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';

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({
    super.key,
    required this.title,
  }) ;

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

class _MyHomePageState extends State<MyHomePage> {
  int _counter = 0;

  void _incrementCounter() {
    setState(() {
      _counter++;
    });
  }

  @override
  Widget build(BuildContext context) {
    print("MyHomePage building");
    
    return Scaffold(
      appBar: AppBar(
        backgroundColor: Theme.of(context).colorScheme.inversePrimary,
        title: Text(widget.title),
      ),
      body: Center(
        // Try changing `count: _counter,` to `_count: 0` and 
        child: CounterValue(
          count: _counter,
          child: const CounterText(),
        )
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: const Icon(Icons.add),
      ),
    );
  }
}

class CounterValue extends InheritedWidget {
  final int count;
  
  const CounterValue({
    super.key,
    required super.child,
    required this.count,
  });
  
  @override
  bool updateShouldNotify(CounterValue oldWidget) => false;
  
  // Change the updateShouldNotify method to the one commented out below to see the CountText update
  
//   @override
//   bool updateShouldNotify(CounterValue oldWidget) => count != oldWidget.count;
  
  // Change the updateShouldNotify method to the one commented out below to always have the CountText build even if nothing changes (see comment above about always setting count to 0)
  
//   @override
//   bool updateShouldNotify(CounterValue oldWidget) => true;
}

class CounterText extends StatelessWidget {
  const CounterText({super.key});
  
  @override
  Widget build(BuildContext context) {
    print("CounterText building");
    
    return Column(
      mainAxisAlignment: MainAxisAlignment.center,
      children: [
        const Text(
          'You have pushed the button this many times:',
        ),
        Text(
          '${context.dependOnInheritedWidgetOfExactType<CounterValue>()!.count}',
          style: Theme.of(context).textTheme.headlineMedium,
        ),
      ],
    );
  }
}