如何支持iOS13及以上和以下版本的UIDatePicker？

1. 首先，在UI控制器或视图控制器类中创建UIDatePicker对象。例如：

let datePicker = UIDatePicker()

2. 针对不同的iOS版本，设置datePicker的preferredDatePickerStyle属性和datePickerMode属性。

• 对于iOS13及以上版本，设置preferredDatePickerStyle为.wheels：

if #available(iOS 13.4, *) {
    datePicker.preferredDatePickerStyle = .wheels
} else {
    // Fallback on earlier versions
}

• 对于iOS13以下版本，设置datePickerMode属性为.countDownTimer：

if #available(iOS 13.4, *) {
    datePicker.preferredDatePickerStyle = .wheels
} else {
    datePicker.datePickerMode = .countDownTimer
}

3. 可选：设置datePicker的日期、时间和格式。

datePicker.date = Date()
datePicker.locale = Locale(identifier: "zh_CN")
datePicker.datePickerMode = .dateAndTime
datePicker.timeZone = TimeZone.current
datePicker.addTarget(self, action: #selector(datePickerValueChanged(_:)), for: .valueChanged)

完整示例代码如下：

import UIKit

class ViewController: UIViewController {

    let datePicker = UIDatePicker()

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        
        datePicker.datePickerMode = .dateAndTime
        datePicker.addTarget(self, action: #selector(datePickerValueChanged(_:)), for: .valueChanged)

        if #available(iOS 13.4, *) {
            datePicker.preferredDatePickerStyle = .wheels
        } else {
            datePicker.datePickerMode = .countDownTimer
        }

        datePicker.date = Date()
        datePicker.locale = Locale(identifier: "zh_CN")
        datePicker.timeZone = TimeZone.current
        
        view.addSubview(datePicker)
        datePicker.translatesAutoresizingMaskIntoConstraints = false
        NSLayoutConstraint.activate([
            datePicker.centerXAnchor.constraint(equalTo: view.centerXAnchor),
            datePicker.centerYAnchor.constraint(equalTo: view.centerYAnchor)
        ])
    }

    @objc func datePickerValueChanged(_ sender: UIDatePicker) {
        print(sender.date)
    }

}