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 AnimatedOpacity'),
    );
  }
}

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

  final String title;

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

class _MyHomePageState extends State<MyHomePage> {
  //透明度を保持する
  double _opacity = 0.8;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(title: Text(widget.title)),
      floatingActionButton: FloatingActionButton(
        child: Text("B"),
        //ボタン押下時状態変化
        onPressed: () => _changeOpacity(),
      ),
      body: Stack(
        children: <Widget>[
          Center(
            child: Text(
              "Stack & Opacity",
              style: TextStyle(fontSize: 30, fontWeight: FontWeight.bold),
            ),
          ),
          Center(
            child: AnimatedOpacity(
              //アニメーションする時間を1秒に設定
              duration: Duration(seconds: 1),
              //透明度を動的に設定
              opacity: _opacity,
              child: Container(
                width: 300,
                height: 100,
                color: Colors.green,
              ),
            ),
          ),
        ],
      ),
    );
  }

  _changeOpacity() {
    setState(() {
      if(_opacity == 0.0){
        _opacity = 0.8;
      }else{
        _opacity = 0.0;
      }
    });
  }
}