👀 Views: 0 💬 Answers: 1 📅 Created: 2025-06-12

ios uicollectionview performance prefetching Swift

I'm upgrading from an older version and I'm performance testing and I've looked through the documentation and I'm still confused about I'm facing performance issues with a `UICollectionView` in my iOS 16 app where the UI becomes unresponsive during prefetching of images from a remote server... I have enabled prefetching by implementing the `UICollectionViewDataSourcePrefetching` protocol, but it seems like the UI hangs when the prefetching runs, especially when scrolling quickly. Here's a brief overview of what I've implemented: ```swift class MyViewController: UIViewController, UICollectionViewDataSource, UICollectionViewDelegate, UICollectionViewDataSourcePrefetching { var collectionView: UICollectionView! var imageUrls: [String] = [] // Array of image URLs override func viewDidLoad() { super.viewDidLoad() setupCollectionView() loadImages() } func setupCollectionView() { let layout = UICollectionViewFlowLayout() collectionView = UICollectionView(frame: view.bounds, collectionViewLayout: layout) collectionView.dataSource = self collectionView.delegate = self collectionView.prefetchDataSource = self collectionView.register(MyCell.self, forCellWithReuseIdentifier: "MyCell") view.addSubview(collectionView) } func loadImages() { // Load image URLs into the imageUrls array... } func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int { return imageUrls.count } func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell { let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "MyCell", for: indexPath) as! MyCell // Configure cell with images... return cell } func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) { for indexPath in indexPaths { // Start loading images asynchronously loadImage(at: indexPath) } } func loadImage(at indexPath: IndexPath) { // Asynchronous image loading implementation... } } ``` The `loadImage(at:)` function fetches images using `URLSession` and updates the cell once the image is downloaded. Despite this being asynchronous, I've noticed that during rapid scrolling, the app becomes sluggish, and I'm seeing the following error in the console: ``` Main thread is blocked for more than 1 second. ``` I've tried using `DispatchQueue.global(qos: .userInitiated).async` to offload the image fetching but it didn’t seem to help. Additionally, I implemented caching with an in-memory cache, but this doesn’t resolve the UI stuttering either. Is there a better way to handle image prefetching in `UICollectionView` to ensure smooth performance during scrolling? Any advice or best practices would be greatly appreciated. This is part of a larger service I'm building. Is there a better approach? The stack includes Swift and several other technologies. I'm coming from a different tech stack and learning Swift. Has anyone else encountered this?