Listen to auth events

override func viewDidLoad() {
    super.viewDidLoad()
    // Do any additional setup after loading the view.

    // Assumes `sink` is declared as an instance variable in your view controller
    sink = Amplify.Hub
        .publisher(for: .auth)
        .sink { payload in
            switch payload.eventName {
            case HubPayload.EventName.Auth.signedIn:
                print("User signed in")
                // Update UI

            case HubPayload.EventName.Auth.sessionExpired:
                print("Session expired")
                // Re-authenticate the user

            case HubPayload.EventName.Auth.signedOut:
                print("User signed out")
                // Update UI

            case HubPayload.EventName.Auth.userDeleted:
                print("User deleted")
                // Update UI

            default:
                break
            }
        }
}