Flutter Change Notifier

Flutter ChangeNotifier Skill

This skill defines how to correctly use ChangeNotifier with the provider package for state management in Flutter.

1. Model

Extend ChangeNotifier to manage state. Keep internal state private and expose unmodifiable views. Call notifyListeners() on every state change.

class CartModel extends ChangeNotifier {
  final List<Item> _items = [];

  UnmodifiableListView<Item> get items => UnmodifiableListView(_items);

  void add(Item item) {
    _items.add(item);
    notifyListeners();
  }

  void removeAll() {
    _items.clear();
    notifyListeners();
  }
}

• Place shared state above the widgets that use it in the widget tree.
• Never directly mutate widgets or call methods on them to change state — rebuild widgets with new data instead.

2. Providing the Model

ChangeNotifierProvider(
  create: (context) => CartModel(),
  child: MyApp(),
)

• ChangeNotifierProvider automatically disposes of the model when it is no longer needed.
• Use MultiProvider when you need to provide multiple models:

MultiProvider(
  providers: [
    ChangeNotifierProvider(create: (_) => CartModel()),
    ChangeNotifierProvider(create: (_) => UserModel()),
  ],
  child: MyApp(),
)

3. Consuming State

Consumer

Consumer<CartModel>(
  builder: (context, cart, child) => Stack(
    children: [
      if (child != null) child,
      Text('Total price: ${cart.totalPrice}'),
    ],
  ),
  child: const SomeExpensiveWidget(), // rebuilt only once
)

• Always specify the generic type (Consumer<CartModel>, not Consumer).
• Use the child parameter to pass widgets that don't depend on the model — they are built once and reused.
• Place Consumer widgets as deep in the widget tree as possible to minimize the scope of rebuilds:

HumongousWidget(
  child: AnotherMonstrousWidget(
    child: Consumer<CartModel>(
      builder: (context, cart, child) {
        return Text('Total price: ${cart.totalPrice}');
      },
    ),
  ),
)

Provider.of with listen: false

Use Provider.of<T>(context, listen: false) when you only need to call methods on the model, not react to state changes:

Provider.of<CartModel>(context, listen: false).removeAll();

4. Optimization Rules

• Do not wrap large widget subtrees in a Consumer if only a small part depends on the model.
• Avoid rebuilding widgets unnecessarily — structure your widget tree and provider usage carefully.
• Use listen: false in callbacks (e.g., onPressed) where you trigger actions but don't need rebuilds.

5. Testing

Write unit tests for your ChangeNotifier models to verify state changes and notifications:

test('adding item updates total', () {
  final cart = CartModel();
  var notified = false;
  cart.addListener(() => notified = true);

  cart.add(Item('Book'));

  expect(cart.items.length, 1);
  expect(notified, isTrue);
});

References

• Flutter Website GitHub Repository

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