Dart Flutter Patterns

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: '[email protected]');
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