How to maintain Flutter Global BloC state using Provider on Hot Reload?

Question

I seem to lose application state whenever I perform a hot reload.

I am using a BloC provider to store application state. This is passed at the App level in the main.dart and consumed on a child page. On the initial load of the view, the value is shown. I can navigate around the application and the state persists. However, when I perform a hot reload, I lose the values and seemingly the state.

How can I fix this issue so that state is preserved on Hot Reload?

Bloc Provider

abstract class BlocBase {
  void dispose();
}

class BlocProvider<T extends BlocBase> extends StatefulWidget {
  BlocProvider({
    Key key,
    @required this.child,
    @required this.bloc,
  }): super(key: key);

  final T bloc;
  final Widget child;

  @override
  _BlocProviderState<T> createState() => _BlocProviderState<T>();

  static T of<T extends BlocBase>(BuildContext context){
    final type = _typeOf<BlocProvider<T>>();
    BlocProvider<T> provider = context.ancestorWidgetOfExactType(type);
    return provider.bloc;
  }

  static Type _typeOf<T>() => T;
}

class _BlocProviderState<T> extends State<BlocProvider<BlocBase>>{
  @override
  void dispose(){
    widget.bloc.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context){
    return widget.child;
  }
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return BlocProvider<ApplicationStateBloc>(
      bloc: ApplicationStateBloc(),
      child: MaterialApp(
        title: 'Handshake',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: LoadingPage(),
      )
    );
  }
}

class ProfileSettings extends StatefulWidget {
  @override
  _ProfileSettingsState createState() => _ProfileSettingsState();
}

class _ProfileSettingsState extends State<ProfileSettings>{
  ApplicationStateBloc _applicationStateBloc;

  @override
  void initState() {
    super.initState();
    _applicationStateBloc = BlocProvider.of<ApplicationStateBloc>(context);
  }

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

  Widget emailField() {
    return StreamBuilder<UserAccount>(
      stream: _applicationStateBloc.getUserAccount,
      builder: (context, snapshot){
        if (snapshot.hasData) {
          return Text(snapshot.data.displayName, style: TextStyle(color: Color(0xFF151515), fontSize: 16.0),);
        }
        return Text('');
      },
    );
  }

  @override
  Widget build(BuildContext context) {

    return BlocProvider<ApplicationStateBloc>(
      bloc: _applicationStateBloc,
      child: Scaffold(
        backgroundColor: Colors.white,
        body: SafeArea(
          child: Column(
            children: <Widget>[
              emailField(),
              .... // rest of code

class ApplicationStateBloc extends BlocBase {

  var userAccountController = BehaviorSubject<UserAccount>();
  Function(UserAccount) get updateUserAccount => userAccountController.sink.add;
  Stream<UserAccount> get getUserAccount => userAccountController.stream;

  @override
  dispose() {
    userAccountController.close();
  }

}

Answer 1

我面临着同样的问题。继承的小部件使得处理集团的资源变得困难。另一方面，有状态小部件允许处置，但在您使用的实现中，它不会将块保留在状态中，从而导致小部件重建时状态丢失。

经过一番试验，我想出了一种将两者结合起来的方法：

class BlocHolder<T extends BlocBase> extends StatefulWidget {
  final Widget child;
  final T Function() createBloc;

  BlocHolder({
    @required this.child,
    @required this.createBloc
  });

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

class _BlocHolderState<T extends BlocBase> extends State<BlocHolder> {
  T _bloc;

  Function hello;

  @override
  void initState() {
    super.initState();
    _bloc = widget.createBloc();
  }

  @override
  Widget build(BuildContext context) {
    return BlocProvider(
      child: widget.child,
      bloc: _bloc,
    );
  }

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

Bloc 持有者在 createState() 中创建 bloc 并持久化它。它还在 dispose() 中处置 bloc 的资源。

class BlocProvider<T extends BlocBase> extends InheritedWidget {
  final T bloc;

  const BlocProvider({
    Key key,
    @required Widget child,
    @required T bloc,
  })
      : assert(child != null),
        bloc = bloc,
        super(key: key, child: child);

  static T of<T extends BlocBase>(BuildContext context) {
    final provider = context.inheritFromWidgetOfExactType(BlocProvider) as BlocProvider;
    return provider.bloc;
  }

  @override
  bool updateShouldNotify(BlocProvider old) => false;
}

BlocProvider，顾名思义，只负责向嵌套小部件提供块。

所有块都扩展 BlocBase 类

abstract class BlocBase {
  void dispose();
}

这是一个使用示例：

class RouteHome extends MaterialPageRoute<ScreenHome> {
 RouteHome({List<ModelCategory> categories, int position}): super(builder: 
    (BuildContext ctx) => BlocHolder(
        createBloc: () => BlocMain(ApiMain()),
        child: ScreenHome(),
      ));
    }