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counter.dart

import 'package:flutter/material.dart';

void main() {
  runApp(const Main());
}

class Main extends StatelessWidget {
  const Main({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      home: const App(),
    );
  }
}

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

  @override
  State createState() => _AppState();
}

class _AppState extends State<App> {
  final countNotifier = ValueNotifier(0);

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

  void _incrementCounter() => countNotifier.value = countNotifier.value + 1;
}