如何在swift中倒计时？

import UIKit

class ViewController: UIViewController {
    @IBOutlet weak var countdownLabel: UILabel!
    
    var countdownTimer: Timer?
    var totalTime = 60 // 倒计时总时长
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        startCountdown()
    }
    
    func startCountdown() {
        countdownTimer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(updateCountdown), userInfo: nil, repeats: true)
    }
    
    @objc func updateCountdown() {
        if totalTime > 0 {
            totalTime -= 1
            countdownLabel.text = "\(totalTime)"
        } else {
            stopCountdown()
        }
    }
    
    func stopCountdown() {
        countdownTimer?.invalidate()
        countdownTimer = nil
    }
}