将默认 Flutter_bloc 8.0 计数器从 Cubit 转换为 Bloc

Question

我正在尝试将 bloc 的简单 cubit 示例转换为 bloc 模式，但是当我按下增量按钮时出现 ...NoSuchMethodError: Class '_Emitter<MyObjectState>... 错误。

我认为我的问题是我用于 emit 参数的 dynamic 类型（将其更改为 MyObjectState 会导致表达式不是函数的错误）。

那应该是什么，这是我唯一的问题吗？

class MyObjectBloc extends Bloc<MyObjectEvent, MyObjectState> {
  MyObjectBloc() : super(const MyObjectState(counter: 99)) {
    on<AddEvent>(_onIncrement);
    on<SubtractEvent>(_onDecrement);
  }

  void _onIncrement(AddEvent event, dynamic emit) => emit(counter: state.counter + 1);
  void _onDecrement(SubtractEvent event, dynamic emit) => emit(counter: state.counter - 1);
  }

  abstract class MyObjectEvent {
    const MyObjectEvent();
  }
  class AddEvent extends MyObjectEvent {}
  class SubtractEvent extends MyObjectEvent {}

 class MyObjectState {
   const MyObjectState({required this.counter});
   final int counter;
 }


class CounterPage extends StatelessWidget {
  const CounterPage({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return BlocProvider(
      create: (_) => MyObjectBloc(),
      child: CounterView(),
    );
  }
}

class CounterView extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final textTheme = Theme.of(context).textTheme;
    return Scaffold(
      body: Center(
        child: BlocBuilder<MyObjectBloc, MyObjectState>(
          builder: (context, state) {
            return Text('${state.counter}', style: textTheme.headline2);
          },
        ),
      ),
      floatingActionButton: Column(
        mainAxisAlignment: MainAxisAlignment.end,
        crossAxisAlignment: CrossAxisAlignment.end,
        children: <Widget>[
          FloatingActionButton(
            child: const Icon(Icons.add),
            onPressed: () => context.read<MyObjectBloc>().add(AddEvent()),
          ),
          FloatingActionButton(
            child: const Icon(Icons.remove),
            onPressed: () => context.read<MyObjectBloc>().add(SubtractEvent()),
          ),
        ],
      ),
    );
  }
}

Answer 1

这里的问题是你的emit是错误的。

TLDR 答案

class MyObjectBloc extends Bloc<MyObjectEvent, MyObjectState> {
  MyObjectBloc() : super(const MyObjectState(counter: 99)) {
    on<AddEvent>(_onIncrement);
    on<SubtractEvent>(_onDecrement);
  }

  void _onIncrement(AddEvent event, Emitter<MyObjectState> emit) => emit(MyObjectState(counter: state.counter + 1));
  void _onDecrement(SubtractEvent event, Emitter<MyObjectState> emit) => emit(MyObjectState(counter: state.counter - 1));
}

abstract class MyObjectEvent {
  const MyObjectEvent();
}
class AddEvent extends MyObjectEvent {}
class SubtractEvent extends MyObjectEvent {}

class MyObjectState {
  const MyObjectState({required this.counter});
  final int counter;
}

说明

如果我们仔细看看 on<EVENT>，它是以下函数的函数：

  void on<E extends Event>(
    EventHandler<E, State> handler, {
    EventTransformer<E>? transformer,
  })

第一个参数是 EventHandler<E, State> 即：

typedef EventHandler<Event, State> = FutureOr<void> Function(
  Event event,
  Emitter<State> emit,
);

所以你应该发出你的状态 class。

将默认 Flutter_bloc 8.0 计数器从 Cubit 转换为 Bloc

Convert Default Flutter_bloc 8.0 Counter from Cubit to Bloc

flutter

flutter-bloc

TLDR 答案

说明