UICollectionViewDiffableDataSource is replacing data instead of updating

I am trying to understand UICollectionViewDiffableDataSource and NSDiffableDataSourceSnapshot.

I have created a very crude version below. Essentially on load it should fetch photos.

On tap of a button in the navigation bar it fetches the next page. This is however just replacing the existing data, I was expecting it to append the vales to the array.

How should I update my data rather than replace it please?

class ViewController: UIViewController {

  lazy var collectionView = UICollectionView(frame: view.frame, collectionViewLayout: UICollectionViewFlowLayout())

  enum Section {
    case main
  }

  typealias DataSource = UICollectionViewDiffableDataSource<Section, AnyHashable>
  typealias DataSourceSnapshot = NSDiffableDataSourceSnapshot<Section, AnyHashable>

  private var dataSource: DataSource!
  private var snapshot = DataSourceSnapshot()

  override func viewDidLoad() {
    super.viewDidLoad()

    let updateButton = UIBarButtonItem(title: "Add", style: .plain, target: self, action: #selector(onTapLoad))
    navigationItem.rightBarButtonItem = updateButton

    view.addSubview(collectionView)
    collectionView.autoresizingMask = [.flexibleHeight, .flexibleWidth]
    collectionView.delegate = self
    collectionView.backgroundColor = .systemBackground
    collectionView.register(PhotoCell.self, forCellWithReuseIdentifier: "PhotoCell")

    dataSource = DataSource(collectionView: collectionView, cellProvider: { (cv, indexPath, object) -> PhotoCell? in

      if let object = object as? Photo {
        let cell = cv.dequeueReusableCell(withReuseIdentifier: "PhotoCell", for: indexPath) as! PhotoCell
        cell.backgroundColor = indexPath.item % 2 == 0 ? .systemTeal : .systemPink
        cell.label.text = object.title
        return cell
      }

      return nil
    })

    load()
  }

  @objc func onTapLoad() {
    load(page: 1)
  }

  func load(page: Int = 0) {
    PhotoLoader.shared.load(page: page) { result in
      if let photos = try? result.get() {
        self.apply(photos)
      }
    }
  }

  func apply(_ photos: [Photo]) {

    snapshot = DataSourceSnapshot()
    snapshot.appendSections([Section.main])
    snapshot.appendItems(photos)

    dataSource.apply(snapshot, animatingDifferences: false)
  }
}

extension ViewController: UICollectionViewDelegateFlowLayout {
  func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
    return .init(width: collectionView.frame.width - 32, height: 100)
  }
}

struct Photo: Decodable, Hashable {
  let id = UUID()
  let title: String
}

final class PhotoLoader {

  static let shared = PhotoLoader()

  func load(page: Int, completion: @escaping (Result<[Photo], Error>) -> Void) {

    URLSession.shared.dataTask(with: URL(string: "https://jsonplaceholder.typicode.com/photos?_start=\(page)&_limit=5")!, completionHandler: { data, response, error in
      if let data = data, let model = try? JSONDecoder().decode([Photo].self, from: data) {
        completion(.success(model))
      }
    }).resume()
  }
}

final class PhotoCell: UICollectionViewCell {
  lazy var label = UILabel(frame: .zero)

  override init(frame: CGRect) {
    super.init(frame: frame)

    label.translatesAutoresizingMaskIntoConstraints = false
    label.numberOfLines = 3
    addSubview(label)
    NSLayoutConstraint.activate([
      label.topAnchor.constraint(equalTo: topAnchor),
      label.leadingAnchor.constraint(equalTo: leadingAnchor),
      label.bottomAnchor.constraint(equalTo: bottomAnchor),
      label.trailingAnchor.constraint(equalTo: trailingAnchor)
    ])

  }

  required init?(coder: NSCoder) {
    return nil
  }

}

Without using an extra array initialize the snapshot in viewDidLoad

override func viewDidLoad() {
    super.viewDidLoad()
    
    ...
    let snapshot = DataSourceSnapshot()
    snapshot.appendSections([Section.main])
    dataSource.apply(snapshot, animatingDifferences: false)
}

and in apply(_ photos) append the photos to the current snapshot rather than creating a new one

func apply(_ photos: [Photo]) {

    var snapshot = dataSource.snapshot()
    snapshot.appendItems(photos, toSection: .main)  
    dataSource.apply(snapshot, animatingDifferences: true)
}

The property is not needed

Note:

Declare the datasource as specific as possible

typealias DataSource = UICollectionViewDiffableDataSource<Section, Photo>
typealias DataSourceSnapshot = NSDiffableDataSourceSnapshot<Section, Photo>

Then you get rid of the unnecessary type check in

dataSource = DataSource(collectionView: collectionView, cellProvider: { (cv, indexPath, photo) -> PhotoCell? in

    let cell = cv.dequeueReusableCell(withReuseIdentifier: "PhotoCell", for: indexPath) as! PhotoCell
    cell.backgroundColor = indexPath.item % 2 == 0 ? .systemTeal : .systemPink
    cell.label.text = photo.title
    return cell
 
})