如何从firebase查询数据并确保每次有新内容时都会更新？

Question

我启用了FIRDatabase.database（）。persistenceEnabled = true，因此即使在离线模式下，缓存的数据也会保留。但是，我确实意识到，即使有数据更改，应用也拒绝更新并添加新内容。

例如，这是我的json树：

{
  "palettes" : {
    "-KTsjCh8nMErdbNXuHqL" : {
      "URL" : --,
      "accessories" : "#141615",
      "bottom" : "#BA9977",
      "overall" : "#676769",
      "shoes" : "#FFFFFF",
      "thisSeason" : "true",
      "title" : "test",
      "top" : "#000000"
    },
    "-KTsjCnNWoUNKoYBlH-j" : {
      "URL" : --,
      "accessories" : "#141615",
      "bottom" : "#BA9977",
      "overall" : "#676769",
      "shoes" : "#FFFFFF",
      "thisSeason" : "false",
      "title" : "test1",
      "top" : "#000000"
    }
  }
}

当条目被添加到＆＃34; palettes＆＃34;时，即使重新启动它，应用程序也不会更新。即使我们在仍然保持离线功能的同时不重启应用程序，我如何启用它会持续监听和更新？

这是我从＃34; palettes＆＃34;

收集数据的功能

func generateDataForRecents()  {
        if URLArrayStringThisSeason.count == 0 {
            self.activityIndicator2.isHidden = false
            self.activityIndicator2.startAnimating()
        }


        let databaseRef = FIRDatabase.database().reference()

        databaseRef.child("palettes").queryLimited(toFirst: 100).observeSingleEvent(of: .value, with: { (snapshot) in

            if let snapDict = snapshot.value as? [String:AnyObject]{

                for each in snapDict as [String:AnyObject]{

                    let URL = each.value["URL"] as! String

                    self.URLArrayStringRecents.append(URL)
                    //print(self.URLArrayString.count)

                    //print(snapshot)

                    //let pictureTitle = each.value["title"] as! String

                }
            }

            self.whatsNewCollectionView?.reloadData() //Reloads data after the number and all the URLs are fetched
            self.activityIndicator2.stopAnimating()
            self.activityIndicator2.isHidden = true
        })
    }

我理解可能是因为我使用observeSingleEventType因此阻止它听取真正的更新。现在它只从本地缓存中提取数据。

编辑：以这种方式添加它是对的吗？

 func generateDataForRecents()  {
        if URLArrayStringThisSeason.count == 0 {
            self.activityIndicator2.isHidden = false
            self.activityIndicator2.startAnimating()
        }


        let databaseRef = FIRDatabase.database().reference()

        databaseRef.queryLimited(toFirst: 100).observe(.childAdded, with: { (snapshot) in

            if let snapDict = snapshot.value as? [String:AnyObject]{

                for each in snapDict as [String:AnyObject]{

                    let URL = each.value["URL"] as! String

                    self.URLArrayStringRecents.append(URL)
                    //print(self.URLArrayString.count)

                    //print(snapshot)

                    //let pictureTitle = each.value["title"] as! String

                }
            }

            self.whatsNewCollectionView?.reloadData() //Reloads data after the number and all the URLs are fetched
            self.activityIndicator2.stopAnimating()
            self.activityIndicator2.isHidden = true
        })


    }