1// This crashes immediately
2let collectionView = UICollectionView(frame: .zero)
3// *** Terminating app due to uncaught exception:
4// 'UICollectionView must be initialized with a non-nil layout parameter'

1let layout = UICollectionViewFlowLayout()
2layout.itemSize = CGSize(width: 100, height: 100)
3layout.minimumInteritemSpacing = 10
4layout.minimumLineSpacing = 10
5
6let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
7collectionView.backgroundColor = .white
8collectionView.register(MyCell.self, forCellWithReuseIdentifier: "cell")

1class GalleryViewController: UIViewController {
2    private var collectionView: UICollectionView!
3
4    override func viewDidLoad() {
5        super.viewDidLoad()
6
7        let layout = UICollectionViewFlowLayout()
8        layout.itemSize = CGSize(width: 120, height: 120)
9        layout.sectionInset = UIEdgeInsets(top: 10, left: 10, bottom: 10, right: 10)
10
11        collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout)
12        collectionView.autoresizingMask = [.flexibleWidth, .flexibleHeight]
13        collectionView.dataSource = self
14        collectionView.delegate = self
15        collectionView.register(UICollectionViewCell.self, forCellWithReuseIdentifier: "cell")
16
17        view.addSubview(collectionView)
18    }
19}

1// WRONG — uses init(nibName:bundle:) which has no layout
2let vc = GalleryCollectionViewController()
3
4// CORRECT — pass a layout
5let layout = UICollectionViewFlowLayout()
6layout.itemSize = CGSize(width: 80, height: 80)
7let vc = GalleryCollectionViewController(collectionViewLayout: layout)

1class GalleryCollectionViewController: UICollectionViewController {
2    // If using init() without parameters, provide a default layout
3    init() {
4        let layout = UICollectionViewFlowLayout()
5        layout.itemSize = CGSize(width: 100, height: 100)
6        super.init(collectionViewLayout: layout)
7    }
8
9    required init?(coder: NSCoder) {
10        super.init(coder: coder)
11    }
12}

1class CustomCollectionView: UICollectionView {
2    required init?(coder: NSCoder) {
3        // The storyboard/nib provides the layout via init(coder:)
4        // This should work automatically IF the layout is configured in IB
5        super.init(coder: coder)
6    }
7
8    // If you need to override the layout after loading from nib:
9    override func awakeFromNib() {
10        super.awakeFromNib()
11        let newLayout = UICollectionViewFlowLayout()
12        newLayout.itemSize = CGSize(width: 150, height: 150)
13        collectionViewLayout = newLayout
14    }
15}

1// Animated layout change
2let newLayout = UICollectionViewFlowLayout()
3newLayout.itemSize = CGSize(width: 200, height: 200)
4
5collectionView.setCollectionViewLayout(newLayout, animated: true) { completed in
6    print("Layout transition completed: \(completed)")
7}
8
9// Immediate layout change (no animation)
10collectionView.collectionViewLayout = newLayout

1let layout = UICollectionViewCompositionalLayout { sectionIndex, environment in
2    let itemSize = NSCollectionLayoutSize(
3        widthDimension: .fractionalWidth(1.0 / 3.0),
4        heightDimension: .fractionalHeight(1.0)
5    )
6    let item = NSCollectionLayoutItem(layoutSize: itemSize)
7    item.contentInsets = NSDirectionalEdgeInsets(top: 5, leading: 5, bottom: 5, trailing: 5)
8
9    let groupSize = NSCollectionLayoutSize(
10        widthDimension: .fractionalWidth(1.0),
11        heightDimension: .absolute(120)
12    )
13    let group = NSCollectionLayoutGroup.horizontal(layoutSize: groupSize, subitems: [item])
14
15    let section = NSCollectionLayoutSection(group: group)
16    return section
17}
18
19let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)

1class WaterfallLayout: UICollectionViewLayout {
2    override var collectionViewContentSize: CGSize {
3        // Return the total content size
4        return CGSize(width: collectionView?.bounds.width ?? 0, height: contentHeight)
5    }
6
7    override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
8        // Return attributes for all items in the visible rect
9        return cachedAttributes.filter { $0.frame.intersects(rect) }
10    }
11
12    override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
13        return cachedAttributes[indexPath.item]
14    }
15}
16
17// Use it
18let collectionView = UICollectionView(frame: .zero, collectionViewLayout: WaterfallLayout())