从FirebaseDatabase下载数据，而不是从JSON下载

Question

我在课程 - >＆gt;中完成了所有工作letsbuildthatapp

但是在课程中 Brian 使用 JSON 。我想使用 FirebaseDatabase 。 如何从 Firebase 下载数据？可能需要使用较少数量的类，但我无法弄清楚如何更改 Firebase 上的下载量。

JSON 结构就像一节课（为了便于理解，我略微削减了 JSON 文件）。 JSON 和 Firebase ：

{
  "categories" : [ {
    "apps" : {
      "4" : {
        "Category" : "Entertainment",
        "Id" : 1,
        "ImageName" : "frozen",
        "Name" : "Disney Build It: Frozen",
        "Price" : 3.99,
        "Screenshots" : [ "frozen_screenshot1", "frozen_screenshot2", "frozen_screenshot3", "frozen_screenshot4", "frozen_screenshot5" ]
      },
      "7" : {
        "Category" : "Health & Fitness",
        "ImageName" : "today",
        "Name" : "Today: Habit tracker for your daily goals and routines"
      }
    },
    "name" : "Best New Apps",
    "type" : ""
  }, {
    "apps" : [ {
      "Category" : "Games",
      "Id" : 2,
      "ImageName" : "telepaint",
      "Name" : "telepaint",
      "Price" : 2.99,
      "Screenshots" : [ "telepaint1", "telepaint2", "telepaint3", "telepaint4", "telepaint5" ]
    }, null, null, {
      "Category" : "Games",
      "ImageName" : "beatstomper",
      "Name" : "Beat Stomper",
      "Price" : 1.99
    } ],
    "name" : "Best New Games",
    "type" : ""
  } ]
}

课程应用

class App {
    var id: String?
    var imageName: String?
    var screenshots = [String]() 

    init (from json: JSON) {
        id = json["Id"].stringValue
        imageName = json["ImageName"].stringValue
    }
    func updateValue(completion: (() -> Void)?) {
        guard let appID = id else { return }
        let url = "https://api.letsbuildthatapp.com/appstore/appdetail?id=\(appID)"
        Alamofire.request(url).responseJSON { response in
            switch response.result {
                case .success(let value):
                    let json = JSON(value)
                    for ssj in json["Screenshots"].arrayValue {
                        if let ss = ssj.string {
                            self.screenshots.append(ss)
                        }
                    }
                case .failure(let error):
                    print(error)
            }
            completion?()
        }
    }
}

班级类别：

class Category {
    var apps = [App]() 

    init (from json: JSON) {
        for appData in json["apps"].arrayValue {
            let app = App(from: appData)
            apps.append(app)
        }
    }
}

班级类别：

class Categoris {

    private let url = "https://api.letsbuildthatapp.com/appstore/featured"
    private var cats = [Category]()

    func numberOfCats() -> Int {
        return cats.count
    }
    func fetchCat(from indexPath: IndexPath) -> Category {
        return cats[indexPath.row]
    }
    func loadData(completion: (() -> Void)?) {
        Alamofire.request(url).responseJSON { response in
            switch response.result {
                case .success(let value):
                    let json = JSON(value)
                    for catData in json["categories"].arrayValue {
                        let cat = Category(from: catData)
                        self.cats.append(cat)
                    }
                case .failure(let error):
                    print(error)
            }
            completion?()
        }
    }
}