On iOS 11, Apple introduced a new way to share auth data between the web and a Mobile App with SFAuthenticationSession.
It would be a bad user experience to show the SFAuthenticationSession prompt to every new users - that might have never used my website - to have them agree and then get nothing out of it and have ask them to login.
The documentation is pretty empty on Apple side. This is the only example I found.
Is there a way to know if there is a cookie available before showing the SFAuthenticationSession prompt? Or maybe, with the Associated Domains enabled, when authenticating with my domain, the system should not show the prompt?
No. Even if there aren't cookies, the user can login entering his username/password and then clicking on "Login"/"Enter" on the website (E.g: Facebook, Instagram).
Accessing the cookies won't be enabled:
When the webpage is presented, it runs in a separate process, so the user and web service are guaranteed that the app has no way to gain access to the user’s credentials. Instead, the app gets a unique authentication token. Official docs
Working example for Instagram-OAuth: https://github.com/dvdhpkns/SFAuthenticationSession-Instagram-Oauth
And the GitHub repo you added was posted by the author to send a bug report about errors in cookie sharing for local servers (rdar://33418129. Original tweet)
Are cookies shared between Safari and SFAuthenticationSession? @DVDHPKNS
They’re supposed to be shared, but we have some timing bugs right now. Please do file bugs about what you’re seeing. @rmondello (Apple employee)
P.S: They added more information to the docs since your original post date.