Counter example

readme.md

Counter example

Created with <3 with dartpad.dev.

main.dart

// Copyright (c) 2019, the Dart project authors.  Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  final String title;

  const MyHomePage({
    Key? key,
    required this.title,
  }) : super(key: key);

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: _State1(
        child: _State2(),
      ),
    );
  }
}

class _State1 extends StatefulWidget {
  const _State1({
    super.key,
    required this.child,
  });

  final Widget child;

  @override
  State<_State1> createState() => _State1State();
}

class _State1State extends State<_State1> {
  var _builds = 0;

  @override
  void initState() {
    super.initState();
    Timer(const Duration(milliseconds: 1000), () {
      setState(() {});
    });
  }

  @override
  Widget build(BuildContext context) {
    _builds++;
    return Column(
      children: [
        Text('Parent builds: $_builds'),
        widget.child,
        // Uncomment each of these separately and see the difference
//         _State2(),
//         const _State2(),
      ],
    );
  }
}

class _State2 extends StatefulWidget {
  const _State2({
    super.key,
  });

  @override
  State<_State2> createState() => _State2State();
}

class _State2State extends State<_State2> {
  var _builds = 0;
  
  @override
  Widget build(BuildContext context) {
    _builds++;
    return Text('Child builds: $_builds');
  }
}