在使用BottomNavigationBar时如何持久化选项卡？

import 'package:flutter/material.dart';
import 'package:shared_preferences/shared_preferences.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: TabScreen(),
        bottomNavigationBar: BottomNavigationBar(
          items: const <BottomNavigationBarItem>[
            BottomNavigationBarItem(icon: Icon(Icons.home), label: 'Home'),
            BottomNavigationBarItem(icon: Icon(Icons.settings), label: 'Settings'),
          ],
          onTap: (index) => _changeTab(index),
          currentIndex: _currentIndex,
        ),
      ),
    );
  }

  int _currentIndex = 0;

  void _changeTab(int index) async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    await prefs.setInt('tab_index', index);
    setState(() {
      _currentIndex = index;
    });
  }

  Future<void> _initTab() async {
    SharedPreferences prefs = await SharedPreferences.getInstance();
    int index = prefs.getInt('tab_index') ?? 0;
    setState(() {
      _currentIndex = index;
    });
  }
}

class TabScreen extends StatefulWidget {
  @override
  _TabScreenState createState() => _TabScreenState();
}

class _TabScreenState extends State<TabScreen> {
  @override
  void initState() {
    super.initState();
    _initTab();
  }

  @override
  Widget build(BuildContext context) {
    return IndexedStack(
      index: MyApp()._currentIndex,
      children: [
        HomeScreen(),
        SettingsScreen(),
      ],
    );
  }
}

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

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