CollectionViewExampleViewController.swift

class ViewController: UIViewController, UICollectionViewDelegate, UICollectionViewDataSource {

    @IBOutlet weak var collectionView: UICollectionView!

    var text = [
        "Some text.",
        "Some longer text text text text text text text text text text.",
        "Lots text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text text."
    ]

    override func viewDidLoad() {
        super.viewDidLoad()

        setupCollectionView()

        DispatchQueue.main.asyncAfter(deadline: DispatchTime.now() + 5) {
            self.text = [
                "a",
                "b",
                "c",
                "d"
            ]
            self.collectionView.reloadData()

            // Only a, b, c rows are displayed unless reloadData() is called once more here
            // self.collectionView.reloadData()
        }
    }

    private func setupCollectionView() {

        collectionView.delegate = self
        collectionView.dataSource = self
        collectionView.register(UINib(nibName: "GenericCell", bundle: nil), forCellWithReuseIdentifier: "GenericCell")
        (collectionView.collectionViewLayout as? UICollectionViewFlowLayout)?.estimatedItemSize = CGSize(width: 100, height: 100)
    }

    // MARK: - UICollecitonViewDataSource

    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return text.count
    }

    // MARK: - UICollectionViewDelegate

    func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {

        guard let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "GenericCell", for: indexPath) as? GenericCell else {
            return UICollectionViewCell()
        }

        cell.configure(labelText: text[indexPath.row])

        return cell
    }
}