Gerenciamento de Estado com StatelessWidget

main.dart

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Sem Statefull-3'),
    );
  }
}

class MyHomePage extends StatelessWidget {
  MyHomePage({this.title});

  final String title;

  @override
  Widget build(BuildContext context) {
    int _counter = 0;
    return Container(child:
        StatefulBuilder(builder: (BuildContext context, StateSetter setState) {
      return Scaffold(
        appBar: AppBar(
          title: Text(title),
        ),
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              Text(
                'You have pushed the button this many times:',
              ),
              Text(
                '$_counter',
                style: Theme.of(context).textTheme.headline4,
              ),
            ],
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            setState(() {
              _counter++;
            });
          },
          tooltip: 'Increment',
          child: Icon(Icons.add),
        ),
      );
    }));
  }
}