collectionViewLayout doesn’t only contains static layout attributes like itemSize, sectionInset, etc. It also contains the dynamic layout info.
Dynamic layout info means collectionViewLayout has frame info for every section and cell of the corresponding UICollectionView.
For example, let’s say the UIColllectionView A has one section and three cells, while UICollectionView B has one section and two cells. If they share one collectionViewLayout, the app will crash with an exception.
Because when A was loaded, the collectionViewLayout will have three layouts for cells at the indexPath (0, 0), (0, 1) and (0, 3). But in B, there only have two cells in (0, 0) and (0, 1). So when collectionViewLayout is applied to B, the layout for (0, 3) is applied to a cell which is not existed.