anchor.diff:  
     void onScaleStart(ScaleStartDetails details) {
       _scaleBefore = scaleStateAwareScale();
  -    _positionBefore = _position;
       _normalizedPosition = details.focalPoint - _position;
       _scaleAnimationController.stop();
       _positionAnimationController.stop();
  
positive.code: class _PhotoViewImageWrapperState extends State<PhotoViewImageWrapper>
      with TickerProviderStateMixin {
    Offset _position;
    Offset _positionBefore;
    Offset _normalizedPosition;
    double _scale;
    double _scaleBefore;
  
negative.code:       ..fling(velocity: 0.4);
    }
  
     void animatePosition(Offset from, Offset to) {
      _positionAnimation = Tween<Offset>(begin: from, end: to)
          .animate(_positionAnimationController);
      _positionAnimationController
        ..value = 0.0
        ..fling(velocity: 0.4);
    }
  
    @override
  