How to zoom a GestureDetector with onPanUpdate call?

Question

I'm working on a drawing app. I want to enable both drawing on the canvas and zooming the drawings. So far, I tried achieving it by wrapping GestureDetector in InteractiveViewer and using AbsorbPointer to turn the zoom mode on and off. See the minimum demo code below.

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

void main() => runApp(new MaterialApp(
  home: new IssueExamplePage(),
  debugShowCheckedModeBanner: false,
));

class IssueExamplePage extends StatefulWidget {
  @override
  _IssueExamplePageState createState() => _IssueExamplePageState();
}

class _IssueExamplePageState extends State<IssueExamplePage> {
  bool drawingBlocked = false;
  List<Offset> points = [];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: SafeArea(
        child: Container(
          color: Colors.grey,
          padding: EdgeInsets.all(5),
          child: Container(
            color: Colors.white,
            child: InteractiveViewer(
              child: AbsorbPointer(
                absorbing: drawingBlocked,
                child: GestureDetector(
                  behavior: HitTestBehavior.translucent,
                  onPanUpdate: (details) {
                    RenderBox renderBox = context.findRenderObject();
                    Offset cursorLocation = renderBox.globalToLocal(details.localPosition);

                    setState(() {
                      points = List.of(points)..add(cursorLocation);
                    });
                  },
                  onPanEnd: (details) {
                    setState(() {
                      points = List.of(points)..add(null);
                    });
                  },
                  child: CustomPaint(
                    painter: MyPainter(points),
                    size: Size.infinite
                  ),
                ),
              ),
            ),
          ),
        ),
      ),
      floatingActionButton: Column(
        mainAxisAlignment: MainAxisAlignment.end,
        children: [
          FloatingActionButton(
            child: Icon(drawingBlocked? CupertinoIcons.hand_raised_fill : CupertinoIcons.hand_raised),
            onPressed: () {
              setState(() {
                drawingBlocked = !drawingBlocked;
              });
            }
          ),
          SizedBox(
            height: 10
          ),
          FloatingActionButton(
            child: Icon(Icons.clear),
            onPressed: () {
              setState(() {
                points = [];
              });
            }
          ), 
          SizedBox(
            height: 20
          ),
        ],
      ),
    );
  }
}

class MyPainter extends CustomPainter {
  MyPainter(this.points);
  List<Offset> points;
  Paint paintBrush = Paint()
    ..color = Colors.blue
    ..strokeWidth = 5
    ..strokeJoin = StrokeJoin.round
    ..strokeCap = StrokeCap.round;

  @override
  void paint(Canvas canvas, Size size) {
    for (int i = 0; i < points.length - 1; i++) {
      if (points[i] != null && points[i + 1] != null) {
        canvas.drawLine(points[i], points[i + 1], paintBrush);
      }
    }
  }
  
  @override
  bool shouldRepaint(MyPainter oldDelegate) {
    return points != oldDelegate.points;
  }
}

However, with this realization OnPanUpdate is working with a delay (see Flutter onPanStart invokes late when the widget is wrapped inside InteractiveViewer). Is there any other way to achieve the expected result (both zooming and drawing) or is it possible to fix the OnPanUpdate delay?

Answer 1

UPDATE: I found some sort of solution. You can use Listener instead of GestureDetector (it has substitues for the most of the parameters: onPanStart -> onPointerDown, onPanUpdate -> onPointerMove, etc.). But it seems like there's no fix for GestureDetector.