AnimatedContainer(
  duration: Duration(milliseconds: 500),
  width: isActive ? 200 : 100,
  height: 100,
  color: isActive ? Colors.blue : Colors.grey,
)

class MyAnimatedBox extends StatefulWidget {
  @override
  _MyAnimatedBoxState createState() => _MyAnimatedBoxState();
}

class _MyAnimatedBoxState extends State<MyAnimatedBox> with SingleTickerProviderStateMixin {
  late AnimationController _controller;
  late Animation<double> _animation;

  @override
  void initState() {
    super.initState();
    _controller = AnimationController(duration: Duration(seconds: 1), vsync: this);
    _animation = Tween(begin: 0.0, end: 1.0).animate(_controller);
    _controller.forward();
  }

  @override
  Widget build(BuildContext context) {
    return FadeTransition(
      opacity: _animation,
      child: Container(width: 200, height: 200, color: Colors.green),
    );
  }

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

Hero(
  tag: 'imageHero',
  child: Image.asset('assets/photo.jpg', width: 100),
)