Question

所以我的应用程序运行正常；我使用以下方式将数据库持久保存在本地：

Database.database().isPersistenceEnabled = true

我什么都没做，但是突然之间它开始返回一个空快照。当我注释掉该行时，一切都再次正常运行，并且界面中的标签正确显示，并且该值在多个设备之间正确同步。这是我的viewDidLoad的代码片段：

import UIKit
import Firebase
import FirebaseDatabase

class ViewController: UIViewController, UITextFieldDelegate {

    @IBOutlet weak var returnedName: UILabel!
    @IBOutlet weak var scannedURLTextField: UITextField!
    @IBOutlet weak var reentries: UILabel!
    @IBOutlet weak var visitors: UILabel!
    @IBOutlet weak var ticketType: UILabel!
    @IBOutlet weak var statusImage: UIImageView!
    @IBOutlet weak var entryOrReentrySwitch: UISwitch!


    var qrDecodedURL : String!
    var ticketNumber : String!
    var eventID : String!
    var numberOfAttendees: String!
    var urlDictionary = [String : Any]()
    var attendeeRef: DatabaseReference!
    var ticketRef : DatabaseReference!
    var entries : DatabaseReference!
    var entriesRef : DatabaseReference!
    var reentriesRef : DatabaseReference!
    var entriesInt : Int!
    var reentriesInt : Int!


    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.
        Database.database().isPersistenceEnabled = true

        attendeeRef = Database.database().reference(withPath: "attendees")
        ticketRef = Database.database().reference(withPath: "tickets")

        entriesRef = Database.database().reference(withPath: "entries")

        reentriesRef = Database.database().reference(withPath: "reentries")

        self.scannedURLTextField .becomeFirstResponder()
        scannedURLTextField.inputView=UIView()//create dummy view to supress keyboard

        //        Incresase size of switch
        entryOrReentrySwitch.transform = CGAffineTransform(scaleX: 1.5, y: 1.5);

        //monitor for changes in entries
        entriesRef.child("entries").observe(.value, with: { (snapshot) in
            print(snapshot);
            self.visitors.text = "Visitors: \(snapshot.value as! String)"
            let tempValue = snapshot.value as! String
            self.entriesInt = Int(tempValue)
        })


        //Monitor for changes in reentries
        reentriesRef.child("reentries").observe(.value, with: { (snapshot) in
            //            print(snapshot);
            self.reentries.text = "Reentries: \(snapshot.value as! String)"
        })

        statusImage.image = UIImage.init(named: "scanQR.png")
    }

生成的错误是：

Could not cast value of type 'NSNull' (0x1fb2f3270) to 'NSString' (0x1fb2fcab8).
2019-10-12 18:05:54.061687-0700 VIM Tickets[621:97014] Could not cast value of type 'NSNull' (0x1fb2f3270) to 'NSString' (0x1fb2fcab8).

在条目更改部分的监视器中的self.visitors.text = "Visitors: \(snapshot.value as! String)"上失败。同样，当我注释掉数据库持久性行时，一切工作都很好，并且快照中有数据。我什至将其移至AppDelegate，但结果相同。

为什么在之前的持久性工作之前会发生这种情况，我有些茫然。任何建议将不胜感激！

Answer 1

启用磁盘持久性后，Firebase客户端会立即使用其对该节点知道的当前值来启动。在您的情况下，听起来好像认为值是null，因此触发该值。如果您与服务器建立了连接，那么稍后它会使用从服务器获得的正确值触发。

处理这种情况的通常方法是先处理check whether the snapshot exists，然后再处理其内容。

永久Firebase实时数据库在快照中创建空值

1 个答案: