使用UIControlEvents.valueChanged的日期选取器不适用于第一个值更改事件

Question

我正在使用下面的代码来了解日期选择器。它有效，但奇怪的是当我第一次滚动日期选择器时，预期的变化不会发生。之后它会正常工作。不知道发生了什么。

import UIKit
import Foundation

class ViewController: UIViewController {

// Place this code inside your ViewController or where you want ;)

var txtField: UITextField = UITextField(frame: CGRect(x: 100, y: 0, width: 300, height: 50))

override func viewDidLoad() {
    super.viewDidLoad()

    // date picker setup
    let datePickerView:UIDatePicker = UIDatePicker()

    // choose the mode you want
    // other options are: DateAndTime, Time, CountDownTimer
    datePickerView.datePickerMode = UIDatePickerMode.countDownTimer

    // choose your locale or leave the default (system)
    //datePickerView.locale = NSLocale.init(localeIdentifier: "it_IT")

    datePickerView.addTarget(self, action: #selector(onDatePickerValueChanged(_:)), for: UIControlEvents.valueChanged)
    txtField.inputView = datePickerView

    // datepicker toolbar setup
    let toolBar = UIToolbar()
    toolBar.barStyle = UIBarStyle.default
    toolBar.isTranslucent = true
    let space = UIBarButtonItem(barButtonSystemItem: UIBarButtonSystemItem.flexibleSpace, target: nil, action: nil)
    let doneButton = UIBarButtonItem(title: "Done", style: UIBarButtonItemStyle.done, target: self, action: #selector(ViewController.doneDatePickerPressed))

    // if you remove the space element, the "done" button will be left aligned
    // you can add more items if you want
    toolBar.setItems([space, doneButton], animated: false)
    toolBar.isUserInteractionEnabled = true
    toolBar.sizeToFit()

    txtField.inputAccessoryView = toolBar

    self.view.addSubview(txtField)
}

func doneDatePickerPressed(){
    self.view.endEditing(true)
}

func onDatePickerValueChanged(_ sender: UIDatePicker) {
    let (h, m, _) = secondsToHoursMinutesSeconds (duration: sender.countDownDuration)
    self.txtField.text = String("\(h) Hours  \(m) Minutes")
}

func secondsToHoursMinutesSeconds (duration : Double) -> (Int, Int, Int) {
    let seconds:Int = Int(duration)
    return (seconds / 3600, (seconds % 3600) / 60, (seconds % 3600) % 60)
}

}