app.dart

class App extends ConsumerWidget {
  const App({Key? key}) : super(key: key);

  List<BottomTabItem> get _items => BottomTabItem.values;

  @override
  Widget build(BuildContext context, WidgetRef ref) {
    final BottomTab bottomTab = ref.watch(bottomTabProvider);
    final int currentIndex = _items.indexOf(bottomTab.item);
    return MaterialApp(
      home: Scaffold(
        body: IndexedStack(
          index: currentIndex,
          children: _items
              .map(
                (BottomTabItem item) => item.page,
              )
              .toList(),
        ),
        bottomNavigationBar: BottomNavigationBar(
          onTap: (int index) => bottomTab.select(_items[index]),
          currentIndex: currentIndex,
          items: _items
              .map(
                (BottomTabItem item) => BottomNavigationBarItem(
                  icon: Icon(item.icon),
                  label: item.label,
                ),
              )
              .toList(),
        ),
      ),
    );
  }
}

bottom_tab.dart

final ChangeNotifierProvider<BottomTab> bottomTabProvider =
    ChangeNotifierProvider<BottomTab>((_) => BottomTab());

class BottomTab extends ChangeNotifier {
  BottomTabItem _item = BottomTabItem.home;
  BottomTabItem _oldItem = BottomTabItem.home;

  BottomTabItem get item => _item;
  BottomTabItem get oldItem => _oldItem;

  void select(BottomTabItem newItem) {
    _oldItem = _item;
    _item = newItem;
    notifyListeners();
  }
}

bottom_tab_item.dart

enum BottomTabItem {
  home,
  list,
  settings,
}

extension BottomTabItemExtension on BottomTabItem {
  IconData get icon {
    switch (this) {
      case BottomTabItem.home:
        return Icons.home;
      case BottomTabItem.list:
        return Icons.list;
      case BottomTabItem.settings:
        return Icons.settings;
    }
  }

  String get label {
    switch (this) {
      case BottomTabItem.home:
        return 'Home';
      case BottomTabItem.list:
        return 'List';
      case BottomTabItem.settings:
        return 'Settings';
    }
  }

  Widget get page {
    switch (this) {
      case BottomTabItem.home:
        return const HomePage();
      case BottomTabItem.list:
        return const LongListPage();
      case BottomTabItem.settings:
        return const SettingsPage();
    }
  }
}