class ViewController: UIViewController, UICollectionViewDelegate {
  func collectionView(_ collectionView: UICollectionView, didSelectItemAt indexPath: IndexPath) {
        // Check whether there is any selected cell or not
        if collectionView.indexPathsForSelectedItems?.count ?? 0 > 0 { // if there is an already selected cells
            // check if selected cell indexPath is one of the indexPathsForSelectedItems
            if collectionView.indexPathsForSelectedItems!.contains(indexPath) {
                // if yes, deselect it
                collectionView.deselectItem(at: indexPath, animated: true)
            } else {
                // if no, not specified what to do here => personal suggestion would be to update the selected cells
                for row in 0..<indexPath.row {
                    collectionView.selectItem(at: IndexPath(row: row, section: indexPath.section), animated: true, scrollPosition: .centeredHorizontally)
                }
            }
        } else { // this is the first cell to select
            // Select all cells ending with the clicked one and starting from the first cell in same section
            for row in 0..<indexPath.row {
                collectionView.selectItem(at: IndexPath(row: row, section: indexPath.section), animated: true, scrollPosition: .centeredHorizontally)
            }
        }
    }
  
  func collectionView(_ collectionView: UICollectionView, shouldSelectItemAt indexPath: IndexPath) -> Bool {
      // You can implement any logic you want here
        return true
    }
}