stiletto_dart/lib/main.dart

import 'package:flutter/material.dart';
import 'package:flutter/widgets.dart';
import 'dart:ui';

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

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Stiletto',
      theme: ThemeData(
        primarySwatch: Colors.red,
      ),
      home: Scaffold(
          appBar: AppBar(
            // Here we take the value from the StilettoCanvas object that was created by
            // the App.build method, and use it to set our appbar title.
            title: Text("Stiletto"),
          ),
          body: Center(
              child: SizedBox.expand(
                  child: StilettoCanvas()
              )
          )
      )
    //  home: Center(
    //      child: SizedBox.expand(
    //          child: StilettoCanvas()
    //      )
    //  )
    );
  }
}

class StilettoCanvas extends StatefulWidget {
  StilettoCanvas({Key? key}) : super(key: key);

  // This widget is the home page of your application. It is stateful, meaning
  // that it has a State object (defined below) that contains fields that affect
  // how it looks.

  // This class is the configuration for the state. It holds the values (in this
  // case the title) provided by the parent (in this case the App widget) and
  // used by the build method of the State. Fields in a Widget subclass are
  // always marked "final".
  @override
  _StilettoCanvasState createState() => _StilettoCanvasState();
}

class _StilettoCanvasState extends State<StilettoCanvas> {
  Offset _lastMousePos = Offset.zero;
  RenderBox? _currentRenderBox = null;

  GlobalKey _globalKey = GlobalKey();

  _afterLayout(Duration) {
      RenderObject? renderObject = _globalKey.currentContext?.findRenderObject();
      if (renderObject is RenderBox) {
          _currentRenderBox = renderObject;
      }
  }

  void _onPointerDown(PointerDownEvent event) {
      print(event.position);
      setState(() {
          Offset? localMousePos = _currentRenderBox?.globalToLocal(event.position);
          if (localMousePos is Offset) {
              _lastMousePos = localMousePos;
          }
      });
  }

  @override
  Widget build(BuildContext context) {
    // This method is rerun every time setState is called, for instance as done
    // by the _incrementCounter method above.
    //
    // The Flutter framework has been optimized to make rerunning build methods
    // fast, so that you can just rebuild anything that needs updating rather
    // than having to individually change instances of widgets.
    WidgetsBinding.instance?.addPostFrameCallback(_afterLayout);
    return Listener(
        onPointerDown: _onPointerDown,
        child: ClipRect(
            child: CustomPaint(
                painter: StilettoPainter(_lastMousePos),
            ),
            key: _globalKey
        )
    );
  }
}

class StilettoPainter extends CustomPainter {
  StilettoPainter(this.sunPos) : super();

  final Offset sunPos;

  Alignment _offsetToAlignment(Offset offset, Rect rect) {
      return Alignment(
          2 * offset.dx / rect.width - 1,
          2 * offset.dy / rect.height - 1
      );
  }

  @override
  void paint(Canvas canvas, Size size) {
    final Rect rect = Offset.zero & size;
    final RadialGradient gradient = RadialGradient(
      center: _offsetToAlignment(sunPos, rect),
      radius: 0.2,
      colors: <Color>[Color(0xFFFFFF00), Color(0xFF0099FF)],
      stops: <double>[0.4, 1.0],
    );
    canvas.drawRect(
      rect,
      Paint()..shader = gradient.createShader(rect),
    );
  }

  @override
  bool shouldRepaint(StilettoPainter oldDelegate) {
      return sunPos != oldDelegate.sunPos;
  }
}