eyes_that_follow.dart

import 'package:flutter/material.dart';

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

class EyesThatFollow extends StatefulWidget {
  const EyesThatFollow({super.key});

  @override
  State<EyesThatFollow> createState() => _EyesThatFollowState();
}

class _EyesThatFollowState extends State<EyesThatFollow> {
  Offset? widgetPosition, cusorPosition;
  final key = GlobalKey();

  @override
  void initState() {
    super.initState();
    WidgetsBinding.instance
        .addPostFrameCallback((_) => _updateWidgetPosition());
  }

  void _updateWidgetPosition() =>
      setState(() => widgetPosition = getWidgetOffset(key));

  @override
  Widget build(BuildContext context) {
    final canUsePupil = widgetPosition != null && cusorPosition != null;
    return MouseRegion(
      onHover: (event) => setState(() => cusorPosition = event.position),
      onExit: (event) => setState(() => cusorPosition = null),
      onEnter: (_) => _updateWidgetPosition(),
      child: Scaffold(
        body: Center(
          child: Stack(
            alignment: AlignmentDirectional.center,
            children: [
              CustomPaint(
                key: key,
                size: const Size(80, 50),
                painter: CirclePainter(fill: !canUsePupil),
              ),
              if (canUsePupil) Pupil(radians(widgetPosition!, cusorPosition!))
            ],
          ),
        ),
      ),
    );
  }
}

class Pupil extends StatelessWidget {
  const Pupil(this.direction, {super.key});

  final double direction;

  @override
  Widget build(BuildContext context) {
    return Transform.translate(
      offset: Offset.fromDirection(direction).scale(24, 10),
      child: CustomPaint(
        size: const Size(20, 15),
        painter: CirclePainter(fill: true, color: Colors.blue),
      ),
    );
  }
}

double radians(Offset element, Offset cursor) => (cursor - element).direction;

Offset? getWidgetOffset(GlobalKey key) {
  final renderBox = key.currentContext?.findRenderObject() as RenderBox?;
  final origin = renderBox?.localToGlobal(Offset.zero);
  return renderBox?.size.center(origin!);
}

class CirclePainter extends CustomPainter {
  CirclePainter({
    bool fill = false,
    Color color = Colors.grey,
  }) : _paint = Paint()
          ..color = color
          ..strokeWidth = 2
          ..style = fill ? PaintingStyle.fill : PaintingStyle.stroke;

  final Paint _paint;

  @override
  void paint(Canvas canvas, Size size) =>
      canvas.drawOval(Rect.fromLTWH(0, 0, size.width, size.height), _paint);

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