DartChildUpdateFromParent

main.dart

import 'package:flutter/material.dart';

const Color darkBlue = Color.fromARGB(255, 18, 32, 47);

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData.dark().copyWith(
        scaffoldBackgroundColor: darkBlue,
      ),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: ParentWid(),
        ),
      ),
    );
  }
}

class ParentWid extends StatefulWidget {
  @override
  State<ParentWid> createState() => _ParentWidState();
}

class _ParentWidState extends State<ParentWid> {
  int counter = 0;
  @override
  Widget build(BuildContext context) {
    return Column(children:[
       IconButton(
       onPressed: () {
         setState(() {
            counter++;
         });
       },
       icon: const Icon(Icons.add),
      ),
      ChildWid(counter)
    ]);
  }
}

class ChildWid extends StatefulWidget {
  final int? counter;
  const ChildWid(this.counter);

  @override
  State<ChildWid> createState() => _ChildWidState();
}

class _ChildWidState extends State<ChildWid> {
  @override
  Widget build(BuildContext context) {
    return Text(
      'Current Counter ${widget.counter ?? 0}',
      style: Theme.of(context).textTheme.headline4,
    );
  }
}