코드로 만든 경우

collectionView.register(CustomCollectionViewCell.self, forCellWithReusepermalink: CustomCollectionViewCell.identifier)

xib로 만든 경우

let nib = UINib(nibName: "CustomCollectionViewCell", bundle: nil)
collectionView.register(nib, forCellWithReusepermalink: CustomCollectionViewCell.identifier)