swift 4中的定时器在后台不工作，有什么办法可以工作吗？

var backgroundTask: UIBackgroundTaskIdentifier = .invalid

backgroundTask = UIApplication.shared.beginBackgroundTask { [weak self] in
    UIApplication.shared.endBackgroundTask(self?.backgroundTask ?? .invalid)
    self?.backgroundTask = .invalid
}

if backgroundTask != .invalid {
    UIApplication.shared.endBackgroundTask(backgroundTask)
    backgroundTask = .invalid
}

UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound, .badge]) { (granted, error) in
    // 处理授权结果
}

let content = UNMutableNotificationContent()
content.title = "定时器通知"
content.body = "定时器触发的操作"
let trigger = UNTimeIntervalNotificationTrigger(timeInterval: 1, repeats: true)
let request = UNNotificationRequest(identifier: "timerNotification", content: content, trigger: trigger)
UNUserNotificationCenter.current().add(request) { (error) in
    // 处理添加通知的结果
}

func userNotificationCenter(_ center: UNUserNotificationCenter, didReceive response: UNNotificationResponse, withCompletionHandler completionHandler: @escaping () -> Void) {
    if response.actionIdentifier == UNNotificationDefaultActionIdentifier {
        // 执行定时器触发的操作
    }
    completionHandler()
}