StatefulWidget demo

main.dart

import 'package:flutter/material.dart';

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

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

class MyHomePage extends StatefulWidget {
  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: MyChild(),
      floatingActionButton: FloatingActionButton(
        onPressed: () => setState(() {}),
        child: Icon(Icons.refresh),
      ),
    );
  }
}

class MyChild extends StatefulWidget {
  @override
  State<MyChild> createState() => _MyChildState();
}

class _MyChildState extends State<MyChild> {
  var i = 0;
  @override
  Widget build(BuildContext context) {
    return Center(
      child: RaisedButton(
        child: Text('$i'),
        onPressed: () => setState(() => i++),
      ),
    );
  }
}