dart-flutter-patterns

name dart-flutter-patterns description Production-ready Dart and Flutter patterns covering null safety, immutable state, async composition, widget architecture, popular state management frameworks (BLoC, Riverpod, Provider), GoRouter navigation, Dio networking, Freezed code generation, and clean architecture. Use when writing or reviewing Dart and Flutter code — state, widgets, navigation, networking, or architecture. metadata {"origin":"ECC"} Dart/Flutter Patterns When to Use Use this skill when: Starting a new Flutter feature and need idiomatic patterns for state management, navigation, or data access Reviewing or writing Dart code and need guidance on null safety, sealed types, or async composition Setting up a new Flutter project and choosing between BLoC, Riverpod, or Provider Implementing secure HTTP clients, WebView integration, or local storage Writing tests for Flutter widgets, Cubits, or Riverpod providers Wiring up GoRouter with authentication guards How It Works This skill provides copy-paste-ready Dart/Flutter code patterns organized by concern: Null safety — avoid ! , prefer ?. / ?? /pattern matching Immutable state — sealed classes, freezed , copyWith Async composition — concurrent Future.wait , safe BuildContext after await Widget architecture — extract to classes (not methods), const propagation, scoped rebuilds State management — BLoC/Cubit events, Riverpod notifiers and derived providers Navigation — GoRouter with reactive auth guards via refreshListenable Networking — Dio with interceptors, token refresh with one-time retry guard Error handling — global capture, ErrorWidget.builder , crashlytics wiring Testing — unit (BLoC test), widget (ProviderScope overrides), fakes over mocks Examples // Sealed state — prevents impossible states sealed class AsyncState<T> {} final class Loading<T> extends AsyncState<T> {} final class Success<T> extends AsyncState<T> { final T data; const Success(this.data); } final class Failure<T> extends AsyncState<T> { final Object error; const Failure(this.error); } // GoRouter with reactive auth redirect final router = GoRouter( refreshListenable: GoRouterRefreshStream(authCubit.stream), redirect: (context, state) { final authed = context.read<AuthCubit>().state is AuthAuthenticated; if (!authed && !state.matchedLocation.startsWith('/login')) return '/login'; return null; }, routes: [...], ); // Riverpod derived provider with safe firstWhereOrNull @riverpod double cartTotal(Ref ref) { final cart = ref.watch(cartNotifierProvider); final products = ref.watch(productsProvider).valueOrNull ?? []; return cart.fold(0.0, (total, item) { final product = products.firstWhereOrNull((p) => p.id == item.productId); return total + (product?.price ?? 0) * item.quantity; }); } Practical, production-ready patterns for Dart and Flutter applications. Library-agnostic where possible, with explicit coverage of the most common ecosystem packages. 1. Null Safety Fundamentals Prefer Patterns Over Bang Operator // BAD — crashes at runtime if null final name = user!.name; // GOOD — provide fallback final name = user?.name ?? 'Unknown'; // GOOD — Dart 3 pattern matching (preferred for complex cases) final display = switch (user) { User(:final name, :final email) => '$name <$email>', null => 'Guest', }; // GOOD — guard early return String getUserName(User? user) { if (user == null) return 'Unknown'; return user.name; // promoted to non-null after check } Avoid late Overuse // BAD — defers null error to runtime late String userId; // GOOD — nullable with explicit initialization String? userId; // OK — use late only when initialization is guaranteed before first access // (e.g., in initState() before any widget interaction) late final AnimationController _controller; @override void initState() { super.initState(); _controller = AnimationController(vsync: this, duration: const Duration(milliseconds: 300)); } 2. Immutable State Sealed Classes for State Hierarchies sealed class UserState {} final class UserInitial extends UserState {} final class UserLoading extends UserState {} final class UserLoaded extends UserState { const UserLoaded(this.user); final User user; } final class UserError extends UserState { const UserError(this.message); final String message; } // Exhaustive switch — compiler enforces all branches Widget buildFrom(UserState state) => switch (state) { UserInitial() => const SizedBox.shrink(), UserLoading() => const CircularProgressIndicator(), UserLoaded(:final user) => UserCard(user: user), UserError(:final message) => ErrorText(message), }; Freezed for Boilerplate-Free Immutability import 'package:freezed_annotation/freezed_annotation.dart'; part 'user.freezed.dart'; part 'user.g.dart'; @freezed class User with _$User { const factory User({ required String id, required String name, required String email, @Default(false) bool isAdmin, }) = _User; factory User.fromJson(Map<String, dynamic> json) => _$UserFromJson(json); } // Usage final user = User(id: '1', name: 'Alice', email: 'alice@example.com'); final updated = user.copyWith(name: 'Alice Smith'); // immutable update final json = user.toJson(); final fromJson = User.fromJson(json); 3. Async Composition Structured Concurrency with Future.wait Future<DashboardData> loadDashboard(UserRepository users, OrderRepository orders) async { // Run concurrently — don't await sequentially final (userList, orderList) = await ( users.getAll(), orders.getRecent(), ).wait; // Dart 3 record destructuring + Future.wait extension return DashboardData(users: userList, orders: orderList); } Stream Patterns // Repository exposes reactive streams for live data Stream<List<Item>> watchCartItems() => _db .watchTable('cart_items') .map((rows) => rows.map(Item.fromRow).toList()); // In widget layer — declarative, no manual subscription StreamBuilder<List<Item>>( stream: cartRepository.watchCartItems(), builder: (context, snapshot) => switch (snapshot) { AsyncSnapshot(connectionState: ConnectionState.waiting) => const CircularProgressIndicator(), AsyncSnapshot(:final error?) => ErrorWidget(error.toString()), AsyncSnapshot(:final data?) => CartList(items: data), _ => const SizedBox.shrink(), }, ) BuildContext After Await // CRITICAL — always check mounted after any await in StatefulWidget Future<void> _handleSubmit() async { setState(() => _isLoading = true); try { await authService.login(_email, _password); if (!mounted) return; // ← guard before using context context.go('/home'); } on AuthException catch (e) { if (!mounted) return; ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text(e.message))); } finally { if (mounted) setState(() => _isLoading = false); } } 4. Widget Architecture Extract to Classes, Not Methods // BAD — private method returning widget, prevents optimization Widget _buildHeader() { return Container( padding: const EdgeInsets.all(16), child: Text(title, style: Theme.of(context).textTheme.headlineMedium), ); } // GOOD — separate widget class, enables const, element reuse class _PageHeader extends StatelessWidget { const _PageHeader(this.title); final String title; @override Widget build(BuildContext context) { return Container( padding: const EdgeInsets.all(16), child: Text(title, style: Theme.of(context).textTheme.headlineMedium), ); } } const Propagation // BAD — new instances every rebuild child: Padding( padding: EdgeInsets.all(16.0), // not const child: Icon(Icons.home, size: 24.0), // not const ) // GOOD — const stops rebuild propagation child: const Padding( padding: EdgeInsets.all(16.0), child: Icon(Icons.home, size: 24.0), ) Scoped Rebuilds // BAD — entire page rebuilds on every counter change class CounterPage extends ConsumerWidget { @override Widget build(BuildContext context, WidgetRef ref) { final count = ref.watch(counterProvider); // rebuilds everything return Scaffold( body: Column(children: [ const ExpensiveHeader(), // unnecessarily rebuilt Text('$count'), const ExpensiveFooter(), // unnecessarily rebuilt ]), ); } } // GOOD — isolate the rebuilding part class CounterPage extends StatelessWidget { const CounterPage({super.key}); @override Widget build(BuildContext context) { return const Scaffold( body: Column(children: [ ExpensiveHeader(), // never rebuilt (const) _CounterDisplay(), // only this rebuilds ExpensiveFooter(), // never rebuilt (const) ]), ); } } class _CounterDisplay extends ConsumerWidget { const _CounterDisplay(); @override Widget build(BuildContext context, WidgetRef ref) { final count = ref.watch(counterProvider); return Text('$count'); } } 5. State Management: BLoC/Cubit // Cubit — synchronous or simple async state class AuthCubit extends Cubit<AuthState> { AuthCubit(this._authService) : super(const AuthState.initial()); final AuthService _authService; Future<void> login(String email, String password) async { emit(const AuthState.loading()); try { final user = await _authService.login(email, password); emit(AuthState.authenticated(user)); } on AuthException catch (e) { emit(AuthState.error(e.message)); } } void logout() { _authService.logout(); emit(const AuthState.initial()); } } // In widget BlocBuilder<AuthCubit, AuthState>( builder: (context, state) => switch (state) { AuthInitial() => const LoginForm(), AuthLoading() => const CircularProgressIndicator(), AuthAuthenticated(:final user) => HomePage(user: user), AuthError(:final message) => ErrorView(message: message), }, ) 6. State Management: Riverpod // Auto-dispose async provider @riverpod Future<List<Product>> products(Ref ref) async { final repo = ref.watch(productRepositoryProvider); return repo.getAll(); } // Notifier with complex mutations @riverpod class CartNotifier extends _$CartNotifier { @override List<CartItem> build() => []; void add(Product product) { final existing = state.where((i) => i.productId == product.id).firstOrNull; if (existing != null) { state = [ for (final item in state) if (item.productId == product.id) item.copyWith(quantity: item.quantity + 1) else item, ]; } else { state = [...state, CartItem(productId: product.id, quantity: 1)]; } } void remove(String productId) => state = state.where((i) => i.productId != productId).toList(); void clear() => state = []; } // Derived provider (selector pattern) @riverpod int cartCount(Ref ref) => ref.watch(cartNotifierProvider).length; @riverpod double cartTotal(Ref ref) { final cart = ref.watch(cartNotifierProvider); final products = ref.watch(productsProvider).valueOrNull ?? []; return cart.fold(0.0, (total, item) { // firstWhereOrNull (from collection package) avoids StateError when product is missing final product = products.firstWhereOrNull((p) => p.id == item.productId); return total + (product?.price ?? 0) * item.quantity; }); } 7. Navigation with GoRouter final router = GoRouter( initialLocation: '/', // refreshListenable re-evaluates redirect whenever auth state changes refreshListenable: GoRouterRefreshStream(authCubit.stream), redirect: (context, state) { final isLoggedIn = context.read<AuthCubit>().state is AuthAuthenticated; final isGoingToLogin = state.matchedLocation == '/login'; if (!isLoggedIn && !isGoingToLogin) return '/login'; if (isLoggedIn && isGoingToLogin) return '/'; return null; }, routes: [ GoRoute(path: '/login', builder: (_, __) => const LoginPage()), ShellRoute( builder: (context, state, child) => AppShell(child: child), routes: [ GoRoute(path: '/', builder: (_, __) => const HomePage()), GoRoute( path: '/products/:id', builder: (context, state) => ProductDetailPage(id: state.pathParameters['id']!),