upstream-flutter-samples/web_embedding/ng-flutter/flutter/lib/pages/counter.dart

import 'package:flutter/material.dart';

class CounterDemo extends StatefulWidget {
  const CounterDemo({super.key, required this.counter});

  final ValueNotifier<int> counter;

  @override
  State<CounterDemo> createState() => _CounterDemoState();
}

class _CounterDemoState extends State<CounterDemo> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: const Text('Counter'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            const Text('You have pushed the button this many times:'),
            ValueListenableBuilder(
              valueListenable: widget.counter,
              builder: (context, value, child) => Text(
                '$value',
                style: Theme.of(context).textTheme.headlineMedium,
              ),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: () {
          widget.counter.value++;
        },
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}