CircleCell.swift

// Cell
final class CircleCell: UICollectionViewCell {
    @IBOutlet private weak var circleView: UIView!
    
    func setCircleView() {
        layoutIfNeeded() // Without this, it doesn't draw a circle correctly.
        circleView.layer.cornerRadius = circleView.frame.width / 2
    }
}

// DataSouce
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell...
    cell.setCircleView()
    return cell
}