Can you explain what is meant by the term 'Compositional Layout' and how it is used?

Compositional Layout is a modern way to create complex and flexible layouts in UICollectionView on iOS, introduced by Apple starting with iOS 13.

It allows describing the collection's structure using a hierarchy of sections, groups, and items, where each part can have its own sizes, insets, and orientation. This simplifies creating complex interfaces with different cell types and layouts without manually calculating positions.

Example of usage:

let itemSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0),
                                      heightDimension: .fractionalHeight(1.0))
let item = NSCollectionLayoutItem(layoutSize: itemSize)

let groupSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1.0),
                                       heightDimension: .absolute(44))
let group = NSCollectionLayoutGroup.horizontal(layoutSize: groupSize, subitems: [item])

let section = NSCollectionLayoutSection(group: group)

let layout = UICollectionViewCompositionalLayout(section: section)
collectionView.collectionViewLayout = layout

Thus, Compositional Layout provides a powerful and declarative tool for building adaptive and complex collections.