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

Prefetching images before actually loading

Loading images before actually needed. Feed them to the table view or collection view.

Overview

You could use ImagePrefetcher to prefetch some images and cache them before you display them on the screen. This is useful when you know a list of image resources you know they would probably be shown later.

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

From iOS 10, Apple introduced a cell prefetching behavior. It could work seamlessly 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.