anchor.diff:      setState(() {
         _locked = scaleState != PhotoViewScaleState.contained;
       });
  +    widget.scaleStateChangedCallback != null
  +        ? widget.scaleStateChangedCallback(scaleState)
  +        : null;
     }
   
     int get actualPage {
  
positive.code:     this.gaplessPlayback = false,
      this.pageController,
      this.onPageChanged,
    }) : super(key: key);
  
    final List<PhotoViewGalleryPageOptions> pageOptions;
  
negative.code:   final bool gaplessPlayback;
    final PageController pageController;
    final PhotoViewGalleryPageChangedCallback onPageChanged;
    final PhotoViewScaleStateChangedCallback scaleStateChangedCallback;
  
    @override
    State<StatefulWidget> createState() {
  