如何在IOS中查找照片资产的标识符

I am writing an app which saves a photo stored in a custom album and associates other information with it in an object. The object is then saved in core data. I would like to store a reference to the photo in the object. Is there a unique identifier that is available or can be derived. I need to be able to access the photo. I am using Photos framework.I am writing an app which saves a photo stored