Flutter bloc：BlocBuilder 在第一次声明 state 时不会重新调用构建器方法

Question

I am trying to change the theme of my flutter app using bloc. But I had a problem since the second time after the state was changed.

Sate is still updated, but the UI has not changed, the builder method has not run again

My log:

Performing hot restart...
Syncing files to device iPhone 11...
Restarted application in 1,386ms.
flutter: builder ThemeState
flutter: builder ThemeState
flutter: build
flutter: _ChangeThemeScreenState AppTheme.BlueDark
flutter: ThemeBloc AppTheme.BlueDark
flutter: ThemeBloc isThemeChange
flutter: builder ThemeState
flutter: _ChangeThemeScreenState AppTheme.BlueLight
flutter: ThemeBloc AppTheme.BlueLight
flutter: ThemeBloc isThemeChange
flutter: _ChangeThemeScreenState AppTheme.GreenDark
flutter: ThemeBloc AppTheme.GreenDark
flutter: ThemeBloc isThemeChange

my code:

class ThemeBloc extends Bloc<ThemeEvent, ThemeState> {
  ThemeBloc() : super(ThemeState(themeData: appThemeData[AppTheme.GreenLight]));

  @override
  Stream<ThemeState> mapEventToState(
    ThemeEvent event,
  ) async* {
    print("ThemeBloc " + (event as ThemeChanged).theme.toString());
    if (event is ThemeChanged) {
      print("ThemeBloc " +'isThemeChange');
      yield ThemeState(themeData: appThemeData[event.theme]);
    }
  }
}

class _MyAppState extends State<MyApp> {
  @override
  Widget build(BuildContext context) {
    return BlocProvider(
      create: (context) => ThemeBloc(),
      child: BlocBuilder<ThemeBloc, ThemeState>(
        builder: (context, state) {
          print("builder " + state.toString());
          return MaterialApp(
              debugShowCheckedModeBanner: false,
              title: 'Bitradez',
              routes: routes,
              theme: state.themeData);
        }
      ),
    );
  }
}

when tab button

onTap: () {
                print("_ChangeThemeScreenState " + itemAppTheme.toString());
                BlocProvider.of<ThemeBloc>(context)
                    .add(ThemeChanged(theme: itemAppTheme));
              },

Answer 1

In your ThemeState class, try passing your themeData property to the props, I am pretty sure that I had the same problem as you a while ago. As far as I understand, the value that you set for the props is what bloc actually looks at when deciding whether to rebuild your widget or not.

I think something like this should work:

List<Object> get props => [themeData]

You obviously still need yield this state correctly.