应用在后台时的Firebase监听器

我的childAdded应用中的通知表有一个Firebase 事件监听器,我想在应用程序在后台时触发推送通知.

这是听众:

@objc func observeNotificationsChildAddedBackground() {
    self.notificationsBackgroundHandler = FIREBASE_REF!.child("notifications/\(Defaults.userUID!)")
    self.notificationsBackgroundHandler!.queryOrdered(byChild: "date").queryLimited(toLast: 99).observe(.childAdded, with: { snapshot in
        let newNotificationJSON = snapshot.value as? [String : Any]
        if let newNotificationJSON = newNotificationJSON {
            let status = newNotificationJSON["status"]
            if let status = status as? Int {
                if status == 1 {
                    self.sendPushNotification()
                }
            }
        }
    })
}

func sendPushNotification() {
    let content = UNMutableNotificationContent()
    content.title = "Here is a new notification"
    content.subtitle = "new notification push"
    content.body = "Someone did something which triggered a notification"
    content.sound = UNNotificationSound.default()

    let request = UNNotificationRequest(identifier: "\(self.notificationBackgroundProcessName)", content: content, trigger: nil)
    NotificationCenter.default.post(name: notificationBackgroundProcessName, object: nil)
    UNUserNotificationCenter.current().delegate = self
    UNUserNotificationCenter.current().add(request){ error in
        if error != nil {
            print("error sending a push notification :\(error?.localizedDescription)")
        }
    }
}

当应用程序位于前台时,这非常有用.然后我在我的app delegates applicationWillResignActive方法中添加一个背景观察器来在后台观察它:

func applicationWillResignActive(_ application: UIApplication) {
     NotificationCenter.default.addObserver(self, selector: #selector(MainNavViewController.observeNotificationsChildAdded), name: notificationBackgroundProcessName, object: nil)
}

但是当应用程序在后台时,事件观察者不会触发.我调查了Firebase Cloud Messenger来解决这个问题并遇到这样的帖子:

是否可以直接从设备使用Firebase云消息传递(FCM)向特殊UDID发送PushNotifications？

但是我并没有尝试从一个用户向另一个用户发送推送通知,我正在尝试UNNotificationRequest从数据库监听器中激活一个.

我还发现这篇文章: FCM背景通知在iOS中不起作用 但是,这与我的用例不同,因为它在FIRMessaging框架内使用.

那么,当应用程序在后台运行时,是否可以运行此数据库侦听器？如果没有,我是否可以让Firebase Cloud Messenger观察表中的更改并发送通知？