func isValidEmail(_ email: String) -> Bool {
   let emailRegEx = 
       "[A-Z0-9a-z._%+-]+@[A-Za-z0-9.-]+\\.[A-Za-z]{2,64}"
   let emailPred = NSPredicate(format:"SELF MATCHES %@",
       emailRegEx)
   return emailPred.evaluate(with: email)
}
  
func isValidPassword(_ password: String) -> Bool {
   let minPasswordLength = 6
   return password.count >= minPasswordLength
}

Auth.auth().currentUser contains the current user, if there is one.
Auth.auth().currentUser.email contains the userID (for email/password)

func isUserLoggedIn() -> Bool {
  return Auth.auth().currentUser != nil
}

if isUserLoggedIn() {
   // Show logout page
} else {
   // Show login page
}