FBSDKAccessToken currentAccessToken nil after quitting app

The problem is because you are calling for FBSDKAccessToken.currentAccessToken() before having called

FBSDKApplicationDelegate.sharedInstance().application(application, didFinishLaunchingWithOptions: launchOptions)

You can check for the access token anytime after calling the above line.

EDIT: Explanation

The above line lets the Facebook SDK process the launchOptions and extract the necessary information which it will require to recognise and persist the user for the application.

In cases where the user is already logged in, this simply initialises the Facebook SDK which in turn logs in the user on the basis of persisted data.

I spent a nice half-day banging my head against this issue. In spite of making sure all the delegate methods were present, the FBSDKAccessToken.current() always returned nil.

It turns out that this is because Keychain Sharing is not enabled ( Xcode 8, iOS 10). To fix, go to App -> Capabilities -> Keychain Sharing and turn ON.

Once this is done, you have to still go through the authorization process and return back to the app. Everything should be fine after.