Collection View Layout Margins Demo

ViewController.swift

private func setupCollectionView() {
        let layout = UICollectionViewCompositionalLayout { [unowned self] section, environment in
            let spacing: CGFloat = 8
            let numberOfColumns: CGFloat = 5
            let numberOfSpacers: CGFloat = numberOfColumns - 1
            
            let contentWidth = collectionView.layoutMarginsGuide.layoutFrame.width
            let itemWidth = (contentWidth - numberOfSpacers * spacing) / numberOfColumns
            
            let layoutItemSize = NSCollectionLayoutSize(widthDimension: .absolute(itemWidth), heightDimension: .fractionalHeight(1))
            let layoutItem = NSCollectionLayoutItem(layoutSize: layoutItemSize)
            
            let layoutGroupSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1), heightDimension: .absolute(itemWidth))
            let layoutGroup = NSCollectionLayoutGroup.horizontal(layoutSize: layoutGroupSize, subitems: [layoutItem])
            layoutGroup.interItemSpacing = .fixed(spacing)
            
            let headerSupplementaryItemSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(1), heightDimension: .estimated(100))
            let headerSupplementaryItem = NSCollectionLayoutBoundarySupplementaryItem(layoutSize: headerSupplementaryItemSize, elementKind: UICollectionView.elementKindSectionHeader, alignment: .top)
            
            let layoutSection = NSCollectionLayoutSection(group: layoutGroup)
            layoutSection.interGroupSpacing = spacing
            layoutSection.contentInsetsReference = .layoutMargins
            layoutSection.boundarySupplementaryItems = [headerSupplementaryItem]
            
            return layoutSection
        }
        
        collectionView = UICollectionView(collectionViewLayout: layout)
        collectionView.delegate = self
        collectionView.preservesSuperviewLayoutMargins = true
        collectionView.backgroundColor = .systemGroupedBackground
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        
        view.addSubview(collectionView)
        
        NSLayoutConstraint.activate([
            collectionView.topAnchor.constraint(equalTo: view.topAnchor),
            collectionView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            collectionView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
        ])
    }