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