【 Flutter 】Flutter を 基礎 から 学習 ( ウィジェット編 ) part118 アニメーション系ウィジェット

import 'package:flutter/material.dart';

void main() {
  runApp(new MaterialApp(home: new MyHomePage()));
}

class MyHomePage extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _MyHomePageState();
  }
}

class _MyHomePageState extends State<MyHomePage>
    with SingleTickerProviderStateMixin {
  AnimationController? _controller;

  Animation<double>? _animation;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(
      vsync: this,
      duration: const Duration(seconds: 3),
    );

    _animation = _controller!.drive(
      Tween(begin: 0, end: 300),
    );
  }

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'テスト中！',
      theme: ThemeData(primarySwatch: Colors.blue),
      debugShowCheckedModeBanner: false,
      home: Scaffold(
        body: Center(
          child: Stack(
            children: <Widget>[
              FadeTransition(
                opacity: _animation!,
                child: AnimatedLogo(
                  key: null,
                  animation: _animation,
                ),
              ),
            ],
          ),
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: () {
            _controller!.forward();
          },
          child: Icon(Icons.add),
        ),
      ),
    );
  }
}

class AnimatedLogo extends AnimatedWidget {
  AnimatedLogo({
    Key? key,
    Animation<double>? animation,
  }) : super(key: key, listenable: animation!);

  Animation<double> get animation => listenable as Animation<double>;

  @override
  Widget build(BuildContext context) {
    // 本書では以下のように実装してありますがどうにも動作しません・・・。
    // final Animation<double> animation = listenable;
    return Center(
      child: Container(
        height: animation.value, width: animation.value,
        // 何度も呼ばれる
        child: const FlutterLogo(),
      ),
    );
  }
}