应为“A”类型的值，但在“Bloc”的 bloc 实例中出现“B”类型之一

Question

我刚开始学习 bloc，我正在尝试制作购物车 bloc，但出现状态错误：

错误：未处理的错误应为“CartSuccess”类型的值，但在“CartBloc”的块实例中出现“CartLoading”类型之一。

即使我删除了yield CartLoading()，错误也会变成：

错误：未处理的错误应为“CartSuccess”类型的值，但在“CartBloc”的 bloc 实例中出现“CartInitial”类型的值。

购物车状态

abstract class CartState extends Equatable {
  const CartState();
  
  @override
  List<Object> get props => [];
}

class CartInitial extends CartState {}

class CartLoading extends CartState {}

class CartSuccess extends CartState {

  final List<ProductModel> carts;

  const CartSuccess(this.carts);

  @override
  List<Object> get props => [carts];

}

class CartFailed extends CartState {}

购物车活动

abstract class CartEvent extends Equatable {
  const CartEvent();

  @override
  List<Object> get props => [];
}

class FetchCart extends CartEvent {}

class AddToCart extends CartEvent {
  final ProductModel product;
  const AddToCart(this.product);
  @override
  List<Object> get props => [product];
}

class RemoveFromCart extends CartEvent {

  final ProductModel product;
  const RemoveFromCart(this.product);
  @override
  List<Object> get props => [product];
}

购物车组

class CartBloc extends Bloc<CartEvent, CartState> {
  CartBloc() : super(CartInitial());

  @override
  Stream<CartState> mapEventToState(CartEvent event) async* {
    
    yield CartLoading();

    if(event is FetchCart){
      final List<ProductModel> carts = List.from((state as CartSuccess).carts);
      yield CartSuccess(carts);
    }

    if(event is AddToCart){
      try{
        final List<ProductModel> carts = List.from((state as CartSuccess).carts)
          ..add(event.product);
        yield CartSuccess(carts);
      }catch(e){
        yield CartFailed();
        print(e);
      }
    }

    if(event is RemoveFromCart){
      try{
        final carts = (state as CartSuccess)
            .carts
            .where((product) => product.id != event.product.id)
            .toList();
        yield CartSuccess(carts);
      }catch(e){
        yield CartFailed();
        print(e);
      }
    }

  }
}

Answer 1

我猜这来自这里：List.from((state as CartSuccess).carts)。您在此之前 yield yield CartLoading();，因此您的状态不再是 CartSuccess 而是 CartLoading。您之前的 CartSuccess 状态因产生 CartLoading 而丢失