如何让imageView占用CollectionView单元？

collectionView.register(CustomCollectionViewCell.self, forCellWithReuseIdentifier: "cellIdentifier")

class CustomCollectionViewCell: UICollectionViewCell {
    let imageView: UIImageView = {
        let imageView = UIImageView()
        imageView.translatesAutoresizingMaskIntoConstraints = false
        imageView.contentMode = .scaleAspectFill
        imageView.clipsToBounds = true
        return imageView
    }()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        addSubview(imageView)
        
        NSLayoutConstraint.activate([
            imageView.topAnchor.constraint(equalTo: topAnchor),
            imageView.leadingAnchor.constraint(equalTo: leadingAnchor),
            imageView.trailingAnchor.constraint(equalTo: trailingAnchor),
            imageView.bottomAnchor.constraint(equalTo: bottomAnchor)
        ])
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "cellIdentifier", for: indexPath) as! CustomCollectionViewCell
    
    // 设置图片
    cell.imageView.image = UIImage(named: "image\(indexPath.item)")
    
    return cell
}