Flutterのホットリロード

#はじめに
今回の記事ではFlutterの特徴でもあるホットリロード機能について、実際に手を動かしながら体験します。
Flutterの環境構築を行っていない方は以下の記事も是非。

**Flutter環境構築**
・[Flutter環境構築 - 1（Flutter SDKインストール編）](https://qiita.com/naru_chan__/items/359fcdfb8aa092e9a44e)
・[Flutter環境構築 - 2（Xcode・iOSシミュレーターセットアップ編）](https://qiita.com/naru_chan__/items/d64455cb7d0e7dd223a5)
・[Flutter環境構築 - 3（Android Studio・Androidエミュレータセットアップ編）](https://qiita.com/naru_chan__/items/94e8e2f85bd1803feddd)
・[Flutter環境構築 - 4（VSCodeセットアップ編）](https://qiita.com/naru_chan__/items/1cf6f3d43beb97698592)

#ホットリロードしてみる
Android StudioでFlutterのサンプルアプリを起動します。
サンプルアプリを作成されていない方は、Android Studio起動画面にて**Start a new Flutter project**をクリックするとFlutterのプロジェクトが作成されます。

初期状態では、main.dartファイルは以下のようになっていると思います。

```main.dart
import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // 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 running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.blue,
        // This makes the visual density adapt to the platform that you run
        // the app on. For desktop platforms, the controls will be smaller and
        // closer together (more dense) than on mobile platforms.
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  // 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
  _MyHomePageState 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(
        // 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.
          //
          // Invoke "debug painting" (press "p" in the console, choose the
          // "Toggle Debug Paint" action from the Flutter Inspector in Android
          // Studio, or the "Toggle Debug Paint" command in Visual Studio Code)
          // to see the wireframe for each widget.
          //
          // 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).
          mainAxisAlignment: MainAxisAlignment.center,
          children: <Widget>[
            Text(
              'You have pushed the button this many times:',
            ),
            Text(
              '$_counter',
              style: Theme.of(context).textTheme.headline4,
            ),
          ],
        ),
      ),
      floatingActionButton: FloatingActionButton(
        onPressed: _incrementCounter,
        tooltip: 'Increment',
        child: Icon(Icons.add),
      ), // This trailing comma makes auto-formatting nicer for build methods.
    );
  }
}
```

Flutterのプロジェクトが起動できたら、iOSシミュレータもしくはAndroidエミュレータを起動し**Run**をクリックします。
![スクリーンショット 2020-04-25 2.17.31.jpg](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/578233/343f7d7a-030a-5581-75c8-15b4dde96896.jpeg)

するとシミュレータの方でFlutterアプリが起動します。
![スクリーンショット 2020-04-25 2.27.58.png](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/578233/1156d1de-7420-cbde-49cd-c5557d04ca5b.png)


ではホットリロードを試してみます。
MyAppクラスの**primarySwatch: Colors**を**red**変更してみましょう。

```main.dart
class MyApp extends StatelessWidget {
  // 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 running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.red,  //←ココ！！！！
        // This makes the visual density adapt to the platform that you run
        // the app on. For desktop platforms, the controls will be smaller and
        // closer together (more dense) than on mobile platforms.
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}
```

変更したら**Command+Sキー**を入力し、ホットリロードします。
するとシミュレーターの配色テーマが先ほどの青色から**赤色**に変わります。
![スクリーンショット 2020-04-25 2.27.47.png](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/578233/879da0c6-c632-3bdc-acd3-7b448196d20a.png)

では次は、画面上方のタイトルを変更してみます。
MyAppクラスの**home: MyHomePage**の**title:**を**Sample App**に変更してみましょう。

```main.dart
class MyApp extends StatelessWidget {
  // 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 running your application with "flutter run". You'll see the
        // application has a blue toolbar. Then, without quitting the app, try
        // changing the primarySwatch below to Colors.green and then invoke
        // "hot reload" (press "r" in the console where you ran "flutter run",
        // or simply save your changes to "hot reload" in a Flutter IDE).
        // Notice that the counter didn't reset back to zero; the application
        // is not restarted.
        primarySwatch: Colors.red,
        // This makes the visual density adapt to the platform that you run
        // the app on. For desktop platforms, the controls will be smaller and
        // closer together (more dense) than on mobile platforms.
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Sample App'),  //←ココ！！！！
    );
  }
}
```

変更したら先ほどと同じく**Command+Sキー**でホットリロードします。
するとシミュレータ画面上方のタイトルが**Sample App**に変更されているのが確認できます。
![スクリーンショット 2020-04-25 2.35.45.png](https://qiita-image-store.s3.ap-northeast-1.amazonaws.com/0/578233/05122418-1ccc-99cb-6943-6e0fc53e3394.png)


#最後に
今回は以上となります。
簡単ではありましたが、ホットリロードがどんなものなのか体感できたと思います。
変更がすぐに確認できて、すごく便利な機能ですね。

最後までご覧くださりありがとうございました。