Flutterラボ リリース3周年記念! プロコースが1ヶ月の料金で2ヶ月間受けられる、実質半額セール (2024年3月17日まで・先着)

Containerのアニメーション版で、一定期間にわたって値が徐々に変化する動作を表現することができます。
import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  const MyApp({Key? key}) : super(key: key);

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      home: const MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

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

  final String title;

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  bool selected = false;

  @override
  Widget build(BuildContext context) {
    return GestureDetector(
      onTap: () {
        setState(() {
          selected = !selected;
        });
      },
      child: Center(
        child: AnimatedContainer(
          width: selected ? 200.0 : 100.0,
          height: selected ? 100.0 : 200.0,
          color: selected ? Colors.red : Colors.blue,
          alignment:
              selected ? Alignment.center : AlignmentDirectional.topCenter,
          duration: const Duration(seconds: 2),
          curve: Curves.fastOutSlowIn,
          child: const FlutterLogo(size: 75),
        ),
      ),
    );
  }
}