Pan and scaling gesture recognizer for flutter

PanAndScalingGestureRecognizer.dart

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

class PanAndScalingGestureRecognizer extends StatefulWidget {
  final Widget child;
  final void Function(Offset initialPoint) onPanStart;
  final void Function(Offset initialPoint, Offset delta) onPanUpdate;
  final void Function() onPanEnd;

  final void Function(Offset initialFocusPoint) onScalingStart;
  final void Function(Offset changedFocusPoint, double scale) onScalingUpdate;
  final void Function() onScalingEnd;

  PanAndScalingGestureRecognizer({
    this.child,
    this.onPanStart,
    this.onPanUpdate,
    this.onPanEnd,
    this.onScalingStart,
    this.onScalingUpdate,
    this.onScalingEnd,
  });

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

class _PanAndScalingGestureRecognizerState
    extends State<PanAndScalingGestureRecognizer> {
  final List<Touch> touches = [];
  double initialScalingDistance;

  @override
  Widget build(BuildContext context) {
    return RawGestureDetector(
      child: widget.child,
      gestures: {
        ImmediateMultiDragGestureRecognizer:
            GestureRecognizerFactoryWithHandlers<
                ImmediateMultiDragGestureRecognizer>(
          () => ImmediateMultiDragGestureRecognizer(),
          (ImmediateMultiDragGestureRecognizer instance) {
            instance.onStart = (Offset offset) {
              final touch = Touch(
                offset,
                (drag, details) => _onTouchUpdate(drag, details),
                (drag, details) => _onTouchEnd(drag, details),
              );
              _onTouchStart(touch);
              return touch;
            };
          },
        ),
      },
    );
  }

  void _onTouchStart(Touch touch) {
    touches.add(touch);
    if (touches.length == 1) {
      widget.onPanStart(touch.startOffset);
    } else if (touches.length == 2) {
      initialScalingDistance =
          (touches[0].currentOffset - touches[1].currentOffset).distance;
      widget.onScalingStart(
          (touches[0].currentOffset + touches[1].currentOffset) / 2);
    } else {
      // Do nothing/ ignore
    }
  }

  void _onTouchUpdate(Touch touch, DragUpdateDetails details) {
    assert(touches.isNotEmpty);
    touch.currentOffset = details.localPosition;

    if (touches.length == 1) {
      widget.onPanUpdate(
        touch.startOffset,
        details.localPosition - touch.startOffset,
      );
    } else {
      // TODO average of ALL offsets, not only 2 first
      var newDistance =
          (touches[0].currentOffset - touches[1].currentOffset).distance;

      widget.onScalingUpdate(
        (touches[0].currentOffset + touches[1].currentOffset) / 2,
        newDistance / initialScalingDistance,
      );
    }
  }

  void _onTouchEnd(Touch touch, DragEndDetails details) {
    touches.remove(touch);
    if (touches.length == 0) {
      widget.onPanEnd();
    } else if (touches.length == 1) {
      widget.onScalingEnd();

      // Restart pan
      touches[0].startOffset = touches[0].currentOffset;
      widget.onPanStart(touches[0].startOffset);
    }
  }
}

class Touch extends Drag {
  Offset startOffset;
  Offset currentOffset;
  final void Function(Drag drag, DragUpdateDetails details) onUpdate;
  final void Function(Drag drag, DragEndDetails details) onEnd;

  Touch(this.startOffset, this.onUpdate, this.onEnd) {
    currentOffset = startOffset;
  }

  @override
  void update(DragUpdateDetails details) {
    super.update(details);
    onUpdate(this, details);
  }

  @override
  void end(DragEndDetails details) {
    super.end(details);
    onEnd(this, details);
  }
}

you have to implement null checkings

if (widget.onPanStart != null)

because I got an exception...

Here is the new one

https://gist.github.com/klaszlo8207/0df97fb3ba7484e20b0061076f3be842

I also added:

final void Function(double dx) onHorizontalDragUpdate;
final void Function(double dy) onVerticalDragUpdate;

Can rotation be calculated with this widget?