Flutter Testing

Flutter Testing Guide

Overview

Write tests following priority order after implementation. Focus on business logic first, UI last.

Announce at start: "I'm using the flutter-testing skill to write tests."

Test Priority Order

Priority 1: Repository & DataSource Unit Tests
  ├── Business logic correctness
  ├── API integration
  └── Data transformation

Priority 2: State Management Unit Tests
  ├── BLoC/Cubit event handling
  ├── Provider state transitions
  └── Error state handling

Priority 3: Widget Tests (Optional)
  ├── User interactions
  ├── Widget rendering
  └── Navigation

Priority 4: Golden Tests (Visual Regression)
  ├── Design system component snapshots
  ├── Pixel-perfect comparison
  └── CI integration

Optional: Integration Tests
  └── Full app flow testing

Priority 1: Repository & DataSource Tests

Repository Test Template

import 'package:flutter_test/flutter_test.dart';
import 'package:mocktail/mocktail.dart';

class MockUserRemoteDataSource extends Mock implements UserRemoteDataSource {}

class MockUserLocalDataSource extends Mock implements UserLocalDataSource {}

void main() {
  late UserRepositoryImpl repository;
  late MockUserRemoteDataSource mockRemoteDataSource;
  late MockUserLocalDataSource mockLocalDataSource;

  setUp(() {
    mockRemoteDataSource = MockUserRemoteDataSource();
    mockLocalDataSource = MockUserLocalDataSource();
    repository = UserRepositoryImpl(
      remoteDataSource: mockRemoteDataSource,
      localDataSource: mockLocalDataSource,
    );
  });

  group('getUser', () {
    const tUserId = '123';
    final tUserModel = UserModel(id: '123', name: 'Test', email: 'test@test.com');
    final tUserEntity = User(id: '123', name: 'Test', email: 'test@test.com');

    test('should return User when remote data source succeeds', () async {
      // Arrange
      when(() => mockRemoteDataSource.getUser(any()))
          .thenAnswer((_) async => tUserModel);

      // Act
      final result = await repository.getUser(tUserId);

      // Assert
      expect(result, equals(tUserEntity));
      verify(() => mockRemoteDataSource.getUser(tUserId));
    });

    test('should throw Exception when remote data source fails', () async {
      // Arrange
      when(() => mockRemoteDataSource.getUser(any()))
          .thenThrow(Exception('Server error'));

      // Act & Assert
      expect(
        () => repository.getUser(tUserId),
        throwsException,
      );
    });
  });
}

DataSource Test Template

import 'package:flutter_test/flutter_test.dart';
import 'package:http/http.dart' as http;
import 'package:mocktail/mocktail.dart';

class MockHttpClient extends Mock implements http.Client {}

void main() {
  late UserRemoteDataSourceImpl dataSource;
  late MockHttpClient mockHttpClient;

  setUpAll(() {
    // mocktail needs a fallback for non-primitive matcher types
    registerFallbackValue(Uri.parse('https://example.com'));
  });

  setUp(() {
    mockHttpClient = MockHttpClient();
    dataSource = UserRemoteDataSourceImpl(client: mockHttpClient);
  });

  group('getUser', () {
    const tUserId = '123';
    final tUserJson = '{"id": "123", "name": "Test", "email": "test@test.com"}';

    test('should return UserModel when response is 200', () async {
      // Arrange
      when(() => mockHttpClient.get(any(), headers: any(named: 'headers')))
          .thenAnswer((_) async => http.Response(tUserJson, 200));

      // Act
      final result = await dataSource.getUser(tUserId);

      // Assert
      expect(result, isA<UserModel>());
      expect(result.id, equals('123'));
    });

    test('should throw ServerException when response is not 200', () async {
      // Arrange
      when(() => mockHttpClient.get(any(), headers: any(named: 'headers')))
          .thenAnswer((_) async => http.Response('Error', 500));

      // Act & Assert
      expect(
        () => dataSource.getUser(tUserId),
        throwsA(isA<ServerException>()),
      );
    });
  });
}

Priority 2: State Management Tests

BLoC Test Template

import 'package:bloc_test/bloc_test.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:mocktail/mocktail.dart';

class MockGetUserUseCase extends Mock implements GetUserUseCase {}

void main() {
  late UserBloc bloc;
  late MockGetUserUseCase mockGetUser;

  setUp(() {
    mockGetUser = MockGetUserUseCase();
    bloc = UserBloc(getUser: mockGetUser);
  });

  tearDown(() {
    bloc.close();
  });

  test('initial state should be UserInitial', () {
    expect(bloc.state, equals(UserInitial()));
  });

  blocTest<UserBloc, UserState>(
    'should emit [Loading, Loaded] when GetUser succeeds',
    build: () {
      when(() => mockGetUser(any()))
          .thenAnswer((_) async => const User(id: '1', name: 'Test'));
      return bloc;
    },
    act: (bloc) => bloc.add(const GetUserEvent('1')),
    expect: () => [
      UserLoading(),
      const UserLoaded(User(id: '1', name: 'Test')),
    ],
  );

  blocTest<UserBloc, UserState>(
    'should emit [Loading, Error] when GetUser fails',
    build: () {
      when(() => mockGetUser(any())).thenThrow(Exception('error'));
      return bloc;
    },
    act: (bloc) => bloc.add(const GetUserEvent('1')),
    expect: () => [
      UserLoading(),
      const UserError('error'),
    ],
  );
}

Provider/Riverpod Test Template

import 'package:flutter_test/flutter_test.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:mocktail/mocktail.dart';

class MockUserRepository extends Mock implements UserRepository {}

void main() {
  late ProviderContainer container;
  late MockUserRepository mockRepository;

  setUp(() {
    mockRepository = MockUserRepository();
    container = ProviderContainer(
      overrides: [
        userRepositoryProvider.overrideWithValue(mockRepository),
      ],
    );
  });

  tearDown(() {
    container.dispose();
  });

  test('should return user when fetchUser succeeds', () async {
    // Arrange
    when(() => mockRepository.getUser(any()))
        .thenAnswer((_) async => const User(id: '1', name: 'Test'));

    // Act
    final result = await container.read(userProvider('1').future);

    // Assert
    expect(result.name, equals('Test'));
  });
}

Priority 3: Widget Tests (Optional)

Widget Test Template

import 'package:bloc_test/bloc_test.dart';
import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:mocktail/mocktail.dart';

class MockUserBloc extends MockBloc<UserEvent, UserState> implements UserBloc {}

void main() {
  late MockUserBloc mockBloc;

  setUp(() {
    mockBloc = MockUserBloc();
  });

  Widget createWidgetUnderTest() {
    return MaterialApp(
      home: BlocProvider<UserBloc>.value(
        value: mockBloc,
        child: const UserScreen(),
      ),
    );
  }

  testWidgets('should display loading indicator when state is Loading',
      (tester) async {
    // Arrange
    when(() => mockBloc.state).thenReturn(UserLoading());

    // Act
    await tester.pumpWidget(createWidgetUnderTest());

    // Assert
    expect(find.byType(CircularProgressIndicator), findsOneWidget);
  });

  testWidgets('should display user name when state is Loaded',
      (tester) async {
    // Arrange
    when(() => mockBloc.state).thenReturn(
      const UserLoaded(User(id: '1', name: 'John Doe')),
    );

    // Act
    await tester.pumpWidget(createWidgetUnderTest());

    // Assert
    expect(find.text('John Doe'), findsOneWidget);
  });

  testWidgets('should call GetUserEvent when button is tapped',
      (tester) async {
    // Arrange
    when(mockBloc.state).thenReturn(UserInitial());

    // Act
    await tester.pumpWidget(createWidgetUnderTest());
    await tester.tap(find.byType(ElevatedButton));

    // Assert
    verify(mockBloc.add(any)).called(1);
  });
}

Priority 4: Golden Tests (Visual Regression)

Verifies the visual consistency of design system components. Prevents unintended UI changes.

Golden Test Template

import 'package:flutter/material.dart';
import 'package:flutter_test/flutter_test.dart';

void main() {
  testWidgets('LoginButton matches golden', (tester) async {
    // Fixed size guarantees consistent screenshots
    await tester.binding.setSurfaceSize(const Size(400, 200));

    await tester.pumpWidget(
      MaterialApp(
        home: Scaffold(
          body: Center(
            child: LoginButton(onPressed: () {}),
          ),
        ),
      ),
    );

    await expectLater(
      find.byType(LoginButton),
      matchesGoldenFile('goldens/login_button.png'),
    );
  });

  testWidgets('UserCard dark mode matches golden', (tester) async {
    await tester.binding.setSurfaceSize(const Size(400, 300));

    await tester.pumpWidget(
      MaterialApp(
        theme: ThemeData.dark(),
        home: Scaffold(
          body: UserCard(
            user: User(id: '1', name: 'Test User', avatar: 'assets/test/avatar.png'),
          ),
        ),
      ),
    );

    await expectLater(
      find.byType(UserCard),
      matchesGoldenFile('goldens/user_card_dark.png'),
    );
  });
}

Golden File Management

# Initial generation / baseline update (after intentional changes)
flutter test --update-goldens

# Comparison run (used in CI/CD)
flutter test --tags golden

# Specific files only
flutter test test/features/auth/presentation/widgets/login_button_golden_test.dart --update-goldens

Test File Naming Convention

Golden test files are distinguished by the _golden_test.dart suffix:

test/features/auth/presentation/widgets/
├── login_button_test.dart         # Functional test (Priority 3)
└── login_button_golden_test.dart  # Golden test (Priority 4)

Tag-Based Execution

Separate golden tests with a tag in dart_test.yaml:

tags:
  golden:
    # Can be run separately in CI

Add the tag to test files:

@Tags(['golden'])
library;

import 'package:flutter_test/flutter_test.dart';
// ...

Caveats When Writing Golden Tests

1. Fixed size: Set a consistent canvas size with setSurfaceSize()
2. Deterministic data: Never use random or time-dependent data
3. Font loading: Preload custom fonts with FontLoader
4. Network images: Replace with mocks (remove network dependency)
5. Platform differences: Rendering can differ across macOS/Linux/Windows → pin the CI environment

When to Write Golden Tests

• Design system components (Button, Card, Input, Badge, etc.)
• Verifying visual impact after theme changes
• Verifying dark mode / light mode switching
• Verifying responsive layouts at each breakpoint

When to Skip Golden Tests

• Business-logic-heavy screens (only the displayed data differs)
• Frequently changing screens (golden file update cost > benefit)
• Screens heavily dependent on external data

Test File Structure

test/
├── features/
│   └── auth/
│       ├── data/
│       │   ├── datasources/
│       │   │   └── auth_remote_datasource_test.dart
│       │   └── repositories/
│       │       └── auth_repository_impl_test.dart
│       └── presentation/
│           ├── bloc/
│           │   └── auth_bloc_test.dart
│           └── widgets/
│               └── login_button_test.dart
└── helpers/
    ├── test_helpers.dart
    └── pump_app.dart

Running Tests

# All tests
flutter test

# Specific feature
flutter test test/features/auth/

# Specific file
flutter test test/features/auth/data/repositories/auth_repository_impl_test.dart

# With coverage
flutter test --coverage

# Generate coverage report
genhtml coverage/lcov.info -o coverage/html

Test Dependencies

# Mocking — mocktail (no codegen; all templates in this skill use it)
flutter pub add dev:mocktail

# State management testing
flutter pub add dev:bloc_test      # If using BLoC (also provides MockBloc)

# Freezed (if using immutable states)
flutter pub add freezed_annotation
flutter pub add dev:freezed
flutter pub add dev:build_runner   # For freezed codegen only

Generate Code (Freezed states etc.)

dart run build_runner build --delete-conflicting-outputs

Key Principles

1. Priority Order: Repository → State → Widget
2. Mock Dependencies: Don't test real APIs or databases
3. Arrange-Act-Assert: Clear test structure
4. One Assertion Focus: Each test tests one thing
5. Descriptive Names: Test names describe behavior

When to Skip Tests

• Skip Widget Tests when:

  • Simple stateless widgets
  • No user interaction logic
  • Pure presentation (no business logic)

• Never Skip:

  • Repository tests (business logic)
  • State management tests (state transitions)
  • Error handling tests

REQUIRED SUB-SKILL

After writing tests, you MUST invoke: → flutter-craft:flutter-verification

Run flutter test and verify all tests pass.