ChangeNotifierProxyProvider update

main.dart

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

void main() {
  runApp(MaterialApp(home: MainPage()));
}

const List<Destination> allDestinations = <Destination>[
  Destination(0, 'Home', Icons.home),
  Destination(1, 'Other', Icons.settings),
];

class MainPage extends StatefulWidget {
  @override
  _MainPageState createState() => _MainPageState();
}

class _MainPageState extends State<MainPage>
    with TickerProviderStateMixin<MainPage> {
  List<GlobalKey> _destinationKeys;
  List<GlobalKey<NavigatorState>> _navigatorKeys;
  int _currentIndex = 0;

  @override
  void initState() {
    super.initState();

    _destinationKeys = List<GlobalKey>.generate(
        allDestinations.length, (int index) => GlobalKey()).toList();
    _navigatorKeys = List<GlobalKey<NavigatorState>>.generate(
        allDestinations.length, (int index) => GlobalKey()).toList();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        fit: StackFit.expand,
        children: allDestinations.map((Destination destination) {
          final Widget view = KeyedSubtree(
            key: _destinationKeys[destination.index],
            child: Navigator(
              key: _navigatorKeys[destination.index],
              onGenerateRoute: (RouteSettings settings) {
                return MaterialPageRoute(
                  settings: settings,
                  builder: (BuildContext context) {
                    if (destination.index == 0) {
                      return Home();
                    } else {
                      return OtherPage();
                    }
                  },
                );
              },
            ),
          );
          return Offstage(
            child: view,
            offstage: destination.index != _currentIndex,
          );
        }).toList(),
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        onTap: (int index) {
          setState(() {
            _currentIndex = index;
          });
        },
        items: allDestinations.map((Destination destination) {
          return BottomNavigationBarItem(
            icon: Icon(destination.icon),
            title: Text(destination.title),
          );
        }).toList(),
      ),
    );
  }
}

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

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

class _HomeState extends State<Home> {
  @override
  Widget build(BuildContext context) {
    return MultiProvider(
      providers: [
        Provider<int>.value(
          value: 0,
        ),
        ChangeNotifierProxyProvider<int, MyNotifier>(
          create: (context) {
            return MyNotifier();
          },
          update: (BuildContext context, int value, MyNotifier previous) {
            print("update called: ${DateTime.now()}");
            return previous;
          },
        ),
      ],
      child: Scaffold(
        body: Builder(builder: (context) {
          final notifier = Provider.of<MyNotifier>(context);
          return Center(
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                Text("MyNotifier ${notifier.hashCode}"),
                OutlineButton(
                  child: Text("setState runs update"),
                  onPressed: () {
                    setState(() {});
                  },
                ),
              ],
            ),
          );
        }),
      ),
    );
  }
}

class MyNotifier extends ChangeNotifier {}



class OtherPage extends StatelessWidget {
  const OtherPage({Key key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(child: Text("Other page")),
    );
  }
}

class Destination {
  const Destination(this.index, this.title, this.icon);
  final int index;
  final String title;
  final IconData icon;
}