DiffableDataSource1.swift

import UIKit

struct Todo: Identifiable {
    var id: UUID
    var title: String
    var done: Bool
}

final class TodoRepository {
    private var todos: [Todo] = (1...30).map { i in
        Todo(id: UUID(), title: "Todo #\(i)", done: false)
    }

    var todoIDs: [Todo.ID] { todos.map(\.id) }

    func todo(id: Todo.ID) -> Todo? {
        todos.first(where: { $0.id == id })
    }
}

final class TodoListViewController: UIViewController {
    enum Section {
        case main
    }

    private var collectionView: UICollectionView!
    private var dataSource: UICollectionViewDiffableDataSource<Section, Todo.ID>!

    private var repository: TodoRepository = .init()

    override func viewDidLoad() {
        super.viewDidLoad()

        configureCollectionView()
        configureDataSource()
        applySnapshot()
    }

    private func configureCollectionView() {
        let layout = UICollectionViewCompositionalLayout { sectionIndex, layoutEnvironment in
            let configuration = UICollectionLayoutListConfiguration(appearance: .plain)
            return NSCollectionLayoutSection.list(using: configuration, layoutEnvironment: layoutEnvironment)
        }
        collectionView = UICollectionView(frame: .null, collectionViewLayout: layout)

        view.addSubview(collectionView)
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        collectionView.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        collectionView.trailingAnchor.constraint(equalTo: view.trailingAnchor).isActive = true
        collectionView.topAnchor.constraint(equalTo: view.topAnchor).isActive = true
        collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
    }

    private func configureDataSource() {
        let todoCellRegistration = UICollectionView.CellRegistration<UICollectionViewListCell, Todo> { cell, indexPath, todo in
            var configuration = cell.defaultContentConfiguration()
            configuration.text = todo.title
            cell.contentConfiguration = configuration

            cell.accessories = [
                .checkmark(displayed: .always, options: .init(isHidden: !todo.done))
            ]
        }

        dataSource = UICollectionViewDiffableDataSource(
            collectionView: collectionView,
            cellProvider: { [weak self] collectionView, indexPath, todoID in
                let todo = self?.repository.todo(id: todoID)
                return collectionView.dequeueConfiguredReusableCell(using: todoCellRegistration, for: indexPath, item: todo)
            }
        )
    }

    private func applySnapshot() {
        var snapshot = NSDiffableDataSourceSnapshot<Section, Todo.ID>()
        snapshot.appendSections([.main])
        snapshot.appendItems(repository.todoIDs, toSection: .main)

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