snipoet by https://flutter.dev/docs/development/ui/animations/tutorial

main.dart

//refer https://flutter.dev/docs/development/ui/animations/tutorial
import 'dart:ui';

import 'package:flutter/animation.dart';
import 'package:flutter/material.dart';

void main() => runApp(LogoApp());

class LogoApp extends StatefulWidget {
  _LogoAppState createState() => _LogoAppState();
}

class _LogoAppState extends State<LogoApp> with SingleTickerProviderStateMixin {
  Animation<double> animation;
  AnimationController controller;

  @override
  void initState() {
    super.initState();
    controller =
        AnimationController(duration: const Duration(seconds: 2), vsync: this);

    animation = CurvedAnimation(parent: controller, curve: Curves.easeIn)
      ..addStatusListener((status) {
        if (status == AnimationStatus.completed) {
          controller.reverse();
        } else if (status == AnimationStatus.dismissed) {
          controller.forward();
        }
      })
      ..addStatusListener((state) => print('$state'));
    controller.forward();
  }

  @override
  Widget build(BuildContext context) => GrowTransition(
        child: LogoWidget(),
        animation: animation,
      );

  @override
  void dispose() {
    controller.dispose();
    super.dispose();
  }
}

class LogoWidget extends StatelessWidget {
  // Leave out the height and width so it fills the animating parent
  Widget build(BuildContext context) => Container(
        margin: EdgeInsets.symmetric(vertical: 10),
        child: FlutterLogo(),
      );
}

class GrowTransition extends StatelessWidget {
  // constructor
  // set classs val by args
  GrowTransition({this.child, this.animation});

  final Widget child;
  final Animation<double> animation;
  // Make the Tweens static because they don't change.
  static final _opacityTween = Tween<double>(begin: 0.1, end: 1);
  static final _sizeTween = Tween<double>(begin: 0, end: 300);

  // declare animation and child(logo widget)
  Widget build(BuildContext context) => Center(
    child: Opacity(
      opacity: _opacityTween.evaluate(animation),
      child: AnimatedBuilder(
            animation: animation,
            builder: (context, child) => Container(
                  color: Colors.lightBlue[200],
                  height: _sizeTween.evaluate(animation),
                  width: _sizeTween.evaluate(animation),
                  //height: animation.value,
                  //width: animation.value,
                  child: child,
                ),
            child: child),
      ));
}