最終更新:2025-01-16 (木) 04:42:13 (23d)  

Flutter/プロジェクト/lib/main.dart
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flutter create

クラス

MyApp?

  • コンストラクタ
  • build

MyHomePage?

  • コンストラクタ
  • createState

_MyHomePage?

コード

  • import 'package:flutter/material.dart';
    
    void main() {
      runApp(const MyApp());
    }
    
    class MyApp extends StatelessWidget {
      const MyApp({super.key});
    
      // This widget is the root of your application.
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          title: 'Flutter Demo',
          theme: ThemeData(
            // This is the theme of your application.
            //
            // TRY THIS: Try running your application with "flutter run". You'll see
            // the application has a purple toolbar. Then, without quitting the app,
            // try changing the seedColor in the colorScheme below to Colors.green
            // and then invoke "hot reload" (save your changes or press the "hot
            // reload" button in a Flutter-supported IDE, or press "r" if you used
            // the command line to start the app).
            //
            // Notice that the counter didn't reset back to zero; the application
            // state is not lost during the reload. To reset the state, use hot
            // restart instead.
            //
            // This works for code too, not just values: Most code changes can be
            // tested with just a hot reload.
            colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
            useMaterial3: true,
          ),
          home: const MyHomePage(title: 'Flutter Demo Home Page'),
        );
      }
    }
    
    class MyHomePage extends StatefulWidget {
      const MyHomePage({super.key, required this.title});
    
      // This widget is the home page of your application. It is stateful, meaning
      // that it has a State object (defined below) that contains fields that affect
      // how it looks.
    
      // This class is the configuration for the state. It holds the values (in this
      // case the title) provided by the parent (in this case the App widget) and
      // used by the build method of the State. Fields in a Widget subclass are
      // always marked "final".
    
      final String title;
    
      @override
      State<MyHomePage> createState() => _MyHomePageState();
    }
    
    class _MyHomePageState extends State<MyHomePage> {
      int _counter = 0;
    
      void _incrementCounter() {
        setState(() {
          // This call to setState tells the Flutter framework that something has
          // changed in this State, which causes it to rerun the build method below
          // so that the display can reflect the updated values. If we changed
          // _counter without calling setState(), then the build method would not be
          // called again, and so nothing would appear to happen.
          _counter++;
        });
      }
    
      @override
      Widget build(BuildContext context) {
        // This method is rerun every time setState is called, for instance as done
        // by the _incrementCounter method above.
        //
        // The Flutter framework has been optimized to make rerunning build methods
        // fast, so that you can just rebuild anything that needs updating rather
        // than having to individually change instances of widgets.
        return Scaffold(
          appBar: AppBar(
            // TRY THIS: Try changing the color here to a specific color (to
            // Colors.amber, perhaps?) and trigger a hot reload to see the AppBar
            // change color while the other colors stay the same.
            backgroundColor: Theme.of(context).colorScheme.inversePrimary,
            // Here we take the value from the MyHomePage object that was created by
            // the App.build method, and use it to set our appbar title.
            title: Text(widget.title),
          ),
          body: Center(
            // Center is a layout widget. It takes a single child and positions it
            // in the middle of the parent.
            child: Column(
              // Column is also a layout widget. It takes a list of children and
              // arranges them vertically. By default, it sizes itself to fit its
              // children horizontally, and tries to be as tall as its parent.
              //
              // Column has various properties to control how it sizes itself and
              // how it positions its children. Here we use mainAxisAlignment to
              // center the children vertically; the main axis here is the vertical
              // axis because Columns are vertical (the cross axis would be
              // horizontal).
              //
              // TRY THIS: Invoke "debug painting" (choose the "Toggle Debug Paint"
              // action in the IDE, or press "p" in the console), to see the
              // wireframe for each widget.
              mainAxisAlignment: MainAxisAlignment.center,
              children: <Widget>[
                const Text(
                  'You have pushed the button this many times:',
                ),
                Text(
                  '$_counter',
                  style: Theme.of(context).textTheme.headlineMedium,
                ),
              ],
            ),
          ),
          floatingActionButton: FloatingActionButton(
            onPressed: _incrementCounter,
            tooltip: 'Increment',
            child: const Icon(Icons.add),
          ), // This trailing comma makes auto-formatting nicer for build methods.
        );
      }
    }
    

解説

  • import 'package:flutter/material.dart';
    
    void main() {
      runApp(const MyApp());
    }
    
    class MyApp extends StatelessWidget {
      const MyApp({super.key});
    
      // アプリケーションのルートウィジェット
      @override
      Widget build(BuildContext context) {
        return MaterialApp(
          title: 'Flutter デモ', // アプリケーションのタイトル
          theme: ThemeData(
            // アプリケーションのテーマを定義
            // 実験: "flutter run" でアプリを起動後、seedColor を Colors.green に変更し、ホットリロードを試してください
            colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
            useMaterial3: true, // Material3 のデザインを適用
          ),
          home: const MyHomePage(title: 'Flutter デモ ホームページ'), // ホーム画面を設定
        );
      }
    }
    
    class MyHomePage extends StatefulWidget {
      const MyHomePage({super.key, required this.title}); // コンストラクタ: 必須の title パラメータを受け取る
    
      // このクラスはホームページの設定を保持
      // 今回の例では title を保持しています
    
      final String title; // ホームページのタイトル
    
      @override
      State<MyHomePage> createState() => _MyHomePageState();
    }
    
    class _MyHomePageState extends State<MyHomePage> {
      int _counter = 0; // ボタンが押された回数を保持するカウンター
    
      // カウンターを増加させるメソッド
      void _incrementCounter() {
        setState(() {
          // setState を呼び出すことで、状態が変更されたことをフレームワークに通知
          _counter++;
        });
      }
    
      @override
      Widget build(BuildContext context) {
        // setState が呼び出されるたびに再実行されます
        return Scaffold(
          appBar: AppBar(
            // アプリの上部バー
            backgroundColor: Theme.of(context).colorScheme.inversePrimary, // 色をテーマから取得
            title: Text(widget.title), // タイトルを表示
          ),
          body: Center(
            // 画面中央にウィジェットを配置
            child: Column(
              mainAxisAlignment: MainAxisAlignment.center, // 子ウィジェットを縦方向に中央揃え
              children: <Widget>[
                const Text(
                  'ボタンを押した回数:', // 説明文
                ),
                Text(
                  '$_counter', // カウンターの値を表示
                  style: Theme.of(context).textTheme.headlineMedium, // テーマのスタイルを適用
                ),
              ],
            ),
          ),
          floatingActionButton: FloatingActionButton(
            onPressed: _incrementCounter, // ボタンが押されたときにカウンターを増加
            tooltip: 'インクリメント', // ツールチップ(長押し時に表示されるテキスト)
            child: const Icon(Icons.add), // プラスアイコン
          ), // trailing comma はコードフォーマットを見やすくします
        );
      }
    }
    

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