = find.text('0');
    expect(finderCount0, findsOneWidget);
  });
}

Answer 1

You can use a library called mocktail.

The state type of MockedMyCubit should be provided by extending MockCubit<int> as follows:

import 'package:mocktail/mocktail.dart';

class MockedMyCubit extends MockCubit<int> implements MyCubit {}

Remember to define how it may behave using when and whenListen.

Issue discussion

Answer 2

I believe this is happening because the mock cubit needs to be told an initial state value. Try this after you instantiate the cubit:

when(() => myCubit.state).thenReturn(0);

Answer 3

First of all, the error 'Null' is not a subtype of type 'Stream' in typecast is given because you are not specifying a state for your mocked bloc/cubit, in this case, an initial state.

Second, what do you want to test? Do you want to test your widget or do you want to test your cubit? Keep in mind that is recommended to test only one thing and the rest of them should be mocked. Therefore, if you are testing your widget then you should mock your cubit. I know that you already mocked your cubit but you are not setting up any stub for the increment and decrement methods of your cubit class.

If you want to test your widget do the following:

flutter dependencies (Use bloc_test instead of mockito to mock your bloc/cubit)

flutter_bloc: ^8.0.1
bloc_test: ^9.0.3

my_cubit.dart (same as yours)

class MyCubit extends Cubit<int> {
  MyCubit() : super(0);

  void increment() {
    emit(state + 1);
  }

  void decrement() {
    emit(state - 1);
  }
}

main.dart (Here you can notice I substituted the BlocBuilder with a BlocConsumer to print the state of the cubit on the listener callback function. This allows you to see the changes in the state of the cubit while running the tests)

void main() {
  MyCubit myCubit = MyCubit();
  runApp(MyAppParent(myCubit: myCubit));
}

class MyAppParent extends StatelessWidget {
  const MyAppParent({
    Key? key,
    required this.myCubit,
  }) : super(key: key);

  final MyCubit myCubit;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: const Text('test'),
        ),
        body: BlocProvider<MyCubit>(
          create: (_) => myCubit,
          child: const MyApp(),
        ),
      ),
    );
  }
}

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    MyCubit myCubit = context.read<MyCubit>();
    return Column(
      children: [
        BlocConsumer<MyCubit, int>(
          bloc: myCubit,
          listener: (BuildContext context, int count) {
            print('COUNTER: $count');
          },
          builder: (BuildContext context, int count) {
            return Text('$count');
          },
        ),
        TextButton(
          onPressed: () {
            myCubit.increment();
          },
          child: const Text('Increment'),
        ),
        TextButton(
          onPressed: () {
            myCubit.decrement();
          },
          child: const Text('Decrement'),
        )
      ],
    );
  }
}

widget_test.dart (As you can see here, I'm creating a mock for the cubit using the MockCubit class offered by bloc_test, and also I'm providing the states I want to test on my widget by using the whenListen stub)

Now you have 100% of test coverage for MyAppParent and MyApp widgets.

class MockMyCubit extends MockCubit<int> implements MyCubit {}

void main() {
  late final MyCubit myCubit;

  setUpAll(() {
    myCubit = MockMyCubit();
  });

  group('Testing', () {
    testWidgets('counter equal to 1', (WidgetTester tester) async {
      whenListen(
        myCubit,
        Stream.fromIterable([0, 1]),
        initialState: 0,
      );

      await tester.pumpWidget(MyAppParent(myCubit: myCubit));

      // The next three lines allows to test the button exists,
      // it can be omitted and this test will still working
      Finder finderIncrement = find.text('Increment');
      expect(finderIncrement, findsOneWidget);
      await tester.tap(finderIncrement);

      await tester.pump();
      Finder finderCount = find.text('1');
      expect(finderCount, findsOneWidget);
    });
  });

  group(
    'Testing',
    () {
      testWidgets(
          'counter equal to 0 after incrementing and decrementing, in that order',
          (WidgetTester tester) async {
        whenListen(
          myCubit,
          Stream.fromIterable([0, 1, 0]),
          initialState: 0,
        );

        await tester.pumpWidget(MyAppParent(myCubit: myCubit));

        // The next three lines allows to test the button exists,
        // it can be omitted and this test will still working
        Finder finderDecrement = find.text('Decrement');
        expect(finderDecrement, findsOneWidget);
        await tester.tap(finderDecrement);

        await tester.pump();
        Finder finderCount = find.text('0');
        expect(finderCount, findsOneWidget);
      });
    },
  );
}

You can see the printed states on the console output:

You can find the entire example here: https://github.com/Abel1027/widget-bloc-testing