draw_atlas.dart

import 'dart:math';
import 'dart:ui' as ui;

import 'package:flutter/material.dart';
import 'package:collection/collection.dart';

void main() => runApp(MaterialApp(home: Scaffold(body: Foo())));

class Foo extends StatefulWidget {
  @override
  State<Foo> createState() => _FooState();
}

class _FooState extends State<Foo> with TickerProviderStateMixin {
  late final controller = AnimationController(vsync: this, duration: Durations.extralong4 * 3);

  @override
  Widget build(BuildContext context) {
    return Column(
      children: [
        ElevatedButton(
          onPressed: () => controller.forward(from: 0),
          child: Text('animate for ${controller.duration!.inMilliseconds}ms'),
        ),
        Expanded(
          child: ClipRect(
            child: CustomPaint(
              painter: FooPainter(controller),
              child: const SizedBox.expand(),
            ),
          ),
        ),
      ],
    );
  }
}

class FooPainter extends CustomPainter {
  FooPainter(this.controller) : super(repaint: controller) {
    final recorder = ui.PictureRecorder();
    final canvas = Canvas(recorder);

    // different shapes of 'point':
    (switch (1) {
      // filled circle
      0 => const ShapeDecoration(color: Colors.black, shape: CircleBorder()),
      // outlined circle
      1 => const ShapeDecoration(shape: CircleBorder(side: BorderSide(color: Colors.black, width: R * 0.6))),
      // 8-pointed star
      _ => const ShapeDecoration(color: Colors.black, shape: StarBorder(points: 8, innerRadiusRatio: 0.6)),
    })
      .createBoxPainter(() {})
      .paint(canvas, Offset.zero, const ImageConfiguration(size: Size.fromRadius(R)));

    image = recorder.endRecording().toImageSync((2 * R).ceil(), (2 * R).ceil());
  }

  static const R = 6.0; // radius
  static const D = 3.0; // delta +/-
  static const N = 1000;

  final AnimationController controller;
  late ui.Image image;
  final rnd = Random();
  List<ui.Offset>? offsets;
  final rects = List.filled(N, Offset.zero & const Size.fromRadius(R));
  final colors = List.generate(N, (i) => HSVColor.fromAHSV(1, 120 * i / N, 1, 0.8).toColor());

  @override
  void paint(Canvas canvas, Size size) {
    offsets ??= [ // initialize offsets
      for (int i = 0; i < N; i++)
        Offset(rnd.nextDouble() * size.width, rnd.nextDouble() * size.height)
    ];
    if (controller.isAnimating) {
      offsets = [ // update offsets by random delta
        ...offsets!.mapIndexed((i, o) {
          final factor = D * ui.lerpDouble(1, 2, colors[i].green / 255)!;
          final x = (o.dx + factor * ui.lerpDouble(-1, 1, rnd.nextDouble())!).clamp(0.0, size.width);
          final y = (o.dy + factor * ui.lerpDouble(-1, 1, rnd.nextDouble())!).clamp(0.0, size.height);
          return Offset(x, y);
        })
      ];
    }
    List<ui.RSTransform> transforms = [
      ...offsets!.map((o) => ui.RSTransform(1, 0, o.dx - R, o.dy - R))
    ];
    canvas.drawAtlas(image, transforms, rects, colors, BlendMode.dstATop, null, Paint());
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) => false;
}