changenotifierprovider_example.dart

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return ChangeNotifierProvider(
      builder: (_) => ValueNotifier<String>(null),
      child: MaterialApp(
        title: 'Example',
        home: A(),
      ),
    );
  }
}

class A extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('A')),
      body: Center(
        child: RaisedButton(
          child: Text('Page B'),
          onPressed: () {
            Navigator.push(context, MaterialPageRoute(builder: (_) => B()));
          },
        ),
      ),
    );
  }
}

class B extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    final notifier = Provider.of<ValueNotifier<String>>(
      context,
    );
    return Scaffold(
      appBar: AppBar(title: Text('B')),
      body: Center(
        child: Column(
          children: <Widget>[
            DropdownButton<String>(
              value: notifier.value,
              items: ['a', 'b', 'c']
                  .map(
                    (s) => DropdownMenuItem<String>(
                      child: Text(s),
                      value: s,
                    ),
                  )
                  .toList(),
              onChanged: (value) {
                notifier.value = value;
              },
            ),
            RaisedButton(
              child: Text('Page C'),
              onPressed: () {
                Navigator.push(context, MaterialPageRoute(builder: (_) => C()));
              },
            ),
          ],
        ),
      ),
    );
  }
}

class C extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text('C')),
      body: Center(
        child: Text(Provider.of<ValueNotifier<String>>(context).value),
      ),
    );
  }
}