abc.dart

child: DropdownButtonHideUnderline(
                              child: StreamBuilder<List<Country>>(
                                  initialData: [],
                                  stream: bloc.countries,
                                  builder: (context, countriesSnapshot) {
                                    return StreamBuilder<Country>(
                                        stream: bloc.country,
                                        builder: (context, snapshot) {
                                          return  countriesSnapshot.hasData ? DropdownButton<Country>(
                                            onChanged: bloc.setCountry,
                                            hint: Text("Country"),
                                            value: snapshot.data ,
                                            items: countriesSnapshot.data
                                                .toList()
                                                .map<
                                                    DropdownMenuItem<
                                                        Country>>((country) =>
                                                    DropdownMenuItem<Country>(
                                                      value: country,
                                                      child: Text(country.name),
                                                    ))
                                                .toList(),
                                          ): CircularProgressIndicator();
                                        });
                                  }),
                            ),
                          ),
                        )
                      : Container(),
                  snapshot.data == 1
                      ? Container(
                          margin: EdgeInsets.only(
                              bottom: screenAwareSize(10.0, context)),
                          child: InputDecorator(
                            decoration: InputDecoration(
                              filled: true,
                              enabledBorder: UnderlineInputBorder(
                                  borderSide: BorderSide(
                                      color: Theme.of(context).primaryColor)),
                            ),
                            child: DropdownButtonHideUnderline(
                              child: StreamBuilder<List<Region>>(
                                  initialData: [],
                                  stream: bloc.regions,
                                  builder: (context, regionSnapshot) {
                                    print("${regionSnapshot.data}");
                                    return  StreamBuilder<Region>(
                                        stream: bloc.region,
                                        builder: (context, snapshot) {
                                          print("${snapshot.data}");
                                          return regionSnapshot.hasData ? DropdownButton<Region>(
                                            value: snapshot.data,
                                            onChanged: bloc.setRegion,
                                            hint: Text("State / Region"),
                                            items: regionSnapshot.data
                                                .map<DropdownMenuItem<Region>>(
                                                    (state) => DropdownMenuItem<
                                                            Region>(
                                                          value: state,
                                                          child:
                                                              Text(state?.name),
                                                        ))
                                                .toList(),
                                          ): Container();
                                        });
                                  }),
                            ),
                          ),
                        )
                      : Container(),
                  snapshot.data == 1
                      ? Container(
                          margin: EdgeInsets.only(
                              bottom: screenAwareSize(10.0, context)),
                          child: InputDecorator(
                            decoration: InputDecoration(
                              filled: true,
                              enabledBorder: UnderlineInputBorder(
                                  borderSide: BorderSide(
                                      color: Theme.of(context).primaryColor)),
                            ),
                            child: DropdownButtonHideUnderline(
                              child: StreamBuilder<List<City>>(
                                  initialData: [],
                                  stream: bloc.cities,
                                  builder: (context, citiesSnapshot) {
                                    return StreamBuilder<City>(
                                        stream: bloc.city,
                                        builder: (context, snapshot) {
                                          return DropdownButton<City>(
                                            value: snapshot.data,
                                            hint: Text("City"),
                                            onChanged: bloc.setCity,
                                            items: citiesSnapshot.data
                                                .map<DropdownMenuItem<City>>(
                                                    (city) =>
                                                        DropdownMenuItem<City>(
                                                          value: city,
                                                          child:
                                                              Text(city.name),
                                                        ))
                                                .toList(),
                                          );
                                        });
                                  }),
                            ),
                          ),
                        )
                      : Container(),

//bloc
class NationalityBloc implements BaseBloc {
  final _countrySubject = BehaviorSubject<Country>();
  final _regionSubject = BehaviorSubject<Region>();
  final _citySubject = BehaviorSubject<City>();
  final _regionsSubject = BehaviorSubject<List<Region>>();
  final _citiesSubject = BehaviorSubject<List<City>>();


  Observable<Country> get country => _countrySubject.stream;

  Observable<Region> get region => _regionSubject.stream;

  Observable<City> get city => _citySubject.stream;

  Observable<List<Country>> countries;

  Observable<List<Region>> get regions => _regionsSubject.stream;

  Observable<List<City>> get cities => _citiesSubject.stream;

  Function(Country) get setCountry => _countrySubject.sink.add;

  Function(Region) get setRegion => _regionSubject.sink.add;

  Function(City) get setCity => _citySubject.sink.add;

  NationalityBloc() {
    countries = Observable.fromFuture(api.getCountries());
    _countrySubject.listen((country) {
      _regionsSubject.sink.addStream(Observable<List<Region>>.fromFuture(
          api.getStates(country.code)));
    });
    _regionSubject.listen((region) {
      _citiesSubject.sink.addStream(Observable<List<City>>.fromFuture(
          api.getCities(region.country, region.name)));
    });
  }
}