main.dart

class BottomBarScreen extends StatefulWidget {
  @override
  _BottomBarScreenState createState() => _BottomBarScreenState();
}

class _BottomBarScreenState extends State<BottomBarScreen> {
  int _currentIndex = 0;

  final List<Widget> _children = [
    PlaceholderWidget(Colors.white),
    PlaceholderWidget(Colors.deepOrange),
    PlaceholderWidget(Colors.green)
  ];

  @override
  Widget build(BuildContext context) {
    return WillPopScope(
      onWillPop: () async {
        if (_currentIndex == 0) {
          return true;
        }
        setState(() {
          _currentIndex = 0;
        });
        return false;
      },
      child: Scaffold(
        appBar: AppBar(
          title: Text('Flutter Bottem Navi '),
        ),
        body: _children[_currentIndex],
        bottomNavigationBar: BottomNavigationBar(
          onTap: onTabTapped,
          currentIndex: _currentIndex,
          items: [
            BottomNavigationBarItem(
              icon: Icon(Icons.home),
              title: Text('Home'),
            ),
            BottomNavigationBarItem(
              icon: Icon(Icons.search),
              title: Text('Search'),
            ),
            BottomNavigationBarItem(
                icon: Icon(Icons.person), title: Text('Profile'))
          ],
        ),
      ),
    );
  }

  void onTabTapped(int index) {
    setState(() {
      _currentIndex = index;
    });
  }
}

class PlaceholderWidget extends StatelessWidget {
  final Color color;

  PlaceholderWidget(this.color);

  @override
  Widget build(BuildContext context) {
    return Container(
      color: color,
    );
  }
}