iOSの壁紙アプリでサムネイル一覧のカクつきを消す — ImageIOダウンサンプリングの実装メモ

import ImageIO
import UIKit
 
func downsampledImage(at url: URL, pointSize: CGSize, scale: CGFloat) -> UIImage? {
    let sourceOptions = [kCGImageSourceShouldCache: false] as CFDictionary
    guard let source = CGImageSourceCreateWithURL(url as CFURL, sourceOptions) else {
        return nil
    }
 
    let maxPixel = max(pointSize.width, pointSize.height) * scale
    let thumbnailOptions = [
        kCGImageSourceCreateThumbnailFromImageAlways: true,
        kCGImageSourceShouldCacheImmediately: true,
        kCGImageSourceCreateThumbnailWithTransform: true,
        kCGImageSourceThumbnailMaxPixelSize: maxPixel
    ] as CFDictionary
 
    guard let cgImage = CGImageSourceCreateThumbnailAtIndex(source, 0, thumbnailOptions) else {
        return nil
    }
    return UIImage(cgImage: cgImage)
}

final class ThumbnailLoader {
    private let queue = DispatchQueue(label: "thumbnail.downsample", qos: .userInitiated, attributes: .concurrent)
    private let cache = NSCache<NSURL, UIImage>()
 
    func loadThumbnail(url: URL, pointSize: CGSize, completion: @escaping (UIImage?) -> Void) {
        if let cached = cache.object(forKey: url as NSURL) {
            completion(cached)
            return
        }
        queue.async { [weak self] in
            let scale = UIScreen.main.scale
            let image = downsampledImage(at: url, pointSize: pointSize, scale: scale)
            if let image = image {
                self?.cache.setObject(image, forKey: url as NSURL)
            }
            DispatchQueue.main.async { completion(image) }
        }
    }
}

extension GalleryViewController: UICollectionViewDataSourcePrefetching {
    func collectionView(_ collectionView: UICollectionView,
                        prefetchItemsAt indexPaths: [IndexPath]) {
        for indexPath in indexPaths {
            let url = items[indexPath.item].fileURL
            loader.loadThumbnail(url: url, pointSize: cellSize) { _ in }
        }
    }
}

func collectionView(_ collectionView: UICollectionView,
                    cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
    let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "Thumb", for: indexPath) as! ThumbCell
    let url = items[indexPath.item].fileURL
    cell.representedURL = url
    loader.loadThumbnail(url: url, pointSize: cell.bounds.size) { image in
        guard cell.representedURL == url else { return }   // 再利用でずれていたら破棄
        cell.imageView.image = image
    }
    return cell
}