Two Screens, One Model

Part of the State Management section of Coddy's Flutter journey. Lesson 13 of 27.

Data shared by two screens is where Provider pays off. The Settings screen changes the name, and the home screen shows it, without the two screens passing anything to each other.

Where the provider sits matters. MaterialApp holds a Navigator, and every screen you push goes into it next to home, not inside it:

ChangeNotifierProvider        <- above MaterialApp: every screen finds it
  MaterialApp
    Navigator
      HomePage
      SettingsScreen          <- pushed next to HomePage

A provider wrapped around HomePage only reaches HomePage. The pushed screen is not below it, so it cannot find the model. Put the provider above MaterialApp, in main, and every screen shares it.

Do not fix the error with a second ChangeNotifierProvider on the other screen. That creates a second model: the error goes away, but the screens no longer share anything.

Challenge

Medium

Tap the gear, type a name and press Save: the app shows an error, because the provider wraps only HomePage.

Move the ChangeNotifierProvider out of MyApp and into main, around MyApp.

Then saving Ada shows Hello, Ada! at home.

Try it yourself

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

class Profile extends ChangeNotifier {
  String _name = 'Guest';

  String get name => _name;

  void rename(String newName) {
    _name = newName;
    notifyListeners();
  }
}

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

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: ChangeNotifierProvider(
        create: (context) => Profile(),
        child: const HomePage(),
      ),
    );
  }
}

class HomePage extends StatelessWidget {
  const HomePage({super.key});

  @override
  Widget build(BuildContext context) {
    final profile = context.watch<Profile>();
    return Scaffold(
      appBar: AppBar(
        title: const Text('Home'),
        actions: [
          IconButton(
            icon: const Icon(Icons.settings),
            onPressed: () {
              Navigator.push(
                context,
                MaterialPageRoute(builder: (context) => const SettingsScreen()),
              );
            },
          ),
        ],
      ),
      body: Center(
        child: Text('Hello, ${profile.name}!', style: const TextStyle(fontSize: 32)),
      ),
    );
  }
}

class SettingsScreen extends StatefulWidget {
  const SettingsScreen({super.key});

  @override
  State<SettingsScreen> createState() => _SettingsScreenState();
}

class _SettingsScreenState extends State<SettingsScreen> {
  final controller = TextEditingController();

  @override
  Widget build(BuildContext context) {
    final profile = context.watch<Profile>();
    return Scaffold(
      appBar: AppBar(title: const Text('Settings')),
      body: Padding(
        padding: const EdgeInsets.all(16),
        child: Column(
          children: [
            Text('Signed in as ${profile.name}'),
            const SizedBox(height: 16),
            TextField(
              controller: controller,
              decoration: const InputDecoration(labelText: 'Your name'),
            ),
            const SizedBox(height: 16),
            ElevatedButton(
              onPressed: () {
                context.read<Profile>().rename(controller.text);
                Navigator.pop(context);
              },
              child: const Text('Save'),
            ),
          ],
        ),
      ),
    );
  }
}

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

All lessons in State Management

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