Kingfisher/Sources/Documentation.docc/Topics/Topic_Prefetch.md

Prefetching images before actually loading

Preloading images before actually required. Feeding them to the table view or collection view to improve the display speed.

Overview

Use ImagePrefetcher to prefetch and cache images that are likely to be displayed later. This improves loading times and ensures smoother image display.

Prefetch some images

let urls = [
    "https://example.com/image1.jpg", 
    "https://example.com/image2.jpg"
].map { URL(string: $0)! }

let prefetcher = ImagePrefetcher(urls: urls) {
    skippedResources, failedResources, completedResources in
    print("These resources are prefetched: \(completedResources)")
}
prefetcher.start()

// Later when you need to display these images:
imageView.kf.setImage(with: urls[0])
anotherImageView.kf.setImage(with: urls[1])

Prefetch images for table view or collection view

Starting with iOS 10, Apple introduced cell prefetching behavior, which can seamlessly integrate with Kingfisher's ImagePrefetcher.

override func viewDidLoad() {
    super.viewDidLoad()
    collectionView?.prefetchDataSource = self
}

extension ViewController: UICollectionViewDataSourcePrefetching {
    func collectionView(_ collectionView: UICollectionView, prefetchItemsAt indexPaths: [IndexPath]) {
        let urls = indexPaths.flatMap { URL(string: $0.urlString) }
        ImagePrefetcher(urls: urls).start()
    }
}

See WWDC 16 - Session 219 for more about changing of it in iOS 10.