ios – Lazy Loading UICompositionalLayout

I’m trying to lazy load sections in a UICollectionView to create a TVGuide. I’m using UICompositionalLayout as well as UIDiffableDataSource to take advantage of these new APIs plus reduce performance overhead.

I have everything working but as soon as the sections are about to reload, you see that there is a redraw issue with the Loading... cell. It’s sized up to the first Event but it should disappear seamlessly as the events come in.

I have an image below which better shows the problem, plus the code snippet below which you can simply copy and paste into a blank project to test. Been trying to figure this out for weeks but just can’t figure out what to do. Any help is greatly appreciated.

import UIKit

class ViewController: UIViewController, UIScrollViewDelegate {
    
    private var collectionView: UICollectionView!
    
    typealias DataSource = UICollectionViewDiffableDataSource<String, String>
    typealias DataSourceSnapshot = NSDiffableDataSourceSnapshot<String, String>
    
    private var dataSource: DataSource!
    private var sections: [SectionViewModel] = []
    private var cellWidths: [Int: [CGFloat]] = [:]
    
    private let operationQueue = OperationQueue()
    private let scheduleWidth: CGFloat = 1500
    
    init() {
        super.init(nibName: nil, bundle: nil)
        
        sections = (0...150).enumerated().map({ index, object in
            let event = EventCellViewModel(name: "Loading...Section-\(index)", width: scheduleWidth)
            let sectionViewModel = SectionViewModel(events: [event])
            return sectionViewModel
        })
        
        view.backgroundColor = .blue
        
        collectionView = UICollectionView(frame: .zero, collectionViewLayout: createLayout())
        collectionView.delegate = self
        collectionView.dataSource = dataSource
        collectionView.translatesAutoresizingMaskIntoConstraints = false
        collectionView.backgroundColor = .clear
        collectionView.bounces = false
        collectionView.register(EventCell.self, forCellWithReuseIdentifier: "EventCell")
        collectionView.isOpaque = true
        
        operationQueue.qualityOfService = .utility
        operationQueue.maxConcurrentOperationCount = 1
        
        view.addSubview(collectionView)
        
        NSLayoutConstraint.activate([
            collectionView.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor),
            collectionView.leadingAnchor.constraint(equalTo: view.leadingAnchor),
            collectionView.trailingAnchor.constraint(equalTo: view.trailingAnchor),
            collectionView.bottomAnchor.constraint(equalTo: view.bottomAnchor)
        ])
        
        configureDataSource()
        createDumbyData()
    }
    
    private func configureDataSource() {
        dataSource = DataSource(collectionView: collectionView, cellProvider: { [weak self] collectionView, indexPath, object in
            let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "EventCell", for: indexPath) as! EventCell
            let viewModel = self?.sections[indexPath.section].events[indexPath.row]
            cell.programmeTitle.text = viewModel?.name
            cell.isOpaque = true
            cell.backgroundColor = .red.withAlphaComponent(0.7)
            return cell
        })
    }
    
    private func applySnapshot() {
        operationQueue.addOperation { [weak self] in
            
            guard let self = self else { return }
            var snapshot = DataSourceSnapshot()
            
            for (sectionIndex, section) in self.sections.enumerated() {
                let sectionId = "Section-\(sectionIndex)"
                snapshot.appendSections([sectionId])
                
                let eventIds = section.events.enumerated().map { "Event-\(sectionIndex)-\($0)" }
                snapshot.appendItems(eventIds, toSection: sectionId)
            }
            
            self.dataSource.apply(snapshot, animatingDifferences: false)
        }
    }
    
    private func updateSnapshot(sectionIndex: Int) {
        guard sections[sectionIndex].isLoading else { return }
        
        let newEvents = (0..<20).map {
            EventCellViewModel(name: "Event-\(sectionIndex)-\($0)")
        }
        
        if cellWidths[sectionIndex] == nil {
            cellWidths[sectionIndex] = newEvents.map { $0.width }
        }
        
        self.sections[sectionIndex].events = newEvents
        self.sections[sectionIndex].isLoading = false
        
        applySnapshot()
    }
    
    private func createDumbyData() {
        let sections = (0...150).map { index in
            SectionViewModel(events: [EventCellViewModel(name: "Loading...\(index)", width: 1500)])
        }
        
        self.sections = sections
        applySnapshot()
    }
    
    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    private func createLayout() -> UICollectionViewLayout {
        let layout = UICollectionViewCompositionalLayout { (sectionIndex, layoutEnvironment) in
            
            let items = self.layoutItems(for: sectionIndex)
            let scheduleWidth = self.scheduleWidth(for: sectionIndex)
            let groupSize = NSCollectionLayoutSize(widthDimension: .absolute(scheduleWidth),
                                                   heightDimension: .absolute(60))
            let group = NSCollectionLayoutGroup.horizontal(layoutSize: groupSize, subitems: items)
            
            // Create the section
            let section = NSCollectionLayoutSection(group: group)
            section.orthogonalScrollingBehavior = .none
            section.contentInsets.bottom = 0
            return section
        }
        return layout
    }
    
    private func layoutItems(for sectionIndex: Int) -> [NSCollectionLayoutItem] {
        return cellWidths(for: sectionIndex).map { width in
            let itemSize = NSCollectionLayoutSize(widthDimension: .absolute(width), heightDimension: .absolute(60))
            let item = NSCollectionLayoutItem(layoutSize: itemSize)
            return item
        }
    }
    
    private func scheduleWidth(for sectionIndex: Int) -> CGFloat {
        sections[sectionIndex].events.map { $0.width }.reduce(0, +)
    }
    
    func cellWidths(for sectionIndex: Int) -> [CGFloat] {
        cellWidths[sectionIndex] ?? [1500]
    }
    
    func isLoading(for section: Int) -> Bool {
        sections[section].isLoading
    }
    
    private func numberOfEvents(for index: Int) -> Int {
        sections[index].events.count
    }
    
    private func viewModel(for sectionIndex: Int, at itemIndex: Int) -> EventCellViewModel? {
        sections[sectionIndex].events[itemIndex]
    }
}

// MARK: - UICollectionViewDelegate
extension ViewController: UICollectionViewDelegate {
    
    func collectionView(_ collectionView: UICollectionView, willDisplay cell: UICollectionViewCell, forItemAt indexPath: IndexPath) {
        reloadCell(at: indexPath.section)
    }
    
    func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
        return sections[section].events.count
    }
    
    func numberOfSections(in collectionView: UICollectionView) -> Int {
        return sections.count
    }

    func reloadCell(at index: Int) {
        self.updateSnapshot(sectionIndex: index)
    }
}

// MARK: - ViewModels
public struct SectionViewModel: Hashable {
    var events: [EventCellViewModel]
    var isLoading = true
}

public struct EventCellViewModel {
    
    var uuid: String { return name }
    public let name: String
    public let width: CGFloat

    init(name: String, width: CGFloat = CGFloat(Int.random(in: 100...400))) {
        self.name = name
        self.width = width
    }
}

extension EventCellViewModel: Hashable {
    
    public func hash(into hasher: inout Hasher) {
        hasher.combine(uuid)
    }
    
    public static func ==(lhs: EventCellViewModel, rhs: EventCellViewModel) -> Bool {
        return lhs.uuid == rhs.uuid
    }
}

// MARK: - Cell
class EventCell: UICollectionViewCell {
    
    let programmeTitle = UILabel()
    
    override init(frame: CGRect) {
        super.init(frame: frame)
        
        programmeTitle.textColor = .white
        programmeTitle.translatesAutoresizingMaskIntoConstraints = false
        
        contentView.addSubview(programmeTitle)
        
        contentView.layer.borderColor = UIColor.blue.withAlphaComponent(0.4).cgColor
        contentView.layer.borderWidth = 1
        
        NSLayoutConstraint.activate([
            programmeTitle.topAnchor.constraint(equalTo: contentView.topAnchor),
            programmeTitle.leadingAnchor.constraint(equalTo: contentView.leadingAnchor, constant: 20),
            programmeTitle.trailingAnchor.constraint(equalTo: contentView.trailingAnchor),
            programmeTitle.bottomAnchor.constraint(equalTo: contentView.bottomAnchor)
        ])
    }
    
    @available(*, unavailable)
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    func configure(title: String?) {
        programmeTitle.text = title
    }
}