Flutter Tester

Flutter Tester

Overview

This skill provides comprehensive guidance for writing consistent, reliable, and maintainable tests for Flutter applications. Follow the testing patterns, mocking strategies, and architectural guidelines to ensure tests are isolated, repeatable, and cover both success and error scenarios. This skill works with any Flutter project using common packages like Riverpod, Mockito, and flutter_test.

When to Use This Skill

Use this skill when:

• Creating new unit tests for repositories, providers, DAOs, or services
• Writing widget tests for UI components and views
• Setting up mocks and test dependencies with Mockito and Riverpod
• Implementing Given-When-Then test structure
• Testing state management with Riverpod providers
• Writing integration tests for multi-layer workflows
• Debugging or fixing existing tests
• Ensuring proper test coverage across data, domain, and presentation layers

Core Testing Principles

1. Clean Architecture Testing

Test each layer in isolation:

• Data Layer → DAOs, APIs, Repositories
• Domain Layer → Models (Freezed), Entities
• Presentation Layer → Providers (Riverpod), Views, Controllers

2. Given-When-Then Structure

Always structure tests using Given-When-Then pattern:

test('Given valid data, When operation executes, Then returns expected result', () async {
  // Arrange (Given)
  when(mockDAO.getData()).thenAnswer((_) async => testData);

  // Act (When)
  final result = await repository.fetchData();

  // Assert (Then)
  expect(result, equals(testData));
  verify(mockDAO.getData()).called(1);
});

3. Test Organization

• Group related tests using group() blocks
• Use setUp() for common initialization
• Use tearDown() for cleanup (reset GetIt, dispose resources)
• Use setUpAll() for one-time expensive setup

Testing Workflow

Step 1: Identify the Layer Under Test

Determine which architectural layer you're testing:

• Repository tests → Mock DAOs and APIs
• Provider tests → Mock services and repositories
• Widget tests → Mock providers and services
• DAO tests → Use FakeDatabase

Step 2: Set Up Dependencies and Mocks

Generate Mocks with Mockito

@GenerateMocks([ILogger, ICarouselRepository, INotificationDAO])
void main() {
  // Test code
}

Important: Never mock providers directly. Override their dependencies instead.

Register with GetIt

setUp(() {
  mockLogger = MockILogger();
  mockRepository = MockICarouselRepository();
  GetIt.I.registerSingleton<ILogger>(mockLogger);
  GetIt.I.registerSingleton<ICarouselRepository>(mockRepository);
});

tearDown(() => GetIt.I.reset());

SharedPreferences Setup

setUpAll(() async {
  SharedPreferences.setMockInitialValues({'key1': 'value1'});
  SharedPrefManager.instance = await SharedPreferences.getInstance();
});

Step 3: Write Tests Following Layer Patterns

Refer to the references/layer_testing_patterns.md file for detailed examples of:

• Repository testing patterns
• Provider testing patterns with Riverpod
• DAO testing patterns with FakeDatabase
• Widget testing patterns with keys and screen size setup

Step 4: Test Error Scenarios

Always test both success and failure paths:

test('Given service throws exception, When called, Then logs error and returns fallback', () async {
  // Arrange
  final exception = Exception('Network error');
  when(mockService.fetchData()).thenThrow(exception);

  // Act
  final result = await repository.getData();

  // Assert
  expect(result, isEmpty); // Or appropriate fallback
  verify(mockLogger.writeExceptionLog('RepositoryName', 'getData', exception, any)).called(1);
});

Widget Testing Essentials

Always Set Screen Size

testWidgets('Test description', (tester) async {
  tester.view.physicalSize = const Size(1000, 1000);
  tester.view.devicePixelRatio = 1.0;

  // Your test code
});

Always Use Keys for Widget Finding

In source code:

ElevatedButton(
  key: const Key('saveButton'),
  onPressed: () {},
  child: const Text('Save'),
);

In test:

await tester.tap(find.byKey(const Key('saveButton')));
await tester.pumpAndSettle();

If a key doesn't exist in the source widget, add it before writing the test.

Loading → Content Transitions

when(mockService.fetchData()).thenAnswer((_) async => data);

await tester.pumpWidget(createTestWidget());
expect(find.byType(CircularProgressIndicator), findsOneWidget);

await tester.pumpAndSettle();
expect(find.byType(DataWidget), findsOneWidget);

Platform-Specific Testing

testWidgets('iOS specific behavior', (tester) async {
  debugDefaultTargetPlatformOverride = TargetPlatform.iOS;

  await tester.pumpWidget(createTestWidget());

  expect(find.byType(CupertinoButton), findsOneWidget);

  debugDefaultTargetPlatformOverride = null;
});

Riverpod Testing

Create Container with Overrides

final container = createContainer(overrides: [
  repoProvider.overrideWith((ref) => mockRepo),
]);

Use the createContainer() helper from test/riverpod_container.dart which auto-disposes on tearDown.

Test Provider State

test('Given valid data, When state updates, Then reflects new value', () async {
  final notifier = container.read(provider.notifier);

  notifier.updateState(newValue);

  expect(container.read(provider).value!.property, newValue);
});

Test Initial State

test('Given empty data, When building initial state, Then returns default state', () async {
  when(mockService.fetchData()).thenAnswer((_) async => []);

  final container = createContainer();
  final state = await container.read(provider.notifier).future;

  expect(state.data, isEmpty);
  expect(state.isLoading, false);
});

Stubbing Patterns

Success Scenarios

when(mockRepo.fetchFromDb()).thenAnswer((_) async => mockData);
when(mockApi.updateData(any, any, any)).thenAnswer((_) async => true);

Failure Scenarios

when(mockRepo.fetchFromDb()).thenThrow(Exception('DB error'));
when(mockApi.updateData(any, any, any)).thenAnswer((_) async => false);

Using Completers for Async Control

final completer = Completer<RegistrationModel>();

when(mockRepo.fetchData(any, any)).thenAnswer((_) => completer.future);

await tester.tap(find.text('Save'));
await tester.pump();

expect(find.byType(CircularProgressIndicator), findsOneWidget);

completer.complete(const RegistrationModel(status: 'success'));
await tester.pump();

expect(find.byType(CircularProgressIndicator), findsNothing);

Fakes vs Mocks

When to Use Fakes

Use fake implementations for consistent dummy behavior:

class FakeLogger extends ILogger {
  @override
  void writeInfoLog(String className, String method, String message) {}

  @override
  void writeErrorLog(String className, String method, dynamic error, StackTrace? stack, [String? msg]) {}
}

Register in test setup:

GetIt.I.registerSingleton<ILogger>(FakeLogger.i);

When to Use Mocks

Use mocks when you need to verify method calls or setup specific behaviors:

when(mockLogger.writeErrorLog(any, any, any, any)).thenReturn(null);
verify(mockLogger.writeErrorLog('ClassName', 'methodName', exception, any)).called(1);

Database Testing with FakeDatabase

late MenuDAO menuDAO;
late Database db;
late IDatabase mockDatabase;

setUp(() async {
  await FakePathProviderPlatform.initialize();
  PathProviderPlatform.instance = FakePathProviderPlatform();

  mockDatabase = FakeDatabase();
  db = await mockDatabase.database;

  menuDAO = MenuDAO(
    dbManager: mockDatabase,
    logger: mockLogger,
  );
});

tearDown() async {
  await menuDAO.deleteTable();
  if (GetIt.I.isRegistered<IDatabase>()) {
    await GetIt.I<IDatabase>().close();
  }
  await GetIt.I.reset();
  await FakePathProviderPlatform.cleanup();
}

Test Checklist

Before submitting tests, ensure:

Setup & Mocking:

• Dependencies mocked (not providers)
• SharedPreferences mocked if used
• GetIt reset in tearDown
• Streams closed in tearDown
• Controllers disposed in tearDown

Widget Tests:

• Keys added & used in widget tests
• Screen size set (physicalSize + devicePixelRatio)
• Platform overrides reset (debugDefaultTargetPlatformOverride = null)
• Navigation tested if applicable
• Dialogs/overlays tested if shown

Test Coverage:

• Success & failure paths covered
• Edge cases tested (null, empty, max values)
• Loading states tested
• Error states tested
• Async handled correctly (await, Completer)

Code Quality:

• Given-When-Then naming used
• verify() or verifyNever() used where appropriate
• No hardcoded delays (use pump/pumpAndSettle)
• Tests are isolated (no dependencies between tests)
• Tests are deterministic (same result every time)

Common Patterns

Verification Patterns

// Single call
verify(mockService.method()).called(1);

// Multiple calls
verify(mockService.method()).called(3);

// Never called
verifyNever(mockService.method());

// Ordered calls
verifyInOrder([
  mockService.method1(),
  mockService.method2(),
]);

Testing Global State

import 'package:your_app/path/to/global_variables.dart' as global_variables;

setUp(() {
  global_variables.someGlobalVariable = initialValue;
});

tearDown(() {
  global_variables.someGlobalVariable = initialValue; // Reset to default
});

Testing Dispose/Cleanup

testWidgets('Given provider disposed, When container disposed, Then unsubscribes and cleans up', () async {
  final container = createContainer();
  final notifier = container.read(provider.notifier);
  await notifier.future;

  container.dispose();

  verify(mockService.unsubscribe(any, any)).called(1);
  verify(mockService.dispose(any)).called(1);
});

Running Tests

Run All Tests

flutter test --coverage
# Or if using FVM:
fvm flutter test --coverage

Run Specific Test File

flutter test test/path/to/your_test.dart
# Or if using FVM:
fvm flutter test test/path/to/your_test.dart

Run Specific Test by Name

flutter test --plain-name "Given valid data"
# Or if using FVM:
fvm flutter test --plain-name "Given valid data"

Generate Coverage Report

flutter test --coverage
genhtml coverage/lcov.info -o coverage/html
# Or if using FVM:
fvm flutter test --coverage
genhtml coverage/lcov.info -o coverage/html

Test Helpers and Utilities

Creating a Test Widget Wrapper

Widget createTestWidget(Widget child) {
  return MaterialApp(
    home: Scaffold(
      body: child,
    ),
  );
}

// With Riverpod
Widget createTestWidgetWithProviders(Widget child, List<Override> overrides) {
  return ProviderScope(
    overrides: overrides,
    child: MaterialApp(
      home: Scaffold(
        body: child,
      ),
    ),
  );
}

Reusable Riverpod Container Helper

ProviderContainer createContainer({List<Override> overrides = const []}) {
  final container = ProviderContainer(overrides: overrides);
  addTearDown(container.dispose);
  return container;
}

Finding Widgets by Type and Text

// Find by type
expect(find.byType(CircularProgressIndicator), findsOneWidget);

// Find by text
expect(find.text('Hello'), findsOneWidget);

// Find by key
expect(find.byKey(const Key('myKey')), findsOneWidget);

// Find descendant
expect(
  find.descendant(
    of: find.byType(Container),
    matching: find.text('Child'),
  ),
  findsOneWidget,
);

// Find ancestor
expect(
  find.ancestor(
    of: find.text('Child'),
    matching: find.byType(Container),
  ),
  findsOneWidget,
);

Matchers for Better Assertions

// Common matchers
expect(value, equals(expected));
expect(value, isNotNull);
expect(value, isNull);
expect(list, isEmpty);
expect(list, isNotEmpty);
expect(list, hasLength(3));
expect(list, contains('item'));
expect(list, containsAll(['a', 'b']));
expect(value, greaterThan(5));
expect(value, lessThan(10));
expect(value, inRange(1, 10));

// Custom matchers
expect(find.byType(Widget), findsOneWidget);
expect(find.byType(Widget), findsNothing);
expect(find.byType(Widget), findsWidgets);
expect(find.byType(Widget), findsNWidgets(3));

Additional Testing Patterns

Testing with GetIt Dependency Injection

setUp(() {
  GetIt.I.registerSingleton<ApiService>(mockApiService);
  GetIt.I.registerLazySingleton<UserRepository>(() => UserRepository());
});

tearDown(() {
  GetIt.I.reset(); // Always reset GetIt between tests
});

Testing Stream-Based Code

test('Given stream emits values, When listening, Then receives all values', () async {
  // Arrange
  final streamController = StreamController<int>();
  final values = <int>[];

  // Act
  streamController.stream.listen(values.add);
  streamController.add(1);
  streamController.add(2);
  streamController.add(3);
  await streamController.close();

  // Wait for stream to complete
  await Future.delayed(Duration.zero);

  // Assert
  expect(values, equals([1, 2, 3]));
});

Testing Timer-Based Code

testWidgets('Given timer completes, When countdown finishes, Then shows message', (tester) async {
  await tester.pumpWidget(MyTimerWidget());

  // Fast-forward time
  await tester.pump(const Duration(seconds: 5));

  expect(find.text('Time is up!'), findsOneWidget);
});

Testing Scrollable Widgets

testWidgets('Given long list, When scrolling, Then finds bottom item', (tester) async {
  await tester.pumpWidget(MyLongListWidget());

  // Scroll until item is visible
  await tester.scrollUntilVisible(
    find.text('Item 99'),
    500.0,
  );

  expect(find.text('Item 99'), findsOneWidget);
});

Resources

This skill includes reference files with detailed patterns and examples:

references/

• layer_testing_patterns.md - Comprehensive examples for testing repositories, providers, DAOs, and services
• widget_testing_guide.md - Detailed widget testing patterns with keys, screen size, and user interactions
• riverpod_testing_guide.md - Advanced Riverpod provider testing patterns and state management testing

Refer to these references when you need specific implementation examples or encounter complex testing scenarios.

Source: ForceInjection/domain-driven-design-skills — distributed by TomeVault.