@9.6 9.7

import 'package:flutter/material.dart';
import 'package:flutter_ui_lesson/screens/first_screen.dart';
import 'package:flutter_ui_lesson/screens/second_screen.dart';
import 'package:flutter_ui_lesson/screens/third_screen.dart';

class MainScreen extends StatefulWidget {
  const MainScreen({Key? key}) : super(key: key);

  @override
  State<MainScreen> createState() => _MainScreenState();
}

class _MainScreenState extends State<MainScreen> {
  final List<Widget> _bodyList = [const FirstScreen(),const SecondScreen(),const ThirdScreen()];
  Widget _bodyWidget = const FirstScreen();
  List<String> _titleList = ['First Screen','Second Screen','Third Screen'];
  String _title = 'First Screen';
  int _selectedIndex = 0;
   @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title:  Text(_title),),
      body: _bodyWidget,
      bottomNavigationBar: SafeArea(
        child: NavigationBar(
          onDestinationSelected: (int index){
            setState(() {
              _selectedIndex = index;
              _bodyWidget = _bodyList[index];
              _title = _titleList[index];
            });
          },
          selectedIndex: _selectedIndex,
          destinations: const [
            NavigationDestination(icon: Icon(Icons.home), label: 'First screen'),
            NavigationDestination(icon: Icon(Icons.video_call), label: 'Second screen'),
            NavigationDestination(icon: Icon(Icons.settings), label: 'Third screen'),
          ],
        ),
      ),
    );
  }
}


theme =>
theme: ThemeData.light().copyWith(
  navigationBarTheme: NavigationBarThemeData(
    backgroundColor: Colors.indigo,
    indicatorColor: Colors.brown,
    iconTheme: MaterialStateProperty.resolveWith((states){
      if(states.contains(MaterialState.selected)){
        return const IconThemeData(color: Colors.white,size: 28);
      }
      return const IconThemeData(color: Colors.grey,size: 23);
    }),
    labelTextStyle: MaterialStateProperty.resolveWith((states){
      if(states.contains(MaterialState.selected)){
        return const TextStyle(color: Colors.white,fontSize: 20);
      }
      return const TextStyle(color: Colors.grey);
    })
  )
),