Flutter: magic, circles

main.dart

/// Just a container for parameters 
/// allowing to draw a circle on a [Canvas] later
class CircleParameters {
  double radius;
  Offset offset;

  CircleParameters({@required this.radius, @required this.offset});
}

/// Renders certain amount of circles on a [Canvas],
/// as per given [CircleParameters] list.
class Circles extends Particle with Fading {
  final List<CircleParameters> circles;

  /// Generates randomly positioned circles in
  /// given count with radius between given bounds.
  factory Circles.random({
    int count = 10,
    double maxRadius = 10,
    double minRadius,
  }) {
    if (minRadius == null) {
      minRadius = maxRadius * .1;
    }

    return Circles(
      circles: List<CircleParameters>.generate(
        count,
        (i) => CircleParameters(
          radius: Randoms.rnd.nextDouble() * maxRadius + minRadius,
          offset: Randoms.offsetFromSize(
            Size(maxRadius, maxRadius),
          ),
        ),
      ),
    );
  }

  Circles({this.circles});

  @override
  void draw(Canvas canvas, Size size) {
    for (var circle in circles) {
      canvas.drawCircle(
        circle.offset,
        circle.radius,
        Paint()..color = Colors.white.withOpacity(opacity),
      );
    }
  }
}