为什么我的CollectionViewController不显示？

class MyCollectionViewController: UICollectionViewController, UICollectionViewDataSource, UICollectionViewDelegate {
    override func viewDidLoad() {
        super.viewDidLoad()
        collectionView.dataSource = self
        collectionView.delegate = self
    }
}

extension MyCollectionViewController {
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return 1
    }

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return items.count // items是你的数据数组
    }
}

override func viewDidLoad() {
    super.viewDidLoad()
    collectionView.register(MyCollectionViewCell.self, forCellWithReuseIdentifier: "cellIdentifier")
}

let layout = UICollectionViewFlowLayout()
layout.itemSize = CGSize(width: 100, height: 100)
let collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout)

override func viewDidLoad() {
    super.viewDidLoad()
    view.addSubview(collectionView)
}

var items: [Item] = [] // 确保items数组中有数据