Exemplo de Restauração de Estado no Flutter

state_restoration_example.dart

import 'package:flutter/material.dart';

void main() => runApp(
      RootRestorationScope(
        restorationId: 'root_id',
        child: App(),
      ),
    );

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      home: HomePage(),
    );
  }
}

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

  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> with RestorationMixin {

  RestorableInt _index = RestorableInt(0);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('Demo'),),
      body: Container(
        color: Colors.primaries[_index.value],
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _index.value,
        selectedItemColor: Colors.blue,
        unselectedItemColor: Colors.black,
        onTap: (index) => setState(() => _index.value = index),
        items: [
          BottomNavigationBarItem(
            icon: Icon(Icons.home),
            label: 'Início'
          ),
          BottomNavigationBarItem(
              icon: Icon(Icons.rss_feed),
              label: 'Notícias'
          ),
          BottomNavigationBarItem(
              icon: Icon(Icons.notifications),
              label: 'Notificações'
          ),
          BottomNavigationBarItem(
              icon: Icon(Icons.settings),
              label: 'Configurações'
          ),
        ],
      ),
    );
  }

  @override
  String get restorationId => 'home_page';

  @override
  void restoreState(RestorationBucket oldBucket, bool initialRestore) {
    registerForRestoration(_index, 'index');
  }
}