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:

What you're testing	Reference file
Repository, DAO, Service logic	`references/layer_testing_patterns.md`
Widget UI, interactions, dialogs, navigation	`references/widget_testing_guide.md`
Riverpod provider state, mutations, lifecycle	`references/riverpod_testing_guide.md`

Core Principles

1. Layer Isolation

Test each layer against its own mocked dependencies:

Layer	What to test	What to mock
Repository	Data coordination between sources	DAOs, APIs, Logger
DAO	Database CRUD operations	Use real in-memory DB, mock Logger
Provider	State management and transitions	Services, Repositories
Service	Business logic and workflows	Repositories, Network clients
Widget	UI behaviour and interactions	Provider dependencies (via overrides)

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

Scenario	Key pattern
Test a repository	Mock DAO + API → inject into repository constructor
Test a DAO	`FakeDatabase` or `openInMemoryDatabase()` in setUp, delete table in tearDown
Test a Riverpod provider	`createContainer(overrides: [serviceProvider.overrideWith(...)])`
Test a widget	Set screen size, use `find.byKey()`, call `pumpAndSettle()`
Test a loading state	Use `Completer`, `pump()` to assert loading, complete, `pump()` again
Test platform-specific UI	`debugDefaultTargetPlatformOverride = TargetPlatform.iOS` — reset after
Test GoRouter navigation	`FakeGoRouter` + `MockGoRouterProvider`

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

Mistake	Fix
Mocking a provider directly	Override its dependencies: `provider.overrideWith(...)`
Missing `GetIt.I.reset()` in `tearDown`	Tests pollute each other — always reset
`await Future.delayed()` in tests	Use `await tester.pumpAndSettle()` or `Completer` instead
Finding widgets by text string	Use `find.byKey(const Key('name'))` — stable across text changes
No screen size in widget tests	Add `tester.view.physicalSize = const Size(1000, 1000)`
Not resetting `debugDefaultTargetPlatformOverride`	Set to `null` at the end of the test
`tearDown()` without a lambda	Write `tearDown(() async { ... })` not `tearDown() async { ... }`

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

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