issue_28814

main.dart

import 'package:flutter/material.dart';
import 'dart:math' show Random;
import 'dart:ui' as ui;

extension PathFromSizeX on Size {
  Path path(double c) => Path()
    ..moveTo(0, height * .5)
    ..quadraticBezierTo(
      width * c,
      height,
      width,
      height * c,
    );
}

extension RandomX on Size {
  Offset randomOffset() => Offset(
        Random().nextInt(width.truncate()).toDouble(),
        Random().nextInt(height.truncate()).toDouble(),
      );
}

void main() => runApp(const MaterialApp(home: MyHomePage()));

class MyHomePage extends StatefulWidget {
  final Size size;
  const MyHomePage({super.key, this.size = const Size(200, 400)});

  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  late final ValueNotifier<int> _dotCount;
  late final ValueNotifier<double> _pathCoefficient;

  void _increaseDotCount() => _dotCount.value++;
  void _updatePathCx() => _pathCoefficient.value += .3;
  @override
  void initState() {
    _dotCount = ValueNotifier(0);
    _pathCoefficient = ValueNotifier(1);
    super.initState();
  }

  @override
  void dispose() {
    _dotCount.dispose();
    _pathCoefficient.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) => Scaffold(
        body: Column(
          mainAxisAlignment: MainAxisAlignment.center,
          children: [
            ValueListenableBuilder<double>(
              valueListenable: _pathCoefficient,
              builder: (context, coefficient, child) =>
                  ValueListenableBuilder<int>(
                valueListenable: _dotCount,
                builder: (context, dotCount, child) => CustomPaint(
                  size: widget.size,
                  painter: MyCustomPainter(
                    path: widget.size.path(coefficient),
                    dots: dotCount,
                  ),
                ),
              ),
            ),
            TextButton(
              onPressed: _increaseDotCount,
              child: const Icon(Icons.plus_one),
            ),
          ],
        ),
        floatingActionButton: FloatingActionButton(
          onPressed: _updatePathCx,
          child: const Icon(Icons.add),
        ),
      );
}

class MyCustomPainter extends CustomPainter {
  final Path path;
  final int dots;
  final Color color;
  final double pathStrokeWidth;
  final double dotsStrokeWidth;

  const MyCustomPainter({
    required this.path,
    required this.dots,
    this.color = Colors.red,
    this.pathStrokeWidth = .8,
    this.dotsStrokeWidth = 4,
  });

  @override
  void paint(Canvas canvas, Size size) {
    canvas.drawPath(
      path,
      Paint()
        ..color = color
        ..style = PaintingStyle.stroke
        ..strokeWidth = pathStrokeWidth,
    );

    canvas.drawPoints(
      ui.PointMode.points,
      [for (int i = 0; i < dots; ++i) size.randomOffset()],
      Paint()
        ..color = Colors.black
        ..strokeWidth = dotsStrokeWidth
        ..strokeCap = StrokeCap.round,
    );
  }

  @override
  bool shouldRepaint(MyCustomPainter oldDelegate) => oldDelegate.path != path;
}