flutter-sport/lib/widgets/bubble.dart

import 'dart:math';

import 'package:flutter/material.dart';

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

 class Bubble {
   Offset speed;
   Offset location;
   double radius;
   double life;
   Color color;
   double opacity;

   double remainingLife;

   Bubble() {
     Random rd = Random();

     this.speed = Offset(-0.5 + rd.nextDouble(), -0.5 + rd.nextDouble());
     this.location = Offset(32.0, 32.0);
     this.radius = 8.0 + rd.nextDouble() * 16;
     this.life = rd.nextDouble() * 20;
     this.remainingLife = this.life;

     this.color = const Color(0xFF6fa1ea);
   }

   move() {
     this.remainingLife -= 0.1;
     this.location = this.location + this.speed;
   }

   display(Canvas canvas) {
     this.opacity = (this.remainingLife / this.life * 100).round() / 100;
     var gradient = RadialGradient(
       colors: [
         Color.fromRGBO(
             this.color.red, this.color.green, this.color.blue, this.opacity),
         Color.fromRGBO(
             this.color.red, this.color.green, this.color.blue, this.opacity),
         Color.fromRGBO(this.color.red, this.color.green, this.color.blue, 0.0)
       ],
       stops: [0.0, 0.5, 1.0],
     );

     Paint painter = Paint()
       ..style = PaintingStyle.fill
       ..shader = gradient.createShader(
           Rect.fromCircle(center: this.location, radius: this.radius));

     canvas.drawCircle(this.location, this.radius, painter);
   }
 }

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;
}