CompositionalLayoutCenterAlign-1.swift

  let layout = UICollectionViewCompositionalLayout(sectionProvider: {
    (sectionIndex: Int, layoutEnvironment: NSCollectionLayoutEnvironment) -> NSCollectionLayoutSection? in

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

    // (1) setting widthDimension to half of collectionView size
    let groupSize = NSCollectionLayoutSize(widthDimension: .fractionalWidth(0.5),
                                           heightDimension: .absolute(50))

    let group = NSCollectionLayoutGroup.horizontal(
      layoutSize: groupSize,
      subitem: item,
      count: 1
    )
    
    // (2) customize layout group here
    
    section.interGroupSpacing = 16

    return section
  })