best-practices

name	best-practices
description	Flutter performance optimization and clean architecture patterns. This skill should be used when writing, reviewing, or refactoring Flutter/Dart code to ensure optimal performance patterns. Triggers on tasks involving Flutter widgets, state management, async patterns, memory management, or architecture design. Use when this capability is needed.
metadata	{"author":"devnogari"}

Flutter Best Practices

Comprehensive performance optimization guide for Flutter applications. Contains 42 rules across 8 categories, prioritized by impact to guide automated refactoring and code generation.

When to Apply

Reference these guidelines when:

• Writing new widgets or screens
• Implementing state management (BLoC, Riverpod, Provider)
• Reviewing code for performance issues
• Refactoring existing Flutter code
• Optimizing build/render performance
• Debugging memory leaks or jank

Rule Categories by Priority

Priority	Category	Impact	Prefix
1	Widget Optimization	CRITICAL	widget-
2	State Management	CRITICAL	state-
3	Async Patterns	HIGH	async-
4	Memory Management	HIGH	mem-
5	Rendering Performance	MEDIUM	render-
6	Architecture	MEDIUM	arch-
7	Testing	MEDIUM	test-
8	Platform Integration	LOW	platform-

Quick Reference

1. Widget Optimization (CRITICAL)

• widget-const - Use const constructors to prevent unnecessary rebuilds
• widget-split - Split widgets to minimize rebuild scope
• widget-key - Use keys correctly for widget identity and state preservation
• widget-builder - Use Builder widgets to limit rebuild context
• widget-repaint - Use RepaintBoundary to isolate expensive paints
• widget-sliver - Use Slivers for efficient scrolling lists

2. State Management (CRITICAL)

• state-bloc - Use BLoC pattern for complex business logic
• state-riverpod - Use Riverpod for reactive state with compile-time safety
• state-provider - Scope Provider to minimize rebuild area
• state-consumer - Use Consumer/BlocBuilder with buildWhen/select
• state-selector - Use Selector to watch specific state properties
• state-rebuild - Avoid unnecessary setState calls

3. Async Patterns (HIGH)

• async-future-builder - Handle all FutureBuilder states correctly
• async-stream - Use StreamBuilder with proper stream lifecycle
• async-cancel - Cancel async operations on widget disposal
• async-error - Handle errors with try-catch and error states
• async-cache - Cache futures to prevent refetch on rebuild
• async-isolate - Use Isolates for heavy computation

4. Memory Management (HIGH)

• mem-dispose - Always dispose controllers and subscriptions
• mem-controller - Dispose TextEditingController, ScrollController, AnimationController
• mem-subscription - Cancel StreamSubscriptions in dispose
• mem-image - Use cached_network_image and precacheImage
• mem-cache - Implement LRU cache for expensive objects
• mem-leak - Avoid closures capturing BuildContext across async gaps

5. Rendering Performance (MEDIUM)

• render-boundary - Use RepaintBoundary for animation isolation
• render-clip - Avoid ClipRRect when possible, use decoration instead
• render-opacity - Use Opacity sparingly, prefer AnimatedOpacity
• render-transform - Use Transform with alignment for GPU acceleration
• render-list - Use ListView.builder for long lists
• render-animation - Use AnimatedBuilder to limit animation rebuilds

6. Architecture (MEDIUM)

• arch-layer - Separate presentation, domain, and data layers
• arch-repository - Use Repository pattern for data abstraction
• arch-usecase - Encapsulate business logic in UseCases
• arch-entity - Use immutable Entities with Equatable
• arch-di - Use dependency injection (get_it, injectable)
• arch-router - Use declarative routing (go_router, auto_route)

7. Testing (MEDIUM)

• test-widget - Write widget tests with WidgetTester
• test-golden - Use golden tests for visual regression
• test-integration - Write integration tests for user flows
• test-mock - Use Mockito for dependency mocking
• test-pump - Use pumpAndSettle for async widget testing
• test-finder - Use semantic finders over positional

8. Platform Integration (LOW)

• platform-channel - Use MethodChannel for native communication
• platform-plugin - Create platform-specific plugins correctly
• platform-native - Handle platform differences with defaultTargetPlatform
• platform-web - Optimize for web with conditional imports
• platform-desktop - Handle desktop-specific input and window management

---

Detailed Rules

1. Widget Optimization (CRITICAL)

widget-const - Use const constructors

Const widgets are cached and reused, preventing unnecessary rebuilds. Always mark widgets as const when possible.

Incorrect (rebuilds on parent rebuild):

class MyWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      padding: EdgeInsets.all(16),
      child: Text('Hello World'),
    );
  }
}

// Usage - creates new instance every build
Scaffold(body: MyWidget())

Correct (const prevents rebuild):

class MyWidget extends StatelessWidget {
  const MyWidget({super.key});

  @override
  Widget build(BuildContext context) {
    return const Padding(
      padding: EdgeInsets.all(16),
      child: Text('Hello World'),
    );
  }
}

// Usage - reuses cached instance
Scaffold(body: const MyWidget())

Impact: 10-50% reduction in widget rebuilds, significant memory savings.

---

widget-split - Split widgets to minimize rebuild scope

Extract parts of the widget tree into separate widgets to isolate rebuilds. When state changes, only the widget containing the state rebuilds.

Incorrect (entire page rebuilds on counter change):

class CounterPage extends StatefulWidget {
  @override
  State<CounterPage> createState() => _CounterPageState();
}

class _CounterPageState extends State<CounterPage> {
  int counter = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Counter')),
      body: Column(
        children: [
          ExpensiveWidget(),  // rebuilds unnecessarily!
          HeavyImageWidget(), // rebuilds unnecessarily!
          Text('Count: $counter'),
          ElevatedButton(
            onPressed: () => setState(() => counter++),
            child: Text('Increment'),
          ),
        ],
      ),
    );
  }
}

Correct (split into separate widgets):

class CounterPage extends StatelessWidget {
  const CounterPage({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: const Text('Counter')),
      body: const Column(
        children: [
          ExpensiveWidget(),  // doesn't rebuild
          HeavyImageWidget(), // doesn't rebuild
          CounterDisplay(),   // only this rebuilds
        ],
      ),
    );
  }
}

class CounterDisplay extends StatefulWidget {
  const CounterDisplay({super.key});

  @override
  State<CounterDisplay> createState() => _CounterDisplayState();
}

class _CounterDisplayState extends State<CounterDisplay> {
  int counter = 0;

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        Text('Count: $counter'),
        ElevatedButton(
          onPressed: () => setState(() => counter++),
          child: const Text('Increment'),
        ),
      ],
    );
  }
}

Impact: 2-10x reduction in rebuild scope, smoother animations.

---

widget-key - Use keys correctly

Keys preserve widget state and identity across rebuilds. Use ValueKey for data-driven widgets, GlobalKey sparingly for accessing state.

Incorrect (state lost on reorder):

ListView(
  children: items.map((item) =>
    ItemTile(item: item)  // state lost when items reorder
  ).toList(),
)

Correct (state preserved with keys):

ListView(
  children: items.map((item) =>
    ItemTile(key: ValueKey(item.id), item: item)
  ).toList(),
)

// For form fields that need external access
final _formKey = GlobalKey<FormState>();
Form(
  key: _formKey,
  child: ...,
)
// Access: _formKey.currentState?.validate()

Impact: Prevents state bugs, correct animations in lists.

---

widget-builder - Use Builder widgets

Builder widgets create a new BuildContext, limiting the scope of InheritedWidget lookups and rebuilds.

Incorrect (entire widget rebuilds on theme change):

class MyPage extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final theme = Theme.of(context);  // subscribes entire widget
    return Scaffold(
      body: Column(
        children: [
          ExpensiveWidget(),
          Text('Themed', style: theme.textTheme.bodyLarge),
        ],
      ),
    );
  }
}

Correct (only Builder child rebuilds):

class MyPage extends StatelessWidget {
  const MyPage({super.key});

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Column(
        children: [
          const ExpensiveWidget(),
          Builder(
            builder: (context) {
              final theme = Theme.of(context);
              return Text('Themed', style: theme.textTheme.bodyLarge);
            },
          ),
        ],
      ),
    );
  }
}

---

widget-repaint - Use RepaintBoundary

RepaintBoundary isolates paint operations, preventing expensive widgets from causing full repaints.

Incorrect (animation causes full repaint):

Stack(
  children: [
    ComplexBackground(),
    AnimatedWidget(),  // causes ComplexBackground to repaint
  ],
)

Correct (animation isolated):

Stack(
  children: [
    const RepaintBoundary(
      child: ComplexBackground(),
    ),
    AnimatedWidget(),  // only repaints its own layer
  ],
)

Impact: 60fps animations, reduced GPU usage.

---

widget-sliver - Use Slivers for lists

Slivers lazily build only visible items, essential for long scrolling lists.

Incorrect (all items built at once):

SingleChildScrollView(
  child: Column(
    children: items.map((item) => ItemWidget(item)).toList(),
  ),
)

Correct (only visible items built):

CustomScrollView(
  slivers: [
    SliverAppBar(title: Text('Items')),
    SliverList(
      delegate: SliverChildBuilderDelegate(
        (context, index) => ItemWidget(items[index]),
        childCount: items.length,
      ),
    ),
  ],
)

// Or simpler with ListView.builder
ListView.builder(
  itemCount: items.length,
  itemBuilder: (context, index) => ItemWidget(items[index]),
)

Impact: O(visible) vs O(n) build time, constant memory usage.

---

2. State Management (CRITICAL)

state-bloc - Use BLoC for complex state

BLoC (Business Logic Component) separates business logic from UI with events and states.

Correct BLoC implementation:

// Events
sealed class UserEvent {}
class LoadUser extends UserEvent {
  final String userId;
  LoadUser(this.userId);
}
class UpdateUser extends UserEvent {
  final User user;
  UpdateUser(this.user);
}

// States
sealed class UserState {}
class UserInitial extends UserState {}
class UserLoading extends UserState {}
class UserLoaded extends UserState {
  final User user;
  UserLoaded(this.user);
}
class UserError extends UserState {
  final String message;
  UserError(this.message);
}

// BLoC
class UserBloc extends Bloc<UserEvent, UserState> {
  final UserRepository _repository;

  UserBloc(this._repository) : super(UserInitial()) {
    on<LoadUser>(_onLoadUser);
    on<UpdateUser>(_onUpdateUser);
  }

  Future<void> _onLoadUser(LoadUser event, Emitter<UserState> emit) async {
    emit(UserLoading());
    try {
      final user = await _repository.getUser(event.userId);
      emit(UserLoaded(user));
    } catch (e) {
      emit(UserError(e.toString()));
    }
  }

  Future<void> _onUpdateUser(UpdateUser event, Emitter<UserState> emit) async {
    emit(UserLoading());
    try {
      await _repository.updateUser(event.user);
      emit(UserLoaded(event.user));
    } catch (e) {
      emit(UserError(e.toString()));
    }
  }
}

---

state-consumer - Use buildWhen/select

Only rebuild when relevant state changes using buildWhen or select.

Incorrect (rebuilds on any state change):

BlocBuilder<UserBloc, UserState>(
  builder: (context, state) {
    // Rebuilds for UserLoading, UserLoaded, UserError...
    if (state is UserLoaded) {
      return UserProfile(user: state.user);
    }
    return const SizedBox.shrink();
  },
)

Correct (rebuild only when needed):

BlocBuilder<UserBloc, UserState>(
  buildWhen: (previous, current) =>
    current is UserLoaded && previous is! UserLoaded,
  builder: (context, state) {
    if (state is UserLoaded) {
      return UserProfile(user: state.user);
    }
    return const SizedBox.shrink();
  },
)

// Or use BlocSelector for specific property
BlocSelector<UserBloc, UserState, String?>(
  selector: (state) => state is UserLoaded ? state.user.name : null,
  builder: (context, name) {
    return Text(name ?? 'Loading...');
  },
)

Impact: 50-90% reduction in unnecessary rebuilds.

---

state-riverpod - Use Riverpod for compile-time safety

Riverpod provides compile-time safety, automatic disposal, and no BuildContext requirement.

Correct Riverpod implementation:

// Define providers
final userRepositoryProvider = Provider((ref) => UserRepository());

final userProvider = FutureProvider.family<User, String>((ref, userId) async {
  final repository = ref.watch(userRepositoryProvider);
  return repository.getUser(userId);
});

final userNotifierProvider = NotifierProvider<UserNotifier, UserState>(() {
  return UserNotifier();
});

class UserNotifier extends Notifier<UserState> {
  @override
  UserState build() => UserInitial();

  Future<void> loadUser(String userId) async {
    state = UserLoading();
    try {
      final user = await ref.read(userRepositoryProvider).getUser(userId);
      state = UserLoaded(user);
    } catch (e) {
      state = UserError(e.toString());
    }
  }
}

// Usage in widget
class UserPage extends ConsumerWidget {
  const UserPage({super.key, required this.userId});
  final String userId;

  @override
  Widget build(BuildContext context, WidgetRef ref) {
    final userAsync = ref.watch(userProvider(userId));

    return userAsync.when(
      loading: () => const CircularProgressIndicator(),
      error: (error, stack) => Text('Error: $error'),
      data: (user) => UserProfile(user: user),
    );
  }
}

---

state-selector - Watch specific properties

Use Selector or select extension to watch only the state properties you need.

Incorrect (rebuilds on any cart change):

Consumer<CartProvider>(
  builder: (context, cart, child) {
    return Text('Items: ${cart.items.length}');
    // Rebuilds when total changes too!
  },
)

Correct (rebuilds only when count changes):

Selector<CartProvider, int>(
  selector: (context, cart) => cart.items.length,
  builder: (context, itemCount, child) {
    return Text('Items: $itemCount');
  },
)

// With Riverpod
ref.watch(cartProvider.select((cart) => cart.items.length))

---

state-rebuild - Avoid unnecessary setState

Only call setState when state actually changes, and batch multiple changes.

Incorrect (multiple rebuilds):

void updateUser() {
  setState(() => _name = newName);
  setState(() => _email = newEmail);  // second rebuild!
  setState(() => _phone = newPhone);  // third rebuild!
}

Correct (single rebuild):

void updateUser() {
  setState(() {
    _name = newName;
    _email = newEmail;
    _phone = newPhone;
  });
}

// Or check if value changed
void setName(String name) {
  if (_name != name) {
    setState(() => _name = name);
  }
}

---

3. Async Patterns (HIGH)

async-future-builder - Handle all states

Always handle waiting, error, and data states in FutureBuilder.

Correct implementation:

FutureBuilder<User>(
  future: _userFuture,
  builder: (context, snapshot) {
    // Handle connection state
    if (snapshot.connectionState == ConnectionState.waiting) {
      return const Center(child: CircularProgressIndicator());
    }

    // Handle error
    if (snapshot.hasError) {
      return Center(
        child: Column(
          mainAxisSize: MainAxisSize.min,
          children: [
            Text('Error: ${snapshot.error}'),
            ElevatedButton(
              onPressed: _retryLoad,
              child: const Text('Retry'),
            ),
          ],
        ),
      );
    }

    // Handle no data
    if (!snapshot.hasData) {
      return const Center(child: Text('No user found'));
    }

    // Success
    return UserProfile(user: snapshot.data!);
  },
)

---

async-cache - Cache futures to prevent refetch

Cache futures in state to prevent refetching on every rebuild.

Incorrect (refetches every rebuild):

class _MyWidgetState extends State<MyWidget> {
  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future: fetchUser(),  // Called every build!
      builder: (context, snapshot) => ...,
    );
  }
}

Correct (cached in state):

class _MyWidgetState extends State<MyWidget> {
  late final Future<User> _userFuture;

  @override
  void initState() {
    super.initState();
    _userFuture = fetchUser();
  }

  @override
  Widget build(BuildContext context) {
    return FutureBuilder(
      future: _userFuture,  // Cached, not refetched
      builder: (context, snapshot) => ...,
    );
  }
}

---

async-cancel - Cancel async operations

Cancel async operations when widget disposes to prevent setState on disposed widget.

Correct implementation:

class _MyWidgetState extends State<MyWidget> {
  CancelableOperation<User>? _operation;

  @override
  void initState() {
    super.initState();
    _loadUser();
  }

  Future<void> _loadUser() async {
    _operation = CancelableOperation.fromFuture(fetchUser());
    try {
      final user = await _operation!.value;
      if (mounted) {
        setState(() => _user = user);
      }
    } catch (e) {
      if (mounted) {
        setState(() => _error = e.toString());
      }
    }
  }

  @override
  void dispose() {
    _operation?.cancel();
    super.dispose();
  }
}

---

async-isolate - Use Isolates for heavy computation

Move CPU-intensive work to Isolates to keep UI responsive.

Correct implementation:

// For simple functions
Future<List<Item>> parseJsonInBackground(String json) async {
  return compute(_parseJson, json);
}

List<Item> _parseJson(String json) {
  final data = jsonDecode(json) as List;
  return data.map((e) => Item.fromJson(e)).toList();
}

// For complex operations with multiple calls
class ImageProcessor {
  late final SendPort _sendPort;

  Future<void> init() async {
    final receivePort = ReceivePort();
    await Isolate.spawn(_isolateEntry, receivePort.sendPort);
    _sendPort = await receivePort.first;
  }

  Future<Uint8List> processImage(Uint8List image) async {
    final response = ReceivePort();
    _sendPort.send([image, response.sendPort]);
    return await response.first;
  }

  static void _isolateEntry(SendPort sendPort) {
    final receivePort = ReceivePort();
    sendPort.send(receivePort.sendPort);

    receivePort.listen((message) {
      final image = message[0] as Uint8List;
      final replyPort = message[1] as SendPort;
      final processed = _heavyImageProcessing(image);
      replyPort.send(processed);
    });
  }
}

Impact: 60fps UI during heavy computation.

---

4. Memory Management (HIGH)

mem-dispose - Always dispose resources

Dispose all controllers, subscriptions, and listeners in dispose().

Correct implementation:

class _MyWidgetState extends State<MyWidget> {
  final _textController = TextEditingController();
  final _scrollController = ScrollController();
  late final AnimationController _animationController;
  StreamSubscription? _subscription;
  Timer? _timer;

  @override
  void initState() {
    super.initState();
    _animationController = AnimationController(vsync: this);
    _subscription = eventStream.listen(_handleEvent);
    _timer = Timer.periodic(Duration(seconds: 1), _tick);
  }

  @override
  void dispose() {
    _textController.dispose();
    _scrollController.dispose();
    _animationController.dispose();
    _subscription?.cancel();
    _timer?.cancel();
    super.dispose();
  }
}

---

mem-leak - Avoid context capture across async gaps

Don't capture BuildContext in closures that outlive the widget.

Incorrect (context captured across async gap):

void _handleTap() async {
  final result = await longRunningOperation();
  // Widget may be disposed by now!
  Navigator.of(context).push(...);  // CRASH!
  ScaffoldMessenger.of(context).showSnackBar(...);  // CRASH!
}

Correct (check mounted, capture early):

void _handleTap() async {
  final navigator = Navigator.of(context);
  final messenger = ScaffoldMessenger.of(context);

  final result = await longRunningOperation();

  if (!mounted) return;

  navigator.push(...);
  messenger.showSnackBar(...);
}

---

mem-image - Optimize image memory

Use cached_network_image and control cache size for images.

Correct implementation:

// Use cached_network_image
CachedNetworkImage(
  imageUrl: url,
  memCacheWidth: 200,  // Decode at display size
  memCacheHeight: 200,
  maxWidthDiskCache: 400,
  maxHeightDiskCache: 400,
  placeholder: (context, url) => const CircularProgressIndicator(),
  errorWidget: (context, url, error) => const Icon(Icons.error),
)

// Precache images
@override
void didChangeDependencies() {
  super.didChangeDependencies();
  precacheImage(AssetImage('assets/logo.png'), context);
}

// Clear image cache when memory pressure
@override
void didReceiveMemoryWarning() {
  imageCache.clear();
  imageCache.clearLiveImages();
}

---

5. Rendering Performance (MEDIUM)

render-list - Use ListView.builder

Use builder constructors for lists to build only visible items.

Incorrect (builds all items):

ListView(
  children: items.map((item) => ItemTile(item)).toList(),
)

Correct (builds only visible):

ListView.builder(
  itemCount: items.length,
  itemBuilder: (context, index) => ItemTile(items[index]),
  // Optional: provide item extent for better performance
  itemExtent: 80,
)

// For very long lists with variable heights
ListView.builder(
  itemCount: items.length,
  itemBuilder: (context, index) => ItemTile(items[index]),
  addAutomaticKeepAlives: false,
  addRepaintBoundaries: true,
)

---

render-animation - Use AnimatedBuilder

Isolate animation rebuilds with AnimatedBuilder.

Incorrect (entire widget rebuilds):

class _MyWidgetState extends State<MyWidget>
    with SingleTickerProviderStateMixin {
  late final AnimationController _controller;

  @override
  Widget build(BuildContext context) {
    return Transform.rotate(
      angle: _controller.value * 2 * pi,
      child: Column(
        children: [
          ExpensiveWidget(),  // rebuilds 60 times/second!
          Icon(Icons.star),
        ],
      ),
    );
  }
}

Correct (only transform rebuilds):

@override
Widget build(BuildContext context) {
  return AnimatedBuilder(
    animation: _controller,
    child: const Column(
      children: [
        ExpensiveWidget(),  // built once
        Icon(Icons.star),
      ],
    ),
    builder: (context, child) {
      return Transform.rotate(
        angle: _controller.value * 2 * pi,
        child: child,  // reused
      );
    },
  );
}

---

render-clip - Avoid ClipRRect when possible

ClipRRect is expensive. Use decoration when you only need rounded corners.

Incorrect (uses clip):

ClipRRect(
  borderRadius: BorderRadius.circular(8),
  child: Container(
    color: Colors.blue,
    child: Text('Hello'),
  ),
)

Correct (uses decoration):

Container(
  decoration: BoxDecoration(
    color: Colors.blue,
    borderRadius: BorderRadius.circular(8),
  ),
  child: Text('Hello'),
)

---

6. Architecture (MEDIUM)

arch-layer - Separate layers

Use clean architecture layers: presentation, domain, data.

Correct structure:

lib/
  features/
    user/
      presentation/
        pages/
          user_page.dart
        widgets/
          user_card.dart
        bloc/
          user_bloc.dart
          user_event.dart
          user_state.dart
      domain/
        entities/
          user.dart
        repositories/
          user_repository.dart
        usecases/
          get_user.dart
          update_user.dart
      data/
        datasources/
          user_remote_datasource.dart
          user_local_datasource.dart
        models/
          user_model.dart
        repositories/
          user_repository_impl.dart

---

arch-repository - Use Repository pattern

Abstract data sources behind repository interfaces.

Correct implementation:

// Domain layer - abstract repository
abstract class UserRepository {
  Future<User> getUser(String id);
  Future<void> updateUser(User user);
  Future<List<User>> searchUsers(String query);
}

// Data layer - concrete implementation
class UserRepositoryImpl implements UserRepository {
  final UserRemoteDataSource _remoteDataSource;
  final UserLocalDataSource _localDataSource;
  final NetworkInfo _networkInfo;

  UserRepositoryImpl({
    required UserRemoteDataSource remoteDataSource,
    required UserLocalDataSource localDataSource,
    required NetworkInfo networkInfo,
  })  : _remoteDataSource = remoteDataSource,
        _localDataSource = localDataSource,
        _networkInfo = networkInfo;

  @override
  Future<User> getUser(String id) async {
    if (await _networkInfo.isConnected) {
      final user = await _remoteDataSource.getUser(id);
      await _localDataSource.cacheUser(user);
      return user;
    } else {
      return _localDataSource.getUser(id);
    }
  }
}

---

arch-di - Use dependency injection

Use get_it or injectable for dependency injection.

Correct implementation:

// injection_container.dart
final sl = GetIt.instance;

Future<void> init() async {
  // BLoCs
  sl.registerFactory(() => UserBloc(sl()));

  // Use Cases
  sl.registerLazySingleton(() => GetUser(sl()));
  sl.registerLazySingleton(() => UpdateUser(sl()));

  // Repositories
  sl.registerLazySingleton<UserRepository>(
    () => UserRepositoryImpl(
      remoteDataSource: sl(),
      localDataSource: sl(),
      networkInfo: sl(),
    ),
  );

  // Data Sources
  sl.registerLazySingleton<UserRemoteDataSource>(
    () => UserRemoteDataSourceImpl(client: sl()),
  );
  sl.registerLazySingleton<UserLocalDataSource>(
    () => UserLocalDataSourceImpl(sharedPreferences: sl()),
  );

  // External
  final sharedPreferences = await SharedPreferences.getInstance();
  sl.registerLazySingleton(() => sharedPreferences);
  sl.registerLazySingleton(() => http.Client());
}

---

7. Testing (MEDIUM)

test-widget - Write widget tests

Use WidgetTester for comprehensive widget testing.

Correct implementation:

void main() {
  group('UserCard', () {
    testWidgets('displays user name', (tester) async {
      final user = User(id: '1', name: 'John Doe', email: '[e-mail]');

      await tester.pumpWidget(
        MaterialApp(
          home: UserCard(user: user),
        ),
      );

      expect(find.text('John Doe'), findsOneWidget);
      expect(find.text('[e-mail]'), findsOneWidget);
    });

    testWidgets('calls onTap when tapped', (tester) async {
      var tapped = false;
      final user = User(id: '1', name: 'John', email: '[e-mail]');

      await tester.pumpWidget(
        MaterialApp(
          home: UserCard(
            user: user,
            onTap: () => tapped = true,
          ),
        ),
      );

      await tester.tap(find.byType(UserCard));
      expect(tapped, isTrue);
    });
  });
}

---

test-pump - Use pumpAndSettle correctly

Use pump for controlled animation frames, pumpAndSettle for waiting animations.

Correct implementation:

testWidgets('shows loading then data', (tester) async {
  await tester.pumpWidget(MyApp());

  // Initially shows loading
  expect(find.byType(CircularProgressIndicator), findsOneWidget);

  // Pump until animations complete and futures resolve
  await tester.pumpAndSettle();

  // Now shows data
  expect(find.byType(CircularProgressIndicator), findsNothing);
  expect(find.byType(UserList), findsOneWidget);
});

testWidgets('animation test with controlled frames', (tester) async {
  await tester.pumpWidget(AnimatedWidget());

  // Advance 100ms
  await tester.pump(const Duration(milliseconds: 100));
  expect(find.byType(FadeTransition), findsOneWidget);

  // Advance to completion
  await tester.pump(const Duration(milliseconds: 200));
});

---

test-mock - Use Mockito for mocking

Mock dependencies with Mockito and mocktail.

Correct implementation:

import 'package:mocktail/mocktail.dart';

class MockUserRepository extends Mock implements UserRepository {}

void main() {
  late UserBloc bloc;
  late MockUserRepository mockRepository;

  setUp(() {
    mockRepository = MockUserRepository();
    bloc = UserBloc(mockRepository);
  });

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

  group('LoadUser', () {
    final testUser = User(id: '1', name: 'John', email: '[e-mail]');

    test('emits [Loading, Loaded] when successful', () async {
      when(() => mockRepository.getUser('1'))
          .thenAnswer((_) async => testUser);

      bloc.add(LoadUser('1'));

      await expectLater(
        bloc.stream,
        emitsInOrder([
          isA<UserLoading>(),
          isA<UserLoaded>().having((s) => s.user, 'user', testUser),
        ]),
      );
    });

    test('emits [Loading, Error] when fails', () async {
      when(() => mockRepository.getUser('1'))
          .thenThrow(Exception('Network error'));

      bloc.add(LoadUser('1'));

      await expectLater(
        bloc.stream,
        emitsInOrder([
          isA<UserLoading>(),
          isA<UserError>(),
        ]),
      );
    });
  });
}

---

8. Platform Integration (LOW)

platform-channel - Use MethodChannel correctly

Implement platform channels with proper error handling.

Correct implementation:

class BatteryService {
  static const _channel = MethodChannel('com.example.app/battery');

  Future<int> getBatteryLevel() async {
    try {
      final int result = await _channel.invokeMethod('getBatteryLevel');
      return result;
    } on PlatformException catch (e) {
      throw BatteryException('Failed to get battery level: ${e.message}');
    }
  }

  Stream<int> batteryLevelStream() {
    const eventChannel = EventChannel('com.example.app/battery_stream');
    return eventChannel
        .receiveBroadcastStream()
        .map((event) => event as int);
  }
}

// Usage
final batteryService = BatteryService();
final level = await batteryService.getBatteryLevel();

---

platform-native - Handle platform differences

Use defaultTargetPlatform for platform-specific behavior.

Correct implementation:

import 'dart:io' show Platform;
import 'package:flutter/foundation.dart' show kIsWeb, defaultTargetPlatform;

Widget buildPlatformWidget() {
  if (kIsWeb) {
    return WebWidget();
  }

  switch (defaultTargetPlatform) {
    case TargetPlatform.iOS:
      return CupertinoWidget();
    case TargetPlatform.android:
      return MaterialWidget();
    case TargetPlatform.macOS:
    case TargetPlatform.windows:
    case TargetPlatform.linux:
      return DesktopWidget();
    default:
      return MaterialWidget();
  }
}

// Platform-specific styling
EdgeInsets get platformPadding {
  if (kIsWeb) return const EdgeInsets.all(24);
  if (Platform.isIOS) return const EdgeInsets.all(16);
  return const EdgeInsets.all(12);
}

---

Integration Workflow

When Writing New Widgets

1. Start with const constructor (widget-const)
2. Identify state boundaries and split widgets (widget-split)
3. Add RepaintBoundary for animations (widget-repaint)
4. Use Builder for InheritedWidget access (widget-builder)

When Implementing State Management

1. Choose BLoC for complex logic (state-bloc) or Riverpod for reactive patterns (state-riverpod)
2. Use buildWhen/select to limit rebuilds (state-consumer, state-selector)
3. Scope providers appropriately (state-provider)

When Working with Async Operations

1. Cache futures in initState (async-cache)
2. Handle all FutureBuilder states (async-future-builder)
3. Cancel operations in dispose (async-cancel)
4. Use Isolates for heavy work (async-isolate)

When Optimizing Performance

1. Profile with Flutter DevTools first
2. Add RepaintBoundary to animations (render-boundary)
3. Use ListView.builder for lists (render-list)
4. Dispose all resources (mem-dispose)

---

References

• Flutter Performance Best Practices
• Effective Dart
• Flutter BLoC Library
• Riverpod Documentation
• Flutter Testing

---

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