swift - 从 Firebase 数据库获取数据？

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，我得到了所有记录，但我的表格 View 没有被填充。我工作得很好，直到我决定向 Vinyl.swift 添加一个新字段。 firebase 数据库的结构没有新字段。这是一个问题吗？这是 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"] 为空或不可用它将从代码的第一行返回，其他人也会发生同样的情况。为了解决这个问题，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 可选)

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 }

与

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 ?? ""

当您使用异步调用时，您还需要更新主队列中的UI，如下所示:

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