Generic cell provider for UICollectionViewDiffableDataSource

ConfigurableCell.swift

import UIKit

protocol ConfigurableCell: UICollectionViewCell {
    associatedtype Item
    func configure(with item: Item, for indexPath: IndexPath)
}

extension ConfigurableCell {
    public static var provider: (UICollectionView, IndexPath, Item) -> Self {
        let registration = registration()
        return { collectionView, indexPath, item in
            collectionView.dequeueConfiguredReusableCell(using: registration, for: indexPath, item: item)
        }
    }

    public static func provider<ID>(_ lookup: @escaping (ID) -> Item?) -> (UICollectionView, IndexPath, ID) -> Self? {
        let registration = registration()
        return { collectionView, indexPath, itemId in
            guard let item = lookup(itemId) else { return nil }
            return collectionView.dequeueConfiguredReusableCell(using: registration, for: indexPath, item: item)
        }
    }
    
    private static func registration() -> UICollectionView.CellRegistration<Self, Item> {
        .init { cell, indexPath, item in
            cell.configure(with: item, for: indexPath)
        }
    }
}

class ViewController: UIViewController {
    private enum Section { case main }

    private typealias DataSource = UICollectionViewDiffableDataSource<Section, Product.ID>
    private typealias Snapshot = NSDiffableDataSourceSnapshot<Section, Product.ID>
    
    private lazy var collectionView: UICollectionView = {
        let configuration = UICollectionLayoutListConfiguration(appearance: .insetGrouped)
        let layout = UICollectionViewCompositionalLayout.list(using: configuration)
        let collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
        collectionView.refreshControl = refreshControl
        return collectionView
    }()
    
    private lazy var dataSource = DataSource(
        collectionView: collectionView,
        cellProvider: ProductCell.provider { Product.byID($0) }
    )

    private lazy var refreshControl = UIRefreshControl(
        frame: .zero,
        primaryAction: UIAction { [unowned self] _ in
            refreshData()
        }
    )
        
    override func loadView() {
        view = collectionView
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        applyInitialSnapshot()
    }
    
    private func applyInitialSnapshot() {
        var snapshot = Snapshot()
        snapshot.appendSections([.main])
        snapshot.appendItems(Product.allProductIDs)
        dataSource.applySnapshotUsingReloadData(snapshot)
    }
    
    private func refreshData() {
        DispatchQueue.main.asyncAfter(deadline: .now() + 1) {
            Product.updatePrices()
            self.reconfigureItems()
            self.refreshControl.endRefreshing()
        }
    }
    
    private func reconfigureItems() {
        var snapshot = dataSource.snapshot()
        snapshot.reconfigureItems(Product.allProductIDs) // preserves the existing cells
        dataSource.apply(snapshot, animatingDifferences: false)
    }
}

class ProductCell: UICollectionViewListCell, ConfigurableCell {
    public func configure(with product: Product, for indexPath: IndexPath) {
        var configuration = defaultContentConfiguration()
        configuration.text = product.name
        configuration.secondaryText = String(product.price)
        configuration.secondaryTextProperties.color = .secondaryLabel
        configuration.prefersSideBySideTextAndSecondaryText = true
        contentConfiguration = configuration
        accessories = [.disclosureIndicator()]
    }
}

struct Product: Identifiable {
    let id: UUID
    let name: String
    var price: Double
    
    static var allProducts = [
        Product(id: UUID(), name: "Americano", price: 2.99),
        Product(id: UUID(), name: "Cappuccino", price: 3.99),
        Product(id: UUID(), name: "Latte", price: 4.99),
    ]
    
    static var allProductIDs: [UUID] {
        allProducts.map(\.id)
    }
    
    static func byID(_ id: UUID) -> Product? {
        allProducts.first(where: { $0.id == id })
    }
    
    static func updatePrices() {
        for index in allProducts.indices {
            allProducts[index].price += 1
        }
    }
}