如何在Swift中每天只显示一次弹出窗口？

import UIKit

class ViewController: UIViewController {
    
    let lastPopupDateKey = "LastPopupDate"
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        // 获取当前日期
        let currentDate = Date()
        
        // 将日期格式化为"yyyy-MM-dd"
        let dateFormatter = DateFormatter()
        dateFormatter.dateFormat = "yyyy-MM-dd"
        let currentDateString = dateFormatter.string(from: currentDate)
        
        // 从持久化存储中获取上次显示弹出窗口的日期
        let lastPopupDate = UserDefaults.standard.string(forKey: lastPopupDateKey)
        
        // 比较当前日期和上次显示弹出窗口的日期
        if currentDateString != lastPopupDate {
            // 显示弹出窗口
            let alertController = UIAlertController(title: "弹出窗口", message: "这是每天只显示一次的弹出窗口", preferredStyle: .alert)
            let okAction = UIAlertAction(title: "确定", style: .default) { _ in
                // 更新上次显示弹出窗口的日期为当前日期
                UserDefaults.standard.set(currentDateString, forKey: self.lastPopupDateKey)
            }
            alertController.addAction(okAction)
            present(alertController, animated: true, completion: nil)
        }
    }
}