Solution for Ch 21 Silver Challenge: Updated Item Sizes in Xcode 9.3

[hkray]

//  PhotosViewController.swift

extension PhotosViewController: UICollectionViewDelegateFlowLayout {
  func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    let collectionViewWidth = collectionView.bounds.size.width
    let numberOfItemsPerRow: CGFloat = 4
    let itemWidth = collectionViewWidth / numberOfItemsPerRow

    return CGSize(width: itemWidth, height: itemWidth)
  }

  func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, insetForSectionAt section: Int) -> UIEdgeInsets {
    return .zero
  }
  
  func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumLineSpacingForSectionAt section: Int) -> CGFloat {
    return 0
  }

  func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, minimumInteritemSpacingForSectionAt section: Int) -> CGFloat {
    return 0
  }

  override func viewWillTransition(to size: CGSize, with coordinator: UIViewControllerTransitionCoordinator) {
    collectionView.collectionViewLayout.invalidateLayout()    // Enhancement by akozin
  }
}