<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 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