我在我的应用程序中应用了 Bloc 模式，但在通过底部导航更改屏幕时遇到了问题。

我尝试进行流式广播，但是在更改屏幕后数据消失了。

这是我的 Home 类，它代表主屏幕。

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  final DepartmentBloc departmentBloc = BlocProvider.getBloc<DepartmentBloc>();
  final PromotionProductBloc promotionProductBloc = BlocProvider.getBloc<PromotionProductBloc>();

  @override
  Widget build(BuildContext context) {
    return SingleChildScrollView(
      physics: BouncingScrollPhysics(),
      child: SafeArea(
        child: Column(
          crossAxisAlignment: CrossAxisAlignment.start,
          children: <Widget>[
            CustomText(
              text: "Promoções",
              fontSize: 20.0,
              padding: EdgeInsets.only(left: 12.0),
            ),
            StreamBuilder(
              stream: promotionProductBloc.outPromotionProducts,
              builder: (context, snapshot) {
                if (snapshot.hasData)
                  return Container(
                    height: 230.0,
                    child: ListView.builder(
                      padding: EdgeInsets.only(
                          left: 12.0, right: 12.0, bottom: 10.0),
                      itemBuilder: (context, index) {
                        return PromotionProductCard(snapshot.data[index]);
                      },
                      itemCount: snapshot.data.length,
                      scrollDirection: Axis.horizontal,
                      physics: BouncingScrollPhysics(),
                    ),
                  );
                else
                  return Container(
                    height: 200.0,
                  );
              },
            ),
            CustomText(
              text: "Categorias",
              fontSize: 20.0,
              padding: EdgeInsets.only(left: 12.0, top: 20.0, bottom: 10.0),
            ),
            StreamBuilder(
              stream: departmentBloc.outDepartments,
              builder: (context, snapshot) {
                if (snapshot.hasData)
                  return Container(
                    height: 120.0,
                    child: ListView.builder(
                      padding: EdgeInsets.only(left: 12.0, right: 12.0),
                      itemBuilder: (context, index) {
                        return CategoryCard(snapshot.data[index]);
                      },
                      itemCount: snapshot.data.length,
                      scrollDirection: Axis.horizontal,
                      physics: BouncingScrollPhysics(),
                    ),
                  );
                else
                  return Container(
                    height: 100.0,
                  );
              },
            ),
          ],
        ),
      ),
    );
  }

这是访问 Service 以获取数据并打开 Stream 导出的类。

class DepartmentBloc extends BlocBase {

  DepartmentService departmentService;

  List<Department> departments;

  final StreamController<List<Department>> _departmentController = StreamController<List<Department>>();
  Stream<List<Department>> get outDepartments => _departmentController.stream;

  DepartmentBloc() {
    departmentService = DepartmentService();
    getAll();
  }

  void getAll() async {
    departments =  await departmentService.getAll();
    _departmentController.sink.add(departments);
  }

  @override
  void dispose() {
    _departmentController.close();
    super.dispose();
  }
}

PromotionProductBloc 与 DepartmentBloc 相同，它只是更改服务访问权限。

我正在使用底部导航，在转到另一个屏幕并返回后出现以下错误:流已被收听。

最佳答案

尝试

  final StreamController<List<Department>> _departmentController = StreamController.broadcast<List<Department>>();

似乎您正在使用单个订阅流 Controller 。他们只能听一次。它们的一个优点是第一个监听器将接收在订阅它之前发出的那些事件。广播流不保证这种行为，但它们可以被多次订阅。

关于flutter - 已经听过流，换屏后报错，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/56606184/