docs: def check_data(self, data):
          """
          Check if data loaded for all sources in data layer.
  
          :param data: data layer from model
          :type data: :class:`~simkit.core.layer.Data`
          :return: dictionary of data sources and objects or `None` if not loaded
          """
          data_objs = {
              data_src: data.objects.get(data_src) for data_src in data.layer
          }
          self._is_data_loaded = all(data_objs.values())
          return data_objs