How to identify user across devices on iOS?

I am working on an app where user will be buying some "credits" as in-app purchases and use them to buy some services inside the app. As user can has more then one device I need to identify him across different devices.

I understand I can create some logging (and store user accounts on my own server) but I would like to avoid this. In best case I would like to use Apple ID but I can't figure out how can I access Apple ID inside an app?

I found similar questions here but no satisfying answer. I do not believe there is no chance to access App开发者_运维问答le ID from the app (I don't need password, just the id) as I found something like AVMetadataiTunesMetadataKeyAppleID in AVFoundation but this framework seems to be only for Mac OS X.

Any suggestion how to solve this?

Sorry. There's no way to do it without implementing your own authentication system on your own server. UDIDs are unique to each device and Apple does not give access to the iTunes user account whatsoever.

Other way is you can use facebook authentication in order to achieve that. Just connect you app with facebook api and u can keep their login address with some additional info.

Furthermore, it might be useful for your app if they can share your app with their facebook friends :p

You can identify user with help of GameKit.