Flutter Tester

Overview

Test each architectural layer in isolation using Given-When-Then structure. Always test both success and error paths. Never mock providers — override their dependencies instead.

Reference Files

Load the relevant file based on what you're testing:

Core Principles

1. Layer Isolation

Test each layer against its own mocked dependencies:

2. Given-When-Then Structure

test('Given valid data, When fetchUsers called, Then returns user list', () async {
  // Arrange (Given)
  when(mockDAO.fetchAll()).thenAnswer((_) async => expectedUsers);

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

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

3. Test Organisation

group('UserRepository', () {
  group('fetchUsers', () {
    setUp(() { /* init mocks, register with GetIt */ });
    tearDown(() => GetIt.I.reset()); // Always reset GetIt

    test('Given success ... When ... Then ...', () { });
    test('Given error  ... When ... Then ...', () { });
  });
});

Standard Test Setup

Generate Mocks

@GenerateMocks([IUserDAO, IUserAPI, ILogger])
void main() { ... }

Run dart run build_runner build after modifying @GenerateMocks.

Register with GetIt

setUp(() {
  mockDAO = MockIUserDAO();
  mockLogger = MockILogger();
  GetIt.I
    ..registerSingleton<IUserDAO>(mockDAO)
    ..registerSingleton<ILogger>(mockLogger);
});

tearDown(() => GetIt.I.reset()); // Critical — always reset

Fakes vs Mocks

• Fakes (class FakeLogger extends ILogger) — silent stubs; use when you don't need to verify calls
• Mocks (MockILogger) — use when you need when(), verify(), or thenThrow()

Quick Reference

Running Tests

flutter test --coverage                       # All tests with coverage
flutter test test/path/to/test.dart           # Specific file
flutter test --plain-name "Given valid data"  # Filter by name
genhtml coverage/lcov.info -o coverage/html   # Generate HTML coverage report
# Prefix any command with `fvm` if using Flutter Version Manager

Common Mistakes

Test Checklist

Setup & Mocking:

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

Widget Tests:

• Keys added to source widgets and used in find.byKey()
• Screen size set (physicalSize + devicePixelRatio)
• Platform overrides reset (debugDefaultTargetPlatformOverride = null)
• Navigation verified if applicable

Test Coverage:

• Success and failure paths covered
• Edge cases tested (null, empty, max values)
• Loading and error states tested
• Async handled correctly (no Future.delayed)

Code Quality:

• Given-When-Then naming used
• verify() or verifyNever() where appropriate
• Tests are isolated and deterministic