StateNotifierProvider

StateNotifierProvider 是一个用于监听和公开StateNotifier的provider(来自 state_notifier package，由Riverpod重新分发)。

它一般用于：

暴露一个 不可变 的状态，在对自定义事件做出反应后可以随时间改变。
修改某些状态的逻辑(又名“业务逻辑”)集中在一个地方，随着时间的推移也能提高可维护性。

信息

我们可以使用 StateNotifierProvider 来实现一个待办清单列表。这样做将允许我们公开 addTodo 等方法，让UI在用户交互时修改todo列表，下面是示例：

// The state of our StateNotifier should be immutable.
// We could also use packages like Freezed to help with the implementation.
@immutable
class Todo {
  const Todo({required this.id, required this.description, required this.completed});

  // All properties should be `final` on our class.
  final String id;
  final String description;
  final bool completed;

  // Since Todo is immutable, we implement a method that allows cloning the
  // Todo with slightly different content.
  Todo copyWith({String? id, String? description, bool? completed}) {
    return Todo(
      id: id ?? this.id,
      description: description ?? this.description,
      completed: completed ?? this.completed,
    );
  }
}

// The StateNotifier class that will be passed to our StateNotifierProvider.
// This class should not expose state outside of its "state" property, which means
// no public getters/properties!
// The public methods on this class will be what allow the UI to modify the state.
class TodosNotifier extends StateNotifier<List<Todo>> {
  // We initialize the list of todos to an empty list
  TodosNotifier(): super([]);

  // Let's allow the UI to add todos.
  void addTodo(Todo todo) {
    // Since our state is immutable, we are not allowed to do `state.add(todo)`.
    // Instead, we should create a new list of todos which contains the previous
    // items and the new one.
    // Using Dart's spread operator here is helpful!
    state = [...state, todo];
    // No need to call "notifyListeners" or anything similar. Calling "state ="
    // will automatically rebuild the UI when necessary.
  }

  // Let's allow removing todos
  void removeTodo(String todoId) {
    // Again, our state is immutable. So we're making a new list instead of
    // changing the existing list.
    state = [
      for (final todo in state)
        if (todo.id != todoId) todo,
    ];
  }

  // Let's mark a todo as completed
  void toggle(String todoId) {
    state = [
      for (final todo in state)
        // we're marking only the matching todo as completed
        if (todo.id == todoId)
          // Once more, since our state is immutable, we need to make a copy
          // of the todo. We're using our `copyWith` method implemented before
          // to help with that.
          todo.copyWith(completed: !todo.completed)
        else
          // other todos are not modified
          todo,
    ];
  }
}

// Finally, we are using StateNotifierProvider to allow the UI to interact with
// our TodosNotifier class.
final todosProvider = StateNotifierProvider<TodosNotifier, List<Todo>>((ref) {
  return TodosNotifier();
});

现在我们已经定义了 StateNotifierProvider ，我们可以使用它来与UI中的待办清单列表交互：

class TodoListView extends ConsumerWidget {
  const TodoListView({super.key});

  @override
  Widget build(BuildContext context, WidgetRef ref) {
    // rebuild the widget when the todo list changes
    List<Todo> todos = ref.watch(todosProvider);

    // Let's render the todos in a scrollable list view
    return ListView(
      children: [
        for (final todo in todos)
          CheckboxListTile(
            value: todo.completed,
            // When tapping on the todo, change its completed status
            onChanged: (value) => ref.read(todosProvider.notifier).toggle(todo.id),
            title: Text(todo.description),
          ),
      ],
    );
  }
}