Riverpod Skill

This skill defines how to correctly use Riverpod for state management in Flutter and Dart applications.

1. Setup

dart
void main() {
  runApp(const ProviderScope(child: MyApp()));
}

• Wrap your app with ProviderScope directly in runApp — never inside MyApp.
• Install and use riverpod_lint to enable IDE refactoring and enforce best practices.

2. Defining Providers

dart
// Functional provider (codegen)
@riverpod
int example(Ref ref) => 0;

// FutureProvider (codegen)
@riverpod
Future<List<Todo>> todos(Ref ref) async {
  return ref.watch(repositoryProvider).fetchTodos();
}

// Notifier (codegen)
@riverpod
class TodosNotifier extends _$TodosNotifier {
  @override
  Future<List<Todo>> build() async {
    return ref.watch(repositoryProvider).fetchTodos();
  }

  Future<void> addTodo(Todo todo) async { ... }
}

• Define all providers as final top-level variables.
• Use Provider, FutureProvider, or StreamProvider based on the return type.
• Use ConsumerWidget or ConsumerStatefulWidget instead of StatelessWidget/StatefulWidget when accessing providers.

3. Using Ref

| Method | Use for | |---|---| | ref.watch | Reactively listen — rebuilds when value changes. Use during build phase only. | | ref.read | One-time access — use in callbacks/Notifier methods, not in build. | | ref.listen | Imperative subscription — prefer ref.watch where possible. | | ref.onDispose | Cleanup when provider state is destroyed. |

dart
// In a widget
class MyWidget extends ConsumerWidget {
  @override
  Widget build(BuildContext context, WidgetRef ref) {
    final value = ref.watch(myProvider);
    return Text('$value');
  }
}

// Cleanup in a provider
final provider = StreamProvider<int>((ref) {
  final controller = StreamController<int>();
  ref.onDispose(controller.close);
  return controller.stream;
});

• Never call ref.watch inside callbacks, listeners, or Notifier methods.
• Use ref.read(yourNotifierProvider.notifier).method() to call Notifier methods from the UI.
• Check context.mounted before using ref after an await in async callbacks.

4. Combining Providers

dart
@riverpod
Future<String> userGreeting(Ref ref) async {
  final user = await ref.watch(userProvider.future);
  return 'Hello, ${user.name}!';
}

• Use ref.watch(asyncProvider.future) to await an async provider's resolved value.
• Providers only execute once and cache the result — multiple widgets listening to the same provider share one computation.

5. Passing Arguments (Families)

dart
@riverpod
Future<Todo> todo(Ref ref, String id) async {
  return ref.watch(repositoryProvider).fetchTodo(id);
}

// Usage
final todo = ref.watch(todoProvider('some-id'));

• Always enable autoDispose for parameterized providers to prevent memory leaks.
• Use Dart 3 records or code generation for multiple parameters — they naturally override ==.
• Avoid passing plain List or Map as parameters (no == override); use const collections, records, or classes with proper equality.
• Use the provider_parameters lint rule from riverpod_lint to catch equality mistakes.

6. Auto Dispose & State Lifecycle

• With codegen: state is destroyed by default when no longer listened to. Opt out with keepAlive: true.
• Without codegen: state is kept alive by default. Use .autoDispose to enable disposal.
• State is always destroyed when a provider is recomputed.

dart
// keepAlive with timer
ref.onCancel(() {
  final link = ref.keepAlive();
  Timer(const Duration(minutes: 5), link.close);
});

• Use ref.onDispose for cleanup; do not trigger side effects or modify providers inside it.
• Use ref.invalidate(provider) to force destruction; use ref.invalidateSelf() from within the provider.
• Use ref.refresh(provider) to invalidate and immediately read the new value — always use the return value.

7. Eager Initialization

Providers are lazy by default. To eagerly initialize:

dart
// In MyApp or a dedicated widget under ProviderScope:
Consumer(
  builder: (context, ref, _) {
    ref.watch(myEagerProvider); // forces initialization
    return const MyApp();
  },
)

• Place eager initialization in a public widget (not main()) for consistent test behavior.
• Use AsyncValue.requireValue to read data directly and throw clearly if not ready.

8. Performing Side Effects

dart
@riverpod
class TodosNotifier extends _$TodosNotifier {
  Future<void> addTodo(Todo todo) async {
    state = const AsyncLoading();
    state = await AsyncValue.guard(() async {
      await ref.read(repositoryProvider).addTodo(todo);
      return [...?state.value, todo];
    });
  }
}

// In UI:
ElevatedButton(
  onPressed: () => ref.read(todosNotifierProvider.notifier).addTodo(todo),
  child: const Text('Add'),
)

• Use ref.read (not ref.watch) in event handlers.
• After a side effect, update state by: setting it directly, calling ref.invalidateSelf(), or manually updating the cache.
• Always handle loading and error states in the UI.
• Do not perform side effects in provider constructors or build methods.

9. Provider Observers

dart
class MyObserver extends ProviderObserver {
  @override
  void didUpdateProvider(ProviderObserverContext context, Object? previousValue, Object? newValue) {
    print('[${context.provider}] updated: $previousValue → $newValue');
  }

  @override
  void providerDidFail(ProviderObserverContext context, Object error, StackTrace stackTrace) {
    // Report to error service
  }
}

runApp(ProviderScope(observers: [MyObserver()], child: MyApp()));

10. Testing

dart
// Unit test
final container = ProviderContainer(
  overrides: [repositoryProvider.overrideWith((_) => FakeRepository())],
);
addTearDown(container.dispose);

expect(await container.read(todosProvider.future), isNotEmpty);

// Widget test
await tester.pumpWidget(
  ProviderScope(
    overrides: [repositoryProvider.overrideWith((_) => FakeRepository())],
    child: const MyApp(),
  ),
);

• Create a new ProviderContainer or ProviderScope for each test — never share state between tests.
• Use container.listen over container.read for autoDispose providers to keep state alive during the test.
• Use overrides to inject mocks or fakes.
• Prefer mocking dependencies (repositories) rather than Notifiers directly.
• If you must mock a Notifier, subclass the original — don't use implements or with Mock.
• Place Notifier mocks in the same file as the Notifier if using code generation.
• Obtain the container in widget tests with ProviderScope.containerOf(tester.element(...)).

References

• Riverpod GitHub Repository