anchor.diff:    final Function(double position)? onSheetMoved;
     final Function(double position, SnappingPosition snappingPosition)?
         onSnapCompleted;
  +
  +  /// This is called when a snapping animation starts
  +  final Function(double position, SnappingPosition snappingPosition)?
  +      onSnapStart;
   
     SnappingSheet({
       Key? key,
  
positive.code:     this.controller,
      this.onSheetMoved,
      this.onSnapCompleted,
    })  : assert(snappingPositions.isNotEmpty),
          super(key: key);
  
  
negative.code:   }
  
    void _snapToPosition(SnappingPosition snappingPosition) {
      widget.onSnapStart?.call(_currentPosition, snappingPosition);
      _animateToPosition(snappingPosition);
      _lastSnappingPosition = snappingPosition;
    }
  