value_listener_sample.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(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatelessWidget {
  final String title;
  
  MyHomePage({Key key, this.title}) : super(key: key);

  final _counter = Lsn(MyState(0));
  
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(title),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            ValueListenableBuilder<MyState>(
              valueListenable: _counter,
              builder: (_context, state, _child) => Text(
                '${state.count}',
              ),
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _counter.increment,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ),
    );
  }
}

class MyState {
  final int count;
  MyState(this.count);
  
  // 以下は Equatable でもっと省略できる
  @override
  bool operator ==(Object other) =>
    identical(this, other) ||
    other is MyState &&
    runtimeType == other.runtimeType &&
    count == other.count;

  @override
  int get hashCode => count.hashCode;
}

class Lsn extends ValueNotifier<MyState> {
  Lsn(value) : super(value);
  
  void increment() {
    value = MyState(value.count + 1);
  }
}