如何在新版本中访问颤振块状态值?

Question

我是这个状态管理领域的新手。所以我试着遵循这个教程(如何使用BloC模式在愿望列表中保存产品- EP10 -电子商务系列)，但是mapEventToState被弃用了，所以我不知道该怎么做。

这是我的状态:

part of 'wishlist_bloc.dart';
abstract class WishlistState extends Equatable {
const WishlistState();
@override
List<Object> get props => [];
}
class WishlistLoading extends WishlistState {}
class WishlistLoaded extends WishlistState {
final WishlistModel wishlist;
const WishlistLoaded({this.wishlist = const WishlistModel()});
@override
List<Object> get props => [wishlist];
}
class WishlistError extends WishlistState {}

这是我的事件:

part of 'wishlist_bloc.dart';
abstract class WishlistEvent extends Equatable {
const WishlistEvent();
@override
List<Object> get props => [];
}
class StartWishlist extends WishlistEvent {}
class AddWishlistProduct extends WishlistEvent {
final ProductModel product;
const AddWishlistProduct(this.product);
@override
List<Object> get props => [product];
}
class RemoveWishlistProduct extends WishlistEvent {
final ProductModel product;
const RemoveWishlistProduct(this.product);
@override
List<Object> get props => [product];
}

这是我的分组:

import '../models/product_model.dart';
import '../models/wishlist_model.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:equatable/equatable.dart';
part 'wishlist_event.dart';
part 'wishlist_state.dart';
class WishlistBloc extends Bloc<WishlistEvent, WishlistState> {
WishlistBloc() : super(WishlistLoading()) {
on<StartWishlist>(_mapStartWishlistToState);
on<AddWishlistProduct>(_mapAddWishlistToState);
on<RemoveWishlistProduct>(_mapRemoveWishlistToState);
}
void _mapStartWishlistToState(event, emit) async {
emit(WishlistLoading());
try {
await Future.delayed(Duration(seconds: 1));
emit(WishlistLoaded());
} catch (_) {}
}
// Error ...
void _mapAddWishlistToState(event, emit) async {
if (state is WishlistLoaded) {
try {
emit(WishlistLoaded(
wishlist: WishlistModel(
products: List.from(state.wishlist.products)
..add(event.product))));
} catch (_) {}
}
}
void _mapRemoveWishlistToState(event, emit) async {}
}

但是我得到这个错误:" getter 'wishlist'不是为'WishlistState'类型定义的。尝试导入定义了'wishlist'的库，将名称修改为现有getter的名称，或者定义getter或字段名'wishlist'"

如何访问新版本的flutter_bloc中的"愿望列表"?谢谢你。

Answer 1

尝试在定义函数时定义参数类型。所以你更新后的代码看起来像这样:

import '../models/product_model.dart';
import '../models/wishlist_model.dart';
import 'package:flutter_bloc/flutter_bloc.dart';
import 'package:equatable/equatable.dart';
part 'wishlist_event.dart';
part 'wishlist_state.dart';
class WishlistBloc extends Bloc<WishlistEvent, WishlistState> {
WishlistBloc() : super(WishlistLoading()) {
on<StartWishlist>(_mapStartWishlistToState);
on<AddWishlistProduct>(_mapAddWishlistToState);
on<RemoveWishlistProduct>(_mapRemoveWishlistToState);
}
void _mapStartWishlistToState(
// Added argument types below
StartWishlist event, Emitter<WishlistState> emit) async {
emit(WishlistLoading());
try {
await Future.delayed(Duration(seconds: 1));
emit(WishlistLoaded());
} catch (_) {}
}
void _mapAddWishlistToState(
// Added argument types below
AddWishlistProduct event, Emitter<WishlistState> emit) async {
if (state is WishlistLoaded) {
try {
emit(WishlistLoaded(
wishlist: WishlistModel(
products: List.from(state.wishlist.products)
..add(event.product))));
} catch (_) {}
}
}
void _mapRemoveWishlistToState(
// Added argument types below
RemoveWishlistProduct event, Emitter<WishlistState> emit) async {}
}

Answer 2

属性的类型提升是不可能的，因为每次调用它们时它们可能返回不同的值。因此，编译器不可能知道stategetter将返回WishlistLoaded实例，即使知道同一个getter在四行之前返回了WishlistLoaded。

解决这个问题的一种方法是将状态赋值给一个局部变量，该局部变量可以进行类型提升。

void _mapAddWishlistToState(AddWishlistProduct event, Emitter<WishlistState> emit) async {
final state = this.state; // local variable
if (state is WishlistLoaded) {
try {
emit(WishlistLoaded(
wishlist: WishlistModel(
products: List.from(state.wishlist.products)
..add(event.product))));
} catch (_) {}
}
}

链接的视频使用了一个参数，该参数也可以进行类型提升。如果在_mapAddWishlistProductToState中直接使用stategetter，也会遇到同样的错误。

Answer 3

您只需要将您的状态转换为如下

void _mapAddWishlistToState(event, emit) async {
if (state is WishlistLoaded) {
try {
emit(WishlistLoaded(
wishlist: WishlistModel(
products: List.from((state as WishlistLoaded).wishlist.products)
..add(event.product))));
} catch (_) {}
}
}