code: def source_group_receiver(self, sender, source, signal, **kwargs):
          
          from imagekit.cachefiles import ImageCacheFile
          source_group = sender
  
          instance = kwargs['instance']
  
          # Ignore signals from unregistered groups.
          if source_group not in self._source_groups:
              return
  
          #HOOK -- update source to point to image file.
          for id in self._source_groups[source_group]:
  
              spec_to_update = generator_registry.get(id, source=source, instance=instance, field=hack_spec_field_hash[id])            
                          
          specs = [generator_registry.get(id, source=source, instance=instance, field=hack_spec_field_hash[id]) for id in
                  self._source_groups[source_group]]
          callback_name = self._signals[signal]
          # print 'callback_name? %s'%(callback_name)
  
          for spec in specs:
              file = ImageCacheFile(spec)
              # print 'SEPC %s file %s'%(spec, file)
              call_strategy_method(file, callback_name)
docs: Relay source group signals to the appropriate spec strategy.