使用计时器从放置在uitextfield中的内容开始倒计时

import UIKit

class ViewController: UIViewController, UITextFieldDelegate {
    @IBOutlet weak var countdownTextField: UITextField!
    
    var countdownTimer: Timer?
    var remainingTime: Int = 0
    
    override func viewDidLoad() {
        super.viewDidLoad()
        countdownTextField.delegate = self
    }
    
    func startCountdown() {
        countdownTimer = Timer.scheduledTimer(timeInterval: 1, target: self, selector: #selector(updateCountdown), userInfo: nil, repeats: true)
    }
    
    @objc func updateCountdown() {
        if remainingTime > 0 {
            remainingTime -= 1
            countdownTextField.text = "\(remainingTime)"
        } else {
            stopCountdown()
            // 倒计时结束后的操作
        }
    }
    
    func stopCountdown() {
        countdownTimer?.invalidate()
        countdownTimer = nil
    }
    
    func textFieldShouldReturn(_ textField: UITextField) -> Bool {
        textField.resignFirstResponder()
        if let inputTime = Int(textField.text ?? "") {
            remainingTime = inputTime
            startCountdown()
        }
        return true
    }
}