Finding sum of digits of a number until sum becomes single digit

Input : 1234
Output : 1
Explanation : The sum of 1+2+3+4 = 10, 
              digSum(x) == 10
              Hence ans will be 1+0 = 1

import UIKit

class ViewController: UIViewController {

var digit = 1234

override func viewDidLoad() {

super.viewDidLoad()

let digits = digit % 9 == 0 ? 9 : digit % 9

print(digits)

// Do any additional setup after loading the view.

}

}