UICollectionView with diffable data source always reload on-screen cells after applying a snapshot.

From what I concluded, a diffable data source is designed to insert/delete/update/move only the differences from the new snapshot (if animatingDifferences is true), for those irrelevent cells, they should not be reloaded.

But whenever I apply a snapshot, even if the new snapshot is exactly the current one from the data source, almost every on-screen cells got reloaded. Here's a simple test view controller:

Code Block swift
import UIKit
class CollectionViewController: UIViewController {
    enum Section: Hashable {
        case main
    }
    struct Item: Hashable {
        let id: String
    }
    private lazy var dataSource: UICollectionViewDiffableDataSource<Section, Item> = {
        return UICollectionViewDiffableDataSource<Section, Item>(collectionView: collectionView) { collectionView, indexPath, item in
            print("data source reloading cell", item, indexPath)
            let cell =  collectionView.dequeueReusableCell(
                withReuseIdentifier: "cell",
                for: indexPath
            ) as! Cell
            cell.item = item
            return cell
        }
    }()
    private lazy var listLayout: UICollectionViewCompositionalLayout = {
        let conf = UICollectionLayoutListConfiguration(appearance: .plain)
        return UICollectionViewCompositionalLayout.list(using: conf)
    }()
    private lazy var regularLayout: UICollectionViewCompositionalLayout = {
        let item = NSCollectionLayoutItem(
            layoutSize: .init(widthDimension: .fractionalWidth(1), heightDimension: .estimated(50))
        )
        let group = NSCollectionLayoutGroup.horizontal(
            layoutSize: .init(widthDimension: .fractionalWidth(1), heightDimension: .estimated(50)),
            subitem: item,
            count: 1
        )
        let section = NSCollectionLayoutSection(group: group)
        return UICollectionViewCompositionalLayout(section: section)
    }()
    private lazy var collectionView: UICollectionView = {
        let view = UICollectionView(frame: .zero, collectionViewLayout: regularLayout)
        view.register(Cell.self, forCellWithReuseIdentifier: "cell")
        view.translatesAutoresizingMaskIntoConstraints = false
        view.backgroundColor = .systemBackground
        return view
    }()
    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(collectionView)
        NSLayoutConstraint.activate([
            collectionView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            collectionView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            collectionView.topAnchor.constraint(equalTo: view.topAnchor),
            collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor),
        ])
        var snapshot = NSDiffableDataSourceSnapshot<Section, Item>()
        snapshot.appendSections([.main])
        snapshot.appendItems((0 ..< 100).map { .init(id: "\($0)") })
        dataSource.apply(snapshot, animatingDifferences: false)
        Timer.scheduledTimer(withTimeInterval: 5, repeats: false) { _ in
            print("\n\nDelayed Reloading")
            var snapshot = self.dataSource.snapshot()
            self.dataSource.apply(snapshot, animatingDifferences: true)
        }
    }
    class Cell: UICollectionViewCell {
        var item: Item? {
            didSet {
                label.text = item?.id
            }
        }
        private(set) lazy var label: UILabel = {
            let view = UILabel()
            view.font = .systemFont(ofSize: 48, weight: .semibold)
            view.translatesAutoresizingMaskIntoConstraints = false
            return view
        }()
        override init(frame: CGRect) {
            super.init(frame: frame)
            print("cell inited")
            contentView.addSubview(label)
            NSLayoutConstraint.activate([
                label.leadingAnchor.constraint(equalTo: contentView.layoutMarginsGuide.leadingAnchor),
                label.topAnchor.constraint(equalTo: contentView.topAnchor, constant: 12),
                label.bottomAnchor.constraint(equalTo: contentView.bottomAnchor, constant: -12),
            ])
        }
        required init?(coder: NSCoder) {
            fatalError("init(coder:) has not been implemented")
        }
    }
}

Output

• Xcode 12.5

• iPhone 11 device (iOS 14.5.1)

Code Block 
Delayed Reloading
data source reloading cell Item(id: "2") [0, 2]
data source reloading cell Item(id: "4") [0, 4]
data source reloading cell Item(id: "6") [0, 6]
data source reloading cell Item(id: "8") [0, 8]
data source reloading cell Item(id: "10") [0, 10]
data source reloading cell Item(id: "12") [0, 12]
data source reloading cell Item(id: "14") [0, 14]
data source reloading cell Item(id: "1") [0, 1]
data source reloading cell Item(id: "3") [0, 3]
data source reloading cell Item(id: "5") [0, 5]
data source reloading cell Item(id: "7") [0, 7]
data source reloading cell Item(id: "9") [0, 9]
data source reloading cell Item(id: "11") [0, 11]
data source reloading cell Item(id: "13") [0, 13]
data source reloading cell Item(id: "15") [0, 15]
data source reloading cell Item(id: "0") [0, 0]
data source reloading cell Item(id: "11") [0, 11]

Is this an expected behavior?

I've also run the same test on UITableView, which is much more reasonable. Only some off-screen cells are reloaded.