Scoped model: https://pub.dartlang.org/packages/scoped_model#-installing-tab-

main_scoped_model.dart

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

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

class CounterModel extends Model {
  int _counter = 0;

  int get counter => _counter;

  void increment() {
    _counter++;
    notifyListeners();
  }
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: ScopedModel(
        model: new CounterModel(),
        child: MyHomePage(title: 'Flutter Demo Home Page'),
      ),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);
  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            new ScopedModelDescendant<CounterModel>(
              builder: (context, child, model) => Text(
                    '${model.counter}',
                    style: Theme.of(context).textTheme.display1,
                  ),
            ),
            TestStateless(),
            TestStateful()
          ],
        ),
      ),
      floatingActionButton: new ScopedModelDescendant<CounterModel>(
        builder: (context, child, model) => FloatingActionButton(
              onPressed: () {
                model.increment();
              },
              tooltip: 'Increment',
              child: Icon(Icons.add),
            ),
      ),
    );
  }
}

class TestStateless extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    print('build TestStateless');
    return Text('TestStateless');
  }
}

class TestStateful extends StatefulWidget {
  @override
  _TestStatefulState createState() => _TestStatefulState();
}

class _TestStatefulState extends State<TestStateful> {
  @override
  Widget build(BuildContext context) {
    print('build TestStateful');
    return Text('_TestStatefulState');
  }
}

pubspec.yaml

dependencies:
  flutter:
    sdk: flutter
  cupertino_icons: ^0.1.2
  scoped_model: ^1.0.1