code: def layer_pre_save(instance, *args, **kwargs):
      
  
      is_valid = True
  
      # we do not need to check validity for WM layers
      if not instance.service.type == 'Hypermap:WorldMap':
  
          # 0. a layer is invalid if its service its invalid as well
          if not instance.service.is_valid:
              is_valid = False
              LOGGER.debug('Layer with id %s is marked invalid because its service is invalid' % instance.id)
  
          # 1. a layer is invalid with an extent within (-2, -2, +2, +2)
          if instance.bbox_x0 > -2 and instance.bbox_x1 < 2 and instance.bbox_y0 > -2 and instance.bbox_y1 < 2:
              is_valid = False
              LOGGER.debug(
                  'Layer with id %s is marked invalid because its extent is within (-2, -2, +2, +2)' % instance.id
              )
  
      instance.is_valid = is_valid
docs: Used to check layer validity.