TableViewCell中的Firebase快照打印

Question

我无法将我的firebase数据显示在tableviewcell中。我已阅读了数十个问题，但似乎没有相同的设置和问题。

我在TableViewController的viewDidLoad中设置了观察者：

 override func viewDidLoad() {

    super.viewDidLoad()
    // Do any additional setup after loading the view, typically from a nib.    

    // Setup observers that perform a closure each time a book is added,
    // removed, or changed.
    func setupDBListeners() {

        // returns each book in order by title.
        let ref = FIRDatabase.database().reference()
        ref.child(K.bookKey)
            .queryOrdered(byChild: K.titleKey)
            .observe(.childAdded, with: { (snapshot) in
                let book = self.convertSnapshotToBookObj(snapshot)
                self.books.append(book)
                self.tableView.reloadData()
            })
        // Listen for changes in book data.
        ref.child(K.bookKey)
            .observe(.childChanged, with: { (snapshot) in
                let book = self.convertSnapshotToBookObj(snapshot)
                for i in 0...self.books.count   {
                    if self.books[i].id == book.id  {
                        self.books[i] = book
                        self.tableView.reloadData()
                        return
                    } }
            })
        // Listen for books removed.
        ref.child(K.bookKey)
            .observe(.childRemoved, with: { (snapshot) in
                let book = self.convertSnapshotToBookObj(snapshot)
                for i in 0...self.books.count   {
                    if self.books[i].id == book.id  {
                        self.books.remove(at: i)
                        self.tableView.reloadData()
                        return
                    } }
    }) }
}

然后在我之外我有一个函数将它转换为数组的对象：

func convertSnapshotToBookObj(_ snap: FIRDataSnapshot) -> Book {

    let bookValues = snap.value as! [String : AnyObject]

    let id = snap.key

    let author = bookValues[K.authorKey] == nil ? "" : bookValues[K.authorKey]! as! String

    let title = bookValues[K.titleKey] == nil ? "" : bookValues[K.titleKey]!
        as! String

    let year = bookValues[K.yearKey] == nil ? "" : bookValues[K.yearKey]! as!
    String

    // Return a book object with the properties set.
    let book = Book()
    book.id = id
    book.author = author
    book.title = title
    book.year = year

    self.tableView.reloadData()
    return book
}