bottombar screens should persist their states

[santhoshvai]

https://www.youtube.com/watch?v=EyLqj9L_Tck

https://github.com/flutter/flutter/blob/master/examples/flutter_gallery/lib/demo/material/bottom_navigation_demo.dart

[jonasermert]

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: MyBottomNavigationBar(),
    );
  }
}

class MyBottomNavigationBar extends StatefulWidget {
  @override
  _MyBottomNavigationBarState createState() => _MyBottomNavigationBarState();
}

class _MyBottomNavigationBarState extends State<MyBottomNavigationBar> with AutomaticKeepAliveClientMixin {
  int _currentIndex = 0;

  @override
  bool get wantKeepAlive => true;

  @override
  Widget build(BuildContext context) {
    super.build(context); // Don't forget to call super.build.

    return Scaffold(
      appBar: AppBar(
        title: Text('Persistent Bottom Bar'),
      ),
      body: PageView(
        controller: PageController(),
        onPageChanged: (index) {
          setState(() {
            _currentIndex = index;
          });
        },
        children: [
          Screen1(),
          Screen2(),
          Screen3(),
        ],
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        onTap: (index) {
          setState(() {
            _currentIndex = index;
          });
        },
        items: [
          BottomNavigationBarItem(
            icon: Icon(Icons.home),
            label: 'Screen 1',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.business),
            label: 'Screen 2',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.school),
            label: 'Screen 3',
          ),
        ],
      ),
    );
  }
}

class Screen1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: Text('Screen 1'),
    );
  }
}

class Screen2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: Text('Screen 2'),
    );
  }
}

class Screen3 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: Text('Screen 3'),
    );
  }
}