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 {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Stiletto',
      theme: ThemeData(
        primarySwatch: Colors.red,
      ),
      home: Scaffold(
          appBar: AppBar(
            title: Text("Stiletto"),
          ),
          body: Center(
              child: SizedBox.expand(
                  child: StilettoCanvas()
              )
          )
      )
    );
  }
}

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

  @override
  _StilettoCanvasState createState() => _StilettoCanvasState();
}

class _StilettoCanvasState extends State<StilettoCanvas> {
  List<Offset> _currentPath = [];
  bool _isDrawing = false;
  RenderBox? _currentRenderBox = null;

  final GlobalKey _globalKey = GlobalKey();

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

  void _onPointerDown(PointerDownEvent event) {
      setState(() {
          _currentPath = [];
          _isDrawing = true;
      });
  }

  void _onPointerUp(PointerUpEvent event) {
      setState(() {
          _isDrawing = false;
      });
  }

  void _onPointerMove(PointerMoveEvent event) {
      setState(() {
          Offset? localMousePos = _currentRenderBox?.globalToLocal(event.position);
          if (localMousePos is Offset) {
              _currentPath.add(localMousePos);
          }
      });
  }

  @override
  Widget build(BuildContext context) {
    WidgetsBinding.instance?.addPostFrameCallback(_afterLayout);
    return Listener(
        onPointerDown: _onPointerDown,
        onPointerUp: _onPointerUp,
        onPointerMove: _onPointerMove,
        child: ClipRect(
            child: CustomPaint(
                painter: StilettoPainter(_currentPath, _isDrawing),
            ),
            key: _globalKey
        )
    );
  }
}

class StilettoPainter extends CustomPainter {
  StilettoPainter(this.currentPath, this.isDrawing) : super();

  final List<Offset> currentPath;
  final bool isDrawing;

  @override
  void paint(Canvas canvas, Size size) {
    final Rect rect = Offset.zero & size;
    canvas.drawRect(
      rect,
      Paint()..color = Color(0xFFFFFFFF)
    );
    final strokePaint = Paint()
        ..color = Colors.black
        ..strokeWidth = 1.0;
    canvas.drawPoints(PointMode.polygon, currentPath, strokePaint);
  }

  @override
  bool shouldRepaint(StilettoPainter oldDelegate) {
      return currentPath != oldDelegate.currentPath || isDrawing;
  }
}