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Flutter

              
                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(
        primarySwatch: Colors.blue,
      ),
      home: MyHomePage(title: 'Flutter Demo Code In The Video'),
    );
  }
}

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

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

// 今回はcontrollerはひとつなので、SingleTickerProviderStateMixinを使用する
class _MyHomePageState extends State<MyHomePage> with TickerProviderStateMixin {
  AnimationController _controller;
  Animation<double> _animation;

  // initState内で_controllerと_animationの初期化を行う
  @override
  void initState() {
    // TODO: implement initState
    super.initState();

    // controllerの初期化
    _controller = AnimationController(
      duration: Duration(seconds: 2),
      vsync: this,
    );

    // animationの初期化
    _animation = Tween(
      begin: 0.0,
      end: 1.0,
    ).animate(_controller);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(widget.title)),
      floatingActionButton: FloatingActionButton(
        // ボタン押下時にアニメーション実行
        onPressed: () => _controller.forward(),
        child: Text('B'),
      ),
      body: Center(
        child: Column(
          mainAxisAlignment: MainAxisAlignment.spaceEvenly,
          children: <Widget>[
            Container(
              height: 100,
              width: 100,
              color: Colors.red,
            ),
            FadeTransition(
              // opacityプロパティに_animationを設定
              opacity: _animation,
              // Containerをフェードインさせる
              child: Container(
                height: 100,
                width: 100,
                color: Colors.blue,
              ),
            ),
            Container(
              height: 100,
              width: 100,
              color: Colors.green,
            ),
          ],
        ),
      ),
    );
  }
}

              
            
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