Ios 如何自动调整单元格大小以适应每行4个单元格

我有一个收集视图，我正在寻找一种方法，找出每行可以容纳4个单元格（在特定屏幕大小上）的单元格大小，并在所有单元格上使用该大小。我有一个可重复使用的电池，我用在其余的电池上

extension LevelSelectController: UICollectionViewDelegateFlowLayout {

// Collection view flow layout setup
func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    // Compute the dimension of a cell for an NxN layout with space S between
    // cells.  Take the collection view's width, subtract (N-1)*S points for
    // the spaces between the cells, and then divide by N to find the final
    // dimension for the cell's width and height.

    let cellsAcross: CGFloat = 4
    let spaceBetweenCells: CGFloat = 0
    let dim = (collectionView.bounds.width - (cellsAcross - 1) * spaceBetweenCells) / cellsAcross

    return CGSize(width: dim, height: dim)
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
    return UIEdgeInsetsMake(0, 0, 0.0, 0.0)
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
    return 0.0
}

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
    return 0.0
}