Auth.auth().createUser(withEmail: emailTextField.text!, password: passwordTextField.text!, completion: {
            (user, error) in

            // user's auth credentials are now created but user can still be nil

Auth.auth().signIn(withEmail: self.emailTextField.text!, password: self.passwordTextField.text!, completion: {
            (user, error) in

            // user's auth credentials are now accessed but user can still be nil

class FirebaseSingleton{

    static let sharedInstance = FirebaseSingleton()

    var dbRef: DatabaseReference? = Database.database().reference()
    var storageDbRef: StorageReference? = Storage.storage().reference(forURL: "gs://blablabla.appspot.com")
    var currentUser: User? = Auth.auth().currentUser
    var currentUserID: String? = Auth.auth().currentUser?.uid
    var currentUserEmail: String? = Auth.auth().currentUser?.email
    var currentUserPhotoUrl: URL? = Auth.auth().currentUser?.photoURL
    var currentUserDisplayName: String? = Auth.auth().currentUser?.displayName
}

Auth.auth().createUser(withEmail: emailTextField.text!, password: passwordTextField.text!, completion: {
                (user, error) in

                if error != nil { return }

                if let user = user{

                    let sharedInstance = FirebaseSingleton.sharedInstance
                    sharedInstance.currentUser = user
                    sharedInstance.currentUserID = user.uid
                    sharedInstance.currentUserEmail = user.email // at this point is it possible for this to be nil?
                    sharedInstance.currentUserPhotoUrl = user.photoURL
                    sharedInstance.currentUserDisplayName = user.displayName
                }else{
                   return
                }

                // is this guard statement necessary?
                guard let email = user.uid else { return }
})

Auth.auth().signIn(withEmail: emailTextField.text!, password: passwordTextField.text!, completion: {
                (user, error) in

                if error != nil { return }

                if let user = user{

                    let sharedInstance = FirebaseSingleton.sharedInstance
                    sharedInstance.currentUser = user
                    sharedInstance.currentUserID = user.uid
                    sharedInstance.currentUserEmail = user.email // at this point is it possible for this to be nil?
                    sharedInstance.currentUserPhotoUrl = user.photoURL
                    sharedInstance.currentUserDisplayName = user.displayName
                }else{
                   return
                }

                // is this guard statements necessary?
                guard let email = user.uid else { return }
})