main.dart

import 'package:bloc/bloc.dart';
import 'package:flutter/material.dart';
import 'package:flutter_bloc/flutter_bloc.dart';

void main() => runApp(BlocProvider<TestBloc>(
      create: (c) => TestBloc(),
      child: MyApp(),
    ));

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: Scaffold(
        body: BlocBuilder<TestBloc, TestState>(builder: (context, state) {
          if (state == null) {
            return Container();
//            throw Exception('state is null');
          }
          return Builder(builder: (c) {
            return Center(
              child: Text('State: ${state.text}'),
            );
          });
        }),
      ),
    );
  }
}

class TestBloc extends Bloc<TestEvent, TestState> {
  @override
  TestState get initialState => TestState('initialState');

  @override
  Stream<TestState> mapEventToState(TestEvent event) async* {
    yield TestState('yield from mapEvent');
  }
}

class TestEvent {}

class TestState {
  TestState(this.text);

  final String text;
}

widget_test.dart

import 'package:bloc_test/bloc_test.dart';
import 'package:fluttebloc/main.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/mockito.dart';

class TestBlocMock extends Mock implements TestBloc {}

void main() {
  testWidgets('Test', (WidgetTester tester) async {
    final testBloc = TestBlocMock();
    when(testBloc.initialState).thenReturn(TestState('Initial mocked'));
    whenListen<TestEvent, TestState>(
      testBloc,
      Stream.fromIterable([
        TestState('First emission'),
        TestState('Second emission'),
      ]),
    );
    await tester.pumpWidget(
        BlocProvider<TestBloc>(create: (c) => testBloc, child: MyApp()));
    await tester.pump(Duration.zero);
//    expect(find.text('First emission'), findsOneWidget);
    expect(find.text('State: Second emission'), findsOneWidget);
  });
}

Hey! This is happening because instead of mocking initialState you need to mock state.

class TestBlocMock extends MockBloc<TestEvent, TestState> implements TestBloc {} // extend MockBloc rather than Mock

void main() {
  testWidgets('Test', (WidgetTester tester) async {
    final testBloc = TestBlocMock();
    when(testBloc.state).thenReturn(TestState('Initial mocked')); // stub state rather than initialState
    whenListen<TestEvent, TestState>(
      testBloc,
      Stream<TestState>.fromIterable([
        TestState('First emission'),
        TestState('Second emission'),
      ]),
    );
    await tester.pumpWidget(
      BlocProvider<TestBloc>(create: (c) => testBloc, child: MyApp()),
    );
    await tester.pump(Duration.zero);
    expect(find.text('State: Second emission'), findsOneWidget);
  });
}

Hope that helps! 👍

How would I go about testing something like this when I am using getIt for dependency injection? I have tried adding:

setUp()
getIt.registerFactory<ProjectFormCubit>(() => MockProjectFormCubit());

And then reset() in teardown.

But I have not had any success. The widget under test is using:
BlocProvider with:
getIt<ProjectFormCubit>()..initialised(optionOf(editedProject)),`

I used
setUpAll(){() => GetIt.registerFactory(() => MockProjectFormCubit())
}
and it works fine.