Design an Image Caching System

let imageCache = ImageCache.shared

// Load image (from cache or download)
imageCache.loadImage(url: imageURL, size: CGSize(width: 300, height: 300)) { image in
    cell.imageView.image = image
}

// Prefetch images
imageCache.prefetch(urls: upcomingImageURLs)

// Clear old cache
imageCache.clearCache(olderThan: 7.days)

class ImageCache {
    // MARK: - Properties
    private let memoryCache: NSCache<NSString, UIImage>
    private let diskCache: DiskCacheManager
    private let downloadQueue: OperationQueue
    private let processingQueue: DispatchQueue
    
    // MARK: - Singleton
    static let shared = ImageCache()
    
    private init() {
        // Memory cache configuration
        memoryCache.countLimit = 100 // Max 100 images
        memoryCache.totalCostLimit = 150 * 1024 * 1024 // 150 MB
        memoryCache.delegate = self
        
        // Download queue configuration
        downloadQueue.maxConcurrentOperationCount = 4
        downloadQueue.qualityOfService = .userInitiated
        
        // Processing queue
        processingQueue = DispatchQueue(
            label: "com.app.image-processing",
            qos: .userInitiated,
            attributes: .concurrent
        )
        
        // Disk cache
        diskCache = DiskCacheManager(maxSize: 500 * 1024 * 1024) // 500 MB
    }
    
    // MARK: - Public API
    func loadImage(url: URL, 
                   size: CGSize? = nil,
                   completion: @escaping (UIImage?) -> Void) {
        let cacheKey = generateCacheKey(url: url, size: size)
        
        // 1. Check memory cache (fastest)
        if let cachedImage = memoryCache.object(forKey: cacheKey as NSString) {
            completion(cachedImage)
            return
        }
        
        // 2. Check disk cache (fast)
        diskCache.retrieveImage(forKey: cacheKey) { [weak self] diskImage in
            if let diskImage = diskImage {
                // Store in memory for next time
                self?.memoryCache.setObject(diskImage, forKey: cacheKey as NSString)
                completion(diskImage)
                return
            }
            
            // 3. Download (slow)
            self?.downloadImage(url: url, size: size, cacheKey: cacheKey, completion: completion)
        }
    }
}

class LRUDiskCache {
    private var accessTimes: [String: Date] = [:]
    private var lock = NSLock()
    
    func retrieveImage(forKey key: String, completion: @escaping (UIImage?) -> Void) {
        lock.lock()
        accessTimes[key] = Date() // Update access time
        lock.unlock()
        
        // Load from disk...
    }
    
    func evictLRU() {
        lock.lock()
        defer { lock.unlock() }
        
        // Sort by access time
        let sorted = accessTimes.sorted { $0.value < $1.value }
        
        // Remove oldest 20%
        let toRemove = Array(sorted.prefix(sorted.count / 5))
        toRemove.forEach { key, _ in
            deleteFromDisk(key: key)
            accessTimes.removeValue(forKey: key)
        }
    }
}

extension UIImage {
    func downscaled(to targetSize: CGSize) -> UIImage {
        let format = UIGraphicsImageRendererFormat()
        format.scale = 1 // Force 1x scale to save memory
        format.opaque = true // Faster rendering if no alpha
        
        let renderer = UIGraphicsImageRenderer(size: targetSize, format: format)
        
        return renderer.image { context in
            self.draw(in: CGRect(origin: .zero, size: targetSize))
        }
    }
}

// Usage in cache:
private func downloadImage(url: URL, size: CGSize?, cacheKey: String, completion: @escaping (UIImage?) -> Void) {
    let operation = ImageDownloadOperation(url: url, size: size) { [weak self] image in
        guard let image = image else {
            completion(nil)
            return
        }
        
        var finalImage = image
        
        // Downscale if size provided
        if let targetSize = size {
            finalImage = image.downscaled(to: targetSize)
        }
        
        // Cache
        self?.memoryCache.setObject(finalImage, forKey: cacheKey as NSString)
        self?.diskCache.store(finalImage, forKey: cacheKey)
        
        completion(finalImage)
    }
    
    downloadQueue.addOperation(operation)
}

class ImageCache {
    private var inFlightRequests: [String: [(UIImage?) -> Void]] = [:]
    private var requestsLock = NSLock()
    
    func loadImage(url: URL, size: CGSize?, completion: @escaping (UIImage?) -> Void) {
        let cacheKey = generateCacheKey(url: url, size: size)
        
        // ... memory/disk cache checks ...
        
        // Check if already downloading
        requestsLock.lock()
        if inFlightRequests[cacheKey] != nil {
            // Add to waiting list
            inFlightRequests[cacheKey]?.append(completion)
            requestsLock.unlock()
            return
        }
        
        // Start new request
        inFlightRequests[cacheKey] = [completion]
        requestsLock.unlock()
        
        // Download
        downloadImage(url: url, size: size) { [weak self] image in
            self?.requestsLock.lock()
            let callbacks = self?.inFlightRequests[cacheKey] ?? []
            self?.inFlightRequests.removeValue(forKey: cacheKey)
            self?.requestsLock.unlock()
            
            // Notify all waiting callbacks
            callbacks.forEach { $0(image) }
        }
    }
}

func compressImage(_ image: UIImage, quality: CGFloat = 0.8) -> Data? {
    // HEIC compression (iOS 11+)
    if #available(iOS 11.0, *),
       let heicData = image.heicData(compressionQuality: quality),
       heicData.count < image.jpegData(compressionQuality: quality)?.count ?? Int.max {
        return heicData
    }
    
    // Fall back to JPEG
    return image.jpegData(compressionQuality: quality)
}

class ThreadSafeImageCache {
    private let cache = NSCache<NSString, UIImage>()
    private let lock = NSLock()
    private let concurrentQueue = DispatchQueue(
        label: "com.app.imagecache",
        attributes: .concurrent
    )
    
    func setImage(_ image: UIImage, forKey key: String) {
        // NSCache is thread-safe, but wrap for consistency
        concurrentQueue.async(flags: .barrier) { [weak self] in
            self?.cache.setObject(image, forKey: key as NSString)
        }
    }
    
    func image(forKey key: String) -> UIImage? {
        var result: UIImage?
        concurrentQueue.sync {
            result = cache.object(forKey: key as NSString)
        }
        return result
    }
}

// Show blurred placeholder, then full resolution
func loadImageProgressively(url: URL, completion: @escaping (UIImage, Bool) -> Void) {
    // 1. Load tiny thumbnail first (instant)
    let thumbnailURL = url.thumbnailVersion()
    loadImage(url: thumbnailURL, size: CGSize(width: 50, height: 50)) { thumbnail in
        if let blur = thumbnail?.applyBlur() {
            completion(blur, false) // Not final
        }
    }
    
    // 2. Load full resolution
    loadImage(url: url) { fullImage in
        if let full = fullImage {
            completion(full, true) // Final
        }
    }
}

override init() {
    super.init()
    
    NotificationCenter.default.addObserver(
        self,
        selector: #selector(handleMemoryWarning),
        name: UIApplication.didReceiveMemoryWarningNotification,
        object: nil
    )
}

@objc private func handleMemoryWarning() {
    // Aggressively clear memory cache
    memoryCache.removeAllObjects()
    
    // Cancel low-priority downloads
    downloadQueue.operations
        .filter { $0.queuePriority == .low }
        .forEach { $0.cancel() }
}

struct ImageCacheMetrics {
    var memoryHitRate: Double    // % of requests served from memory
    var diskHitRate: Double      // % served from disk
    var downloadRate: Double     // % that needed download
    var averageLoadTime: TimeInterval
    var cacheSize: (memory: Int, disk: Int)
}