BasicFlowLayoutViewController.swift

extension BasicFlowLayoutViewController: UICollectionViewDelegateFlowLayout {
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
        return UIEdgeInsets(top: 0, left: 15, bottom: 100, right: 15)
    }
    
    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let noOfCellsInRow = 2
        let flowLayout = collectionViewLayout as! UICollectionViewFlowLayout
        let totalSpace = flowLayout.sectionInset.left
            + flowLayout.sectionInset.right
            + (flowLayout.minimumInteritemSpacing * CGFloat(noOfCellsInRow - 1))

        let size = Int((collectionView.bounds.width - totalSpace) / CGFloat(noOfCellsInRow)) - 20
        return CGSize(width: size, height: size)
    }
}