Flutter rebuild provider above MaterialApp

Question

In my application, I defined a provider Song above MaterialApp in order to have access to it through my application.

---

Need: I did this because I have a PageA which contains a songTitle variable and a button to go to PageB. On PageB, I have a button that calls my provider Song and updates PageA. So when I'm on PageB and do a Navigator.pop (context) I come back to PageA and see the updated songTitle variable. To be able to update the PageA from the PageB I have to put my provider Song above MaterialApp.

==> It works.

---

My problem: I want to be able to reset my provider when I call PageA. So if my songTitle variable had been updated and I quit pageA, I want my songTitle variable to return to its default value when I initialize the provider Song. For the moment the songTitle variable remains updated all the time ...

---

Here the router:

abstract class RouterClass{

  static Route<dynamic> generate(RouteSettings settings){
    final args = settings.arguments;

    switch(settings.name){

      case RouterName.kMenu:
        return CupertinoPageRoute(
            builder: (context) => Menu()
        );
      case RouterName.kPageA:
        return CupertinoPageRoute(
            builder: (context) => PageA()
        );
      case RouterName.kPageB:
        return CupertinoPageRoute(
            builder: (context) => PageB()
        );

      default:
        return CupertinoPageRoute(
            builder: (context) => Error404View(title: "Error")
        );
    }
  }
}

---

The Menu:

class Menu extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Scaffold(
        appBar: AppBar(
          title: Text('Menu'),
        ),
        body : Center(
          child: MaterialButton(
            onPressed: () {
              Navigator.pushNamed(
                    context,
                    RouterName.kPageA,
                  ),
            },
            child: Text('Button'),
          ),
        ),
      ),
    );
  }
}

---

The PageA:

class PageA extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Scaffold(
        appBar: AppBar(
        title: Text('pageA'),
      ),
        body : Center(
          child: Consumer<Song>(builder: (context, song, child) {
            print('Consumer() : ${song.songTitle}');

            return Column(
              children: <Widget>[
                // SONG TITLE
                Text(song.songTitle),
                // Button
                MaterialButton(
                  onPressed: () => Navigator.pushNamed(
                    context,
                    RouterName.kPageB,
                  ),
                  child: Text('Button'),
                ),
              ],
            );
          }),
        ),
      ),
    );
  }
}

---

The PageB:

class PageB extends StatelessWidget {

  @override
  Widget build(BuildContext context) {
    return SafeArea(
      child: Scaffold(
        appBar: AppBar(
          title: Text('pageB'),
        ),
        body : Center(
          child: MaterialButton(
            onPressed: () {
              Provider.of<Song>(context, listen: false).updateSongTitle('New Title');
            },
            child: Text('Button'),
          ),
        ),
      ),
    );
  }
}

---

The provider Song:

class Song extends ChangeNotifier {
  late String songTitle;

  Song(){
    _initialise();
  }

  Future _initialise() async
  {
    songTitle = "Title";
    notifyListeners();
  }

  void updateSongTitle(String newTitle) {
    songTitle = newTitle;

    notifyListeners();
  }
}

Answer 1

Use create in PageA:

child: ChangeNotifierProvider(
  create: (_) => Song(),
  child: Consumer<Song>(
    builder: (context, song, child) {
      print('Consumer() : ${song.songTitle}');
...

Pass your song object to PageB:

Navigator.pushNamed(
  context,
  '/pageB',
  arguments: song,
);

Get song in PageB:

final song = ModalRoute.of(context)!.settings.arguments as Song;

Full code:

// ignore_for_file: avoid_print

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

void main() => runApp(const MyApp());

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      routes: {
        '/': (_) => const HomePage(),
        '/pageA': (_) => const PageA(),
        '/pageB': (_) => const PageB(),
      },
    );
  }
}

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

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

class _HomePageState extends State<HomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: GestureDetector(
          onTap: () {
            Navigator.pushNamed(
              context,
              '/pageA',
            );
          },
          child: Container(
            width: 100,
            height: 100,
            color: Colors.red,
          ),
        ),
      ),
    );
  }
}

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

  @override
  State<PageA> createState() => _PageAState();
}

class _PageAState extends State<PageA> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: ChangeNotifierProvider(
          create: (_) => Song(),
          child: Consumer<Song>(
            builder: (context, song, child) {
              print('Consumer() : ${song.songTitle}');
              return Column(
                mainAxisSize: MainAxisSize.min,
                children: <Widget>[
                  // SONG TITLE
                  Text(song.songTitle),
                  // Button
                  MaterialButton(
                    onPressed: () {
                      Navigator.pushNamed(
                        context,
                        '/pageB',
                        arguments: song,
                      );
                    },
                    child: const Text('Button'),
                  ),
                ],
              );
            },
          ),
        ),
      ),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    final song = ModalRoute.of(context)!.settings.arguments as Song;
    return SafeArea(
      child: Scaffold(
        appBar: AppBar(
          title: const Text('pageB'),
        ),
        body: Center(
          child: MaterialButton(
            onPressed: () {
              song.updateSongTitle('New Title');
            },
            child: const Text('Button'),
          ),
        ),
      ),
    );
  }
}

class Song extends ChangeNotifier {
  late String songTitle;

  Song() {
    _initialise();
  }

  Future _initialise() async {
    songTitle = "Title";
    notifyListeners();
  }

  void updateSongTitle(String newTitle) {
    songTitle = newTitle;

    notifyListeners();
  }
}