如何在屏幕加载时只调用一次 bloc 事件

Question

我正在使用flutter_bloc库。我的屏幕小部件代码看起来像这样：

class RegisterScreen extends StatefulWidget {
  RegisterScreen();

  @override
  _RegisterScreenState createState() => _RegisterScreenState();
}

class _RegisterScreenState extends State<RegisterScreen> {
  @override
  Widget build(BuildContext context) {
    return BlocProvider<RegisterBloc>(
      create: (BuildContext context) {
        // RegisterBloc.get(context).add(RegisterGetCitiesEvent());<=== This gives me error `BlocProvider.of() called with a context that does not contain a Cubit of type RegisterBloc. \n No ancestor could be found starting from the context that was passed to BlocProvider.of<RegisterBloc>().`
        return RegisterBloc();
      },
      child: BlocConsumer<RegisterBloc, RegisterState>(
        listener: (BuildContext context, RegisterState state) {},
        builder: (BuildContext context, RegisterState state) {
          // RegisterBloc.get(context).add(RegisterGetCitiesEvent());<=== This line is executed infinite times
          return Scaffold(
            floatingActionButton: FloatingActionButton(
              onPressed: () {
                RegisterBloc.get(context).add(RegisterGetCitiesEvent());
              },
              child: new Icon(Icons.refresh),
            ),
            body: Conteiner(
              child: Text(RegisterBloc.get(context).cities==null?'':RegisterBloc.get(context).cities,)
            ),
          );
        },
      ),
    );
  }
}

而bloc文件是这样的：

class RegisterBloc extends Bloc<RegisterEvent, RegisterState> {
  static RegisterBloc get(BuildContext context) => BlocProvider.of(context);
  RegisterBloc() : super(RegisterInitial());

  String cities = '';

  @override
  Stream<RegisterState> mapEventToState(
    RegisterEvent event,
  ) async* {
    if (event is RegisterGetCitiesEvent) {
      yield* getCities();
    }
  }

  Stream<RegisterGetCitiesState> getCities() async* {
    yield await (CustomerServer.getCities()).then((res) {
      cities = res;
      return RegisterGetCitiesState();
    });
  }
}

在我的代码中，我必须点击FAB来调用RegisterGetCitiesEventbloc 事件。如何RegisterBloc.get(context).add(RegisterGetCitiesEvent())在不需要用户单击任何按钮的情况下立即调用该事件？

我试过的：

我试图在BlocConsumerbuilder 函数中添加该行，但该函数执行了无限次。
我试图在BlocProvidercreate 函数中添加该行，但它给了我一个错误BlocProvider.of() called with a context that does not contain a Cubit of type RegisterBloc.

我应该在哪里放置这条线RegisterBloc.get(context).add(RegisterGetCitiesEvent())，以便RegisterGetCitiesEvent在屏幕显示时自动调用事件（无需单击任何按钮）？

Answer 1

我建议您完整阅读BLoC文档。但这里有一个对这个问题的快速回答：您应该在BlocProvider的 create 函数中执行此操作。像这样：

class RegisterScreen extends StatefulWidget {
  RegisterScreen();

  @override
  _RegisterScreenState createState() => _RegisterScreenState();
}

class _RegisterScreenState extends State<RegisterScreen> {
  @override
  Widget build(BuildContext context) {
    return BlocProvider<RegisterBloc>(
      create: (context) => RegisterBloc()..add(RegisterGetCitiesEvent())
    );
  }
}