changeNotifier.dart

import 'package:flutter/material.dart';


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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final CounterModel counterModel = CounterModel();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: AnimatedBuilder(
            animation: counterModel,
            builder: (context, _) {
              return Text(
                'Counter ChangeNotifier - ${counterModel.value}',
              );
            }),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            AnimatedBuilder(
                animation: counterModel,
                builder: (context, _) {
                  return Text(
                    '${counterModel.value}',
                    style: Theme.of(context).textTheme.bodyText1,
                  );
                }),
          ],
        ),
      ),
      floatingActionButton: Container(
        width: 100.0,
        height: 100.0,
        child: FloatingActionButton(
          onPressed: counterModel.increment
          ,
          tooltip: 'Increment',
          child: Icon(Icons.add),
        ),
      ),
    );
  }
}


class CounterModel extends ChangeNotifier {
  
  var value = 0;
   
  void increment() {
    value++;
    
    //executa todas as funções!
    notifyListeners();
  }
}