bubbles.dart

class Bubbles extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _BubblesState();
  }
}

class _BubblesState extends State<Bubbles> with TickerProviderStateMixin {
  AnimationController animationController;
  final bubbles = <Bubble>[];

  @override
  void initState() {
    super.initState();

    List.generate(5, (i) {
      bubbles.add(Bubble());
    });

    animationController =
        AnimationController(vsync: this, duration: Duration(milliseconds: 1000))
          ..addListener(() {
            for (int i = 0; i < bubbles.length; i++) {
              bubbles[i].move();

              if (bubbles[i].remainingLife < 0 || bubbles[i].radius < 0) {
                bubbles[i] = Bubble();
              }
            }
          })
          ..repeat();
  }

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

  @override
  Widget build(BuildContext context) {
    return AnimatedBuilder(
            animation: animationController,
            builder: (context, child) => CustomPaint(
                  size: Size(64.0, 64.0),
                  painter: _BubblePainter(bubbles),
                ),
          );
  }
}

class _BubblePainter extends CustomPainter {
  final List<Bubble> bubbles;

  _BubblePainter(this.bubbles);

  @override
  void paint(Canvas canvas, Size size) {
    for (var bubble in bubbles) {
      bubble.display(canvas);
    }
  }

  @override
  bool shouldRepaint(_BubblePainter oldDelegate) => true;
}