如何在 initState 方法中访问 flutter bloc？

Question

在下面显示的代码中，在获取 BuildContext 对象后从 build 方法中调用 dispatch 事件。如果我希望做的是在 initState 方法本身内的页面开始处理期间分派事件怎么办？

如果我使用 didChangeDependencies 方法，那么我会收到这个错误： BlocProvider.of() called with a context that does not contain a Bloc of type FileManagerBloc.如何解决这个问题？

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: BlocProvider<FileManagerBloc>(
          builder: (context)=>FileManagerBloc(),
          child: SafeArea(
      child: Container(
          child: Column(
            children: <Widget>[
              Container(color: Colors.blueGrey, child: TopMenuBar()),
              Expanded(
                child: BlocBuilder<FileManagerBloc,FileManagerState>(
                  builder: (context , state){
                    return GridView.count(
                      scrollDirection: Axis.vertical,
                      physics: ScrollPhysics(),
                      crossAxisCount: 3,
                      crossAxisSpacing: 10,
                      children: getFilesListWidget(context , state),
                    );
                  },
                ),
              )
            ],
          ),
      ),
    ),
        ));
  }

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

  @override
  void didChangeDependencies() {
    logger.i('Did change dependency Called');
    final FileManagerBloc bloc = BlocProvider.of<FileManagerBloc>(context) ;
    Messenger.sendGetHomeDir()
        .then((path) async {
      final files = await Messenger.sendListDir(path);
        bloc.dispatch(SetCurrentWorkingDir(path)) ;
        bloc.dispatch(UpdateFileSystemCacheMapping(path , files)) ;
    });
  }

Answer 1

问题是您正在初始化FileManagerBloc内部的实例，BlocProvider这当然是父小部件无法访问的。我知道这有助于自动清理，Bloc但如果你想在内部访问它，initState或者didChangeDependencies你必须像这样在父级初始化它，

FileManagerBloc _fileManagerBloc;

@override
void initState() {
  super.initState();
  _fileManagerBloc= FileManagerBloc();
  _fileManagerBloc.dispatch(LoadEducation());
}

@override
  Widget build(BuildContext context) {
    return Scaffold(
        body: BlocProvider<FileManagerBloc>(
          builder: (context)=> _fileManagerBloc,
          child: SafeArea(
      child: Container(
          child: Column(
            children: <Widget>[
              Container(color: Colors.blueGrey, child: TopMenuBar()),
              Expanded(
                child: BlocBuilder<FileManagerBloc,FileManagerState>(
                  builder: (context , state){
                    return GridView.count(
                      scrollDirection: Axis.vertical,
                      physics: ScrollPhysics(),
                      crossAxisCount: 3,
                      crossAxisSpacing: 10,
                      children: getFilesListWidget(context , state),
                    );
                  },
                ),
              )
            ],
          ),
      ),
    ),
        ));
  }

  @override
  void dispose() {
    _fileManagerBloc.dispose();
    super.dispose();
  }

  @override
  void didChangeDependencies() {
    logger.i('Did change dependency Called');
    Messenger.sendGetHomeDir()
        .then((path) async {
      final files = await Messenger.sendListDir(path);
        _fileManagerBloc.dispatch(SetCurrentWorkingDir(path)) ;
        _fileManagerBloc.dispatch(UpdateFileSystemCacheMapping(path , files)) ;
    });
  }

或者，如果FileManagerBloc在祖父母处提供/初始化，Widget则可以this通过以下方式轻松访问BlocProvider.of<CounterBloc>(context);

Answer 2

您可以在didChangeDependenciesmethod 而不是 中使用它initState。

例子

 @override
  void didChangeDependencies() {
    final CounterBloc counterBloc = BlocProvider.of<CounterBloc>(context);
    //do whatever you want with the bloc here.
    super.didChangeDependencies();
  }