Flutter: magic, fading circle color

main.dart

class FadingCircle extends Particle with Fading {
  final double radius;
  final Color color;

  FadingCircle({
    this.radius = 10,
    this.color = Colors.white,
  });

  @override
  void draw(Canvas canvas, Size size) {
    canvas.drawCircle(
      Offset.zero,
      radius,
      Paint()..color = color.withOpacity(opacity),
    );
  }
}