从Firebase数据库中获取数据？

Question

我有这段代码，我想从FirebaseDatase加载一些数据

func fetchVinylsData(){
    SVProgressHUD.show()
    guard let currentUID = Auth.auth().currentUser?.uid else { return }

    dbRef.child("vinyls").child(currentUID).observe(.childAdded) { (snapshot) in
        guard let dictionary = snapshot.value as? Dictionary<String,AnyObject> else { return }
        let vinyl = Vinyl(dictionary: dictionary)
        print(dictionary)
        self.vinyls = [Vinyl]()
        self.vinyls.append(vinyl)
        self.vinyls.sort(by: { (vinyl1, vinyl2) -> Bool in
            return vinyl1.artist < vinyl2.artist
        })
        self.tableView.reloadData()
    }
    SVProgressHUD.dismiss()
}

当我打印字典时，我可以很好地获得所有记录，但是我的表视图不会被填充。我一直工作得很好，直到我决定向Vinyl.swift中添加一个新字段。firebase数据库的结构没有新的field.is这是个问题吗？以下是Vinyl.swift的代码：

import UIKit
import Firebase

class Vinyl {
    var vinylID : String!
    var title : String!
    var artist : String!
    var label : String!
    var vinylCountry : String!
    var isOut : Bool = false
    var vinylLocation : String!
    var year : String!
    var vinylAutoID : String!
    var vinylFormat : String!

    init(dictionary : Dictionary<String,AnyObject>) {
        guard let title = dictionary["title"] as? String else { return }
        guard let artist = dictionary["artist"] as? String else { return }
        guard let label = dictionary["label"] as? String else { return }
        guard let vinylID = dictionary["vinylID"] as? String else { return }
        guard let vinylCountry = dictionary["vinylCountry"] as? String else { return }
        guard let vinylLocation = dictionary["vinylLocation"] as? String else { return }
        guard let year = dictionary["vinylYear"] as? String else { return }
        guard let autoID = dictionary["autoID"] as? String else { return }
        guard let isOut = dictionary["isOut"] as? Bool else { return }
        guard let vinylFormat = dictionary["format"] as? String else { return }

        self.vinylID = vinylID
        self.title = title
        self.artist = artist
        self.label = label
        self.vinylCountry = vinylCountry
        self.isOut = isOut
        self.vinylLocation = vinylLocation
        self.year = year
        self.vinylAutoID = autoID
        self.vinylFormat = vinylFormat
    }
}

Answer 1

我不建议在init方法中使用guard let，因为假设dictionary["title"]为空或不可用，那么它将从代码的第一行返回，其他代码也会发生同样的情况。要解决这个问题，您可以使用Nil-Coalescing Operator (这意味着您可以指定默认值nil或任何其他值，如果something["something"]为nil或optional)

替换

guard let title = dictionary["title"] as? String else { return }
guard let artist = dictionary["artist"] as? String else { return }
guard let label = dictionary["label"] as? String else { return }
guard let vinylID = dictionary["vinylID"] as? String else { return }
guard let vinylCountry = dictionary["vinylCountry"] as? String else { return }
guard let vinylLocation = dictionary["vinylLocation"] as? String else { return }
guard let year = dictionary["vinylYear"] as? String else { return }
guard let autoID = dictionary["autoID"] as? String else { return }
guard let isOut = dictionary["isOut"] as? Bool else { return }
guard let vinylFormat = dictionary["format"] as? String else { return }

使用

let title = dictionary["title"] as? String ?? ""
let artist = dictionary["artist"] as? String ?? ""
let label = dictionary["label"] as? String ?? ""
let vinylID = dictionary["vinylID"] as? String ?? ""
let vinylCountry = dictionary["vinylCountry"] as? String ?? ""
let vinylLocation = dictionary["vinylLocation"] as? String ?? ""
let year = dictionary["vinylYear"] as? String ?? ""
let autoID = dictionary["autoID"] as? String ?? ""
let isOut = dictionary["isOut"] as? Bool ?? false
let vinylFormat = dictionary["format"] as? String ?? ""

在处理async调用时，您还需要更新main queue中的UI，如下所示：

DispatchQueue.main.async {
    self.tableView.reloadData()
}