从Firebase获取新数据后重新加载页面视图控制器

Question

我使用Firebase作为移动应用的数据库。我想从Firebase获取数据并将其推送到本地阵列，然后将该数组用作我的页面视图控制器的数据源。

问题在于，当运行下面的代码时，我得到索引超出范围的异常。我确保将正确的数据加载到数组中。

import UIKit
import Firebase

class PageViewController: UIPageViewController, UIPageViewControllerDataSource {

    let ref =  FIRDatabase.database().reference(withPath: "posts")
    var items: [Humm] = []

    override func viewDidAppear(_ animated: Bool) {
        super.viewDidAppear(animated)

    }

    override func viewDidLoad() {
        super.viewDidLoad()
        dataSource = self

        items.append(Humm(poster: "Test", posterUID: "tst", genre: "Rock", audioFilePath: "sss"))
        items.append(Humm(poster: "Hello", posterUID: "tst", genre: "Rock", audioFilePath: "sss"))

        ref.observe(.value, with: { snapshot in
            var newItems: [Humm] = []

            if let posts = snapshot.value as? [String: AnyObject] {
                for post in posts as [String: AnyObject] {
                    self.ref.child(post.0).observe(.value, with: { (snap) in
                        if let hummItem = snap.value as? [String: AnyObject] {
                            for item in hummItem as [String: AnyObject] {
                                let newHumm = Humm(posterUID: post.0, key: item.key, snapshot: snap, data: item.value as! [String : AnyObject])
                                newItems.append(newHumm)
                            }
                        }
                    })
                }
            }

            self.items = newItems
        })

        self.setViewControllers([getViewControllerAtIndex(index: 0)] as [UIViewController], direction: UIPageViewControllerNavigationDirection.forward, animated: true, completion: nil)
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


    func getViewControllerAtIndex(index: NSInteger) -> HomeViewController {
        let pageContentViewController = self.storyboard?.instantiateViewController(withIdentifier: "Home") as! HomeViewController

        pageContentViewController.name = self.items[index].poster
        pageContentViewController.pageIndex = index
        return pageContentViewController
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerAfter viewController: UIViewController) -> UIViewController? {
        let pageContent: HomeViewController = viewController as! HomeViewController
        var index = pageContent.pageIndex

        if (index == NSNotFound) {
            return nil;
        }

        index += 1;

        if (index == items.count) {
            return nil;
        }
        return getViewControllerAtIndex(index: index)
    }

    func pageViewController(_ pageViewController: UIPageViewController, viewControllerBefore viewController: UIViewController) -> UIViewController? {

        return nil
    }
}