code: def index_changed_aggregation_layer_combo(self, index):
          
          if index == 0:
              # No aggregation layer, we should display the user requested extent
              self.aggregation = None
  
              extent = setting('user_extent', None, str)
              if extent:
                  extent = QgsGeometry.fromWkt(extent)
                  if not extent.isGeosValid():
                      extent = None
  
              crs = setting('user_extent_crs', None, str)
              if crs:
                  crs = QgsCoordinateReferenceSystem(crs)
                  if not crs.isValid():
                      crs = None
  
              mode = setting('analysis_extents_mode', HAZARD_EXPOSURE_VIEW)
              if crs and extent and mode == HAZARD_EXPOSURE_BOUNDINGBOX:
                  self.extent.set_user_extent(extent, crs)
  
          else:
              # We have one aggregation layer. We should not display the user
              # extent.
              self.extent.clear_user_analysis_extent()
              self.aggregation = layer_from_combo(self.aggregation_layer_combo)
  
          self.validate_impact_function()
docs: Automatic slot executed when the Aggregation combo is changed.
  
          :param index: The index number of the selected exposure layer.
          :type index: int