def on_touch_down(self, touch):
if touch.is_mouse_scrolling:
if touch.button == 'scrolldown':
if self.index > tls._MIN:
self.index = self.index - _SPEED
elif touch.button == 'scrollup':
if self.index < tls._MAX:
self.index = self.index + _SPEED
GridLayout.on_touch_down(self, touch)
