import 'package:flutter/material.dart';

void main() {
  runApp(const MyApp());
}

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.deepPurple),
        useMaterial3: true,
      ),
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatefulWidget {
  @override
  State<MyHomePage> createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  final double _size = 160;
  Offset _position = Offset.zero;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Stack(
        children: [
          GestureDetector(
            onPanUpdate: (DragUpdateDetails details) {
              final RenderBox? box = context.findRenderObject() as RenderBox?;

              setState(() {
                _position = _calculateFocalPointOffset(
                  box,
                  details.globalPosition,
                );
              });
            },
            child: Wrap(
              children: [
                for (int i = 1; i <= 100; i++)
                  Container(
                    width: 32,
                    height: 32,
                    color: i % 2 == 0 ? Colors.red : Colors.green,
                  ),
              ],
            ),
          ),
          Align(
            alignment: Alignment.topRight,
            child: RawMagnifier(
              decoration: MagnifierDecoration(
                shape: CircleBorder(
                  side: BorderSide(
                    color: Theme.of(context).primaryColor,
                    width: 5,
                  ),
                ),
              ),
              size: Size(_size, _size),
              focalPointOffset: _position,
              magnificationScale: 2.0,
            ),
          ),
        ],
      ),
    );
  }

  Offset _calculateFocalPointOffset(RenderBox? box, Offset globalTouchPoint) {
    return Offset(
      globalTouchPoint.dx - (box?.size.width ?? 0) + (_size / 2),
      globalTouchPoint.dy - (_size / 2),
    );
  }
}