使用 blocbuilder 检查 bloc 状态

Question

我正在从 API 获取项目列表并使用 BlocBuilder 构建它们。它可以工作并且构建了小部件列表，但是当我打印出来检查正在执行的状态部分时，我得到如下所示。为什么会出现“Nothing”？

ProductInitial
fetching product //from bloc when fetching api
Nothing
fetching complete //from bloc after fetching api
ProductSuccess

主要

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MultiBlocProvider(
        providers: [
          BlocProvider<ProductBloc>(
            create: (BuildContext context) => ProductBloc()..add(FetchProduct())
          ),
        ],
        child: MaterialApp(
          debugShowCheckedModeBanner: false,
          home: MainScreen(),
        )
      );
  }
}

列表屏幕

Widget build(BuildContext context) {
    return Container(
      child: Padding(
        padding: EdgeInsets.all(10.0),
        child: BlocBuilder<ProductBloc, ProductState>(
          builder: (context, state) {
            if(state is ProductInitial){
              print('ProductInitial');
              return buildLoadingWidget();
            }
            if(state is ProductSuccess){
              print('ProductSuccess');
              return _buildProductListWidget(state.products);
            }
            if(state is ProductFailed){
              print('ProductFailed');
              return Center(
                child: Text('Something went wrong'),
              );
            }
            print('Nothing');
            return buildLoadingWidget();
          }
        )
      )
    );
  }

更新

添加了bloc代码以供参考。

集团

class ProductBloc extends Bloc<ProductEvent, ProductState> {
  ProductBloc() : super(ProductInitial());

  ProductRepository _repository = ProductRepository();

  @override
  Stream<ProductState> mapEventToState(ProductEvent event,) async* {
    
    if(event is FetchProduct){
      yield ProductLoading();
      try{
        print('fetching product');
        final List<ProductModel> products = await _repository.getProducts();
        yield ProductSuccess(products);
        print('fetching complete');
      }catch(e){
        yield ProductFailed();
        print(e);
        print('fetching failed');
      }
    }

  }
}

Answer 1

使用以下代码更新您的集团：

BLOC

class ProductBloc extends Bloc<ProductEvent, ProductState> {
ProductBloc() : super(ProductInitial());
   
 ProductRepository _repository = ProductRepository();

  @override
  Stream<ProductState> mapEventToState(ProductEvent event,) async* {
    
    if(event is FetchProduct){
      yield ProductInitial();
      try{
        print('fetching product');
        final List<ProductModel> products = await _repository.getProducts();
        yield ProductSuccess(products);
        print('fetching complete');
      }catch(e){
        yield ProductFailed();
        print(e);
        print('fetching failed');
      }
    }
  }
}

在您的代码中，您处于 yield ProductLoading 状态，但没有在 BlocBuilder 中处理该状态，这就是它绕过所有 if 语句并打印 Nothing 的原因。

所以另一种方法是在你的块构建中处理ProductLoading，如下所示

列表屏幕

  Widget build(BuildContext context) {
        return Container(
          child: Padding(
            padding: EdgeInsets.all(10.0),
            child: BlocBuilder<ProductBloc, ProductState>(
              builder: (context, state) {
                if(state is ProductInitial){
                  print('ProductInitial');
                  return buildLoadingWidget();
                }
                if(state is ProductLoading){
                  print('ProductLoading');
                  return buildLoadingWidget();
                }
                if(state is ProductSuccess){
                  print('ProductSuccess');
                  return _buildProductListWidget(state.products);
                }
                if(state is ProductFailed){
                  print('ProductFailed');
                  return Center(
                    child: Text('Something went wrong'),
                  );
                }
                print('Nothing');
                return buildLoadingWidget();
              }
            )
          )
        );
      }