Rork Max の SwiftUI グリッドが画像でカクつくときに効く、スクロール最適化

import ImageIO
import UIKit
 
enum ImageDownsampler {
    /// 指定したポイントサイズに合わせたサムネイルを生成します。
    /// フル解像度を展開しないため、メモリ使用量とデコード時間を大きく抑えられます。
    static func thumbnail(at url: URL, pointSize: CGSize, scale: CGFloat) -> CGImage? {
        let sourceOptions = [kCGImageSourceShouldCache: false] as CFDictionary
        guard let source = CGImageSourceCreateWithURL(url as CFURL, sourceOptions) else {
            return nil
        }
        // 長辺をターゲットのピクセル数に合わせる
        let maxPixel = max(pointSize.width, pointSize.height) * scale
        let options = [
            kCGImageSourceCreateThumbnailFromImageAlways: true,
            kCGImageSourceShouldCacheImmediately: true,   // デコードをこの場で済ませる
            kCGImageSourceCreateThumbnailWithTransform: true,
            kCGImageSourceThumbnailMaxPixelSize: maxPixel
        ] as CFDictionary
        return CGImageSourceCreateThumbnailAtIndex(source, 0, options)
    }
}

struct WallpaperCell: View, Equatable {
    let item: Wallpaper       // Identifiable
    let side: CGFloat
 
    @State private var image: UIImage?
 
    var body: some View {
        Color(.secondarySystemBackground)
            .overlay {
                if let image {
                    Image(uiImage: image)
                        .resizable()
                        .scaledToFill()
                }
            }
            .frame(width: side, height: side)
            .clipped()
            .task(id: item.id) {        // セルが再利用されると前のタスクは自動でキャンセルされる
                let target = CGSize(width: side, height: side)
                let scale = UIScreen.main.scale
                let url = item.fileURL
                let cg = await Task.detached(priority: .userInitiated) {
                    ImageDownsampler.thumbnail(at: url, pointSize: target, scale: scale)
                }.value
                if let cg {
                    image = UIImage(cgImage: cg)
                }
            }
    }
 
    static func == (lhs: WallpaperCell, rhs: WallpaperCell) -> Bool {
        lhs.item.id == rhs.item.id && lhs.side == rhs.side
    }
}

struct Wallpaper: Identifiable, Hashable {
    let id: String
    let fileURL: URL
}
 
struct WallpaperGrid: View {
    let items: [Wallpaper]
    private let columns = [GridItem(.adaptive(minimum: 110), spacing: 2)]
 
    var body: some View {
        ScrollView {
            LazyVGrid(columns: columns, spacing: 2) {
                ForEach(items) { item in
                    WallpaperCell(item: item, side: 110)
                        .equatable()      // 同じ値なら body を再評価しない
                }
            }
            .padding(.horizontal, 2)
        }
    }
}

final class ThumbnailCache {
    static let shared = ThumbnailCache()
    private let cache = NSCache<NSString, UIImage>()
 
    private init() {
        cache.countLimit = 200          // 同時に保持するサムネイル数の上限
    }
 
    func image(for key: String) -> UIImage? { cache.object(forKey: key as NSString) }
    func set(_ image: UIImage, for key: String) { cache.setObject(image, forKey: key as NSString) }
}