An animated box shadow button with Dart 2.3 collection if/for

main.dart

class CircleStack extends StatefulWidget {
  @override
  _CircleStackState createState() => _CircleStackState();
}

class _CircleStackState extends State<CircleStack> {
  bool open = true;

  @override
  Widget build(BuildContext context) {
    return Center(
      child: AnimatedContainer(
        width: 80,
        height: 80,
        duration: Duration(seconds: 2),
        curve: Curves.easeOutQuart,
        decoration: BoxDecoration(
          shape: BoxShape.circle,
          color: Colors.purple,
          boxShadow: [
            if (open) // 👈
              for (var i = 0; i < 5; i += 1) // 👈
                BoxShadow(
                    spreadRadius: i * 40.0,
                    color: Colors.purple.withAlpha((255 / (i + 1)).toInt()))
          ],
        ),
        child: IconButton(
          icon: Icon(Icons.mic, color: Colors.white),
          onPressed: () {
            setState(() {
              open = !open;
            });
          },
        ),
      ),
    );
  }
}