我正在尝试建立一个像iPhone一样的图片库，但是我在使用CollectionView时遇到了问题。标题不显示，我不知道为什么。

override func viewDidLoad() {
    super.viewDidLoad()
    
    
    layout.itemSize = CGSize(width: view.frame.width/3, height: view.frame.width/3)
    
    collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
    collectionView.translatesAutoresizingMaskIntoConstraints = false
    collectionView.backgroundColor = .white
    collectionView.register(ImageCell.self, forCellWithReuseIdentifier: "ImageCell")
    collectionView.dataSource = self
    collectionView.delegate = self
    
    view.addSubview(titleView)
    view.addSubview(collectionView)
    view.addSubview(button)
    
    NSLayoutConstraint.activate([
        titleView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 16),
        titleView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor,constant: 16),
        titleView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor,constant: 16),
        titleView.bottomAnchor.constraint(equalTo: collectionView.topAnchor, constant: -16),
        
        collectionView.topAnchor.constraint(equalTo: titleView.bottomAnchor, constant: 200),
        collectionView.leadingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.leadingAnchor),
        collectionView.trailingAnchor.constraint(equalTo: view.safeAreaLayoutGuide.trailingAnchor),
        collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
        
        button.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor, constant: -8),
        button.rightAnchor.constraint(equalTo: view.rightAnchor, constant: -8),
        button.widthAnchor.constraint(equalToConstant: 50),
        button.heightAnchor.constraint(equalToConstant: 50)
    ])
}