test.dart

// main.dart
void main() {
  runApp(
    BlocProvider<HomeBloc>(
      baloc: HomeBloc(),
      child: MyApp(),
    ),
  );
}
//app.dart
class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print("my app buidl metod called");
    return MaterialApp(
      title: 'Jk GAD',
      theme: ThemeData(brightness: Brightness.dark, fontFamily: "RobotoMono"),
      home: HomePage(),
      routes: routes,
    );
  }

  Map<String, WidgetBuilder> get routes => <String, WidgetBuilder>{
        'orderDetials': (BuildContext context) => OrderDetails()
      };
}

//home bloc 
class HomeBloc extends BlocBase {
  final _navigationSubject = BehaviorSubject<int>(seedValue: 0);

  Sink<int> get navigationIndex => _navigationSubject.sink;

  Observable<int> get currentNavigationIndex => _navigationSubject.stream;

  HomeBloc() {
    // _navigationSubject.listen((index) {
    //   _navigationSubject.sink.add(index);
    // });
  }

  @override
  void dispose() {
    _navigationSubject.close();
  }
}
//homepage.dart
class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> {
  var ordersBloc;
  var noticesBloc;

  @override
  void initState() {
    super.initState();
    print('called initstate in homepage');
    ordersBloc = OrdersBloc();
    noticesBloc = NoticesBloc();
  }

  @override
  Widget build(BuildContext context) {
    final bloc = BlocProvider.of<HomeBloc>(context);
    print(bloc);
    print("home page buidld method called ");
    return StreamBuilder<int>(
      stream: bloc.currentNavigationIndex,
      builder: (BuildContext context, AsyncSnapshot<int> snapshot) {
        if (snapshot.hasData) {
          return Scaffold(
            appBar: AppBar(
              title: Text("...."),
            ),
            drawer: Drawer(),
            bottomNavigationBar: BottomNavigationBar(
              items: <BottomNavigationBarItem>[
                BottomNavigationBarItem(
                    title: Text("Orders"), icon: Icon(Icons.view_headline)),
                BottomNavigationBarItem(
                    title: Text("Notice Board"),
                    icon: Icon(Icons.notifications)),
                BottomNavigationBarItem(
                    title: Text("Calendar"), icon: Icon(Icons.calendar_today)),
              ],
              type: BottomNavigationBarType.fixed,
              currentIndex: snapshot.data,
              onTap: (index) {
                bloc.navigationIndex.add(index);
              },
            ),
            body: getPage(snapshot.data),//page changes here
          );
        } else {
          return Text('alskdjfl');
        }
      },
    );
  }

  @override
  void dispose() {
    // ordersBloc?.dispose();
    // noticesBloc?.dispose();
    super.dispose();
  }

  Widget getPage(int currentIndex) {
    if (currentIndex == 0) {
      return BlocProvider<OrdersBloc>(bloc: ordersBloc, child: OrdersPage());
    } else if (currentIndex == 1) {
      return BlocProvider<NoticesBloc>(bloc: noticesBloc, child: HomePage());
    } else {
      return CalendarPage();
    }
  }
}