Overloads the base QGraphicsScene wheelEvent method to \
        use scrolling as a zooming mechanism, if the system \
        is currently in the view mode.
        
        :param      event   <QWheelEvent>

def wheelEvent( self, event ):
        """
        Overloads the base QGraphicsScene wheelEvent method to \
        use scrolling as a zooming mechanism, if the system \
        is currently in the view mode.
        
        :param      event   <QWheelEvent>
        """
        # check to see if the system is
        # currently in view mode
        if ( self.inViewMode() ):
            if ( event.delta() < 0 ):
                self.zoomOut()
            else:
                self.zoomIn()
            
            event.accept()
            self.mainView().setFocus()
        
        else:
            super(XNodeScene, self).wheelEvent(event)