RootPage where located bottomNavigationBar

RootPage.dart

class RootPage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _RootPage();
  }
}

class _RootPage extends State<RootPage> {
  int _currentIndex = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
          body: SafeArea(
            child: _tabs[context ,_currentIndex],
          ),
          bottomNavigationBar: BottomNavigationBar(
            type: BottomNavigationBarType.fixed,
            onTap: (index) => setState(() => _currentIndex = index),
            currentIndex: _currentIndex,
            
            items: <BottomNavigationBarItem>[
              //all BottomNavigationBarItem
            ],
          ),
        );
  }
}

Widget _tabs(BuildContext rootContext, int _tabIndex){
  switch (_tabIndex) {
    case 0:
      return MainTabOrDetailPage(rootContext);
      break;
    case 1: 
      return OtherTab();
      break;
  }
}