anchor.diff:      _controller = widget.pageController ?? PageController();
       _locked = false;
       super.initState();
  +    WidgetsBinding.instance.addPostFrameCallback((_) => _controller.jumpToPage(widget.defaultPageIndex) );
     }
   
     void scaleStateChangedCallback(PhotoViewScaleState scaleState) {
  
positive.code:   /// Mirror to [PhotoView.enableRotation]
    final bool enableRotation;
  
    @override
    State<StatefulWidget> createState() {
      return _PhotoViewGalleryState();
  
negative.code:     this.onPageChanged,
      this.scaleStateChangedCallback,
      this.enableRotation = false,
      this.defaultPageIndex = 0,
    }) : super(key: key);
  
    /// A list of options to describe the items in the gallery
  