sgl0v

struct Theme {
    static let light = Theme(type: .light, colors: .light)
    static let dark = Theme(type: .dark, colors: .dark)
    @available(iOS 13.0, *)
    static let adaptive = Theme(type: .adaptive, colors: .adaptive)

    enum `Type` {
        case light
        case dark
        @available(iOS 13.0, *)

sgl0v

protocol ThemeProvider: class {
    var theme: Theme { get }
    func register<Observer: Themeable>(observer: Observer)
    func toggleTheme()
}

@available(iOS 13.0, *)
public class DefaultThemeProvider: NSObject, ThemeProvider {
    static let shared = DefaultThemeProvider()
    let theme: Theme = .adaptive

sgl0v

extension Themeable where Self: UITraitEnvironment {
    var themeProvider: ThemeProvider {
        if #available(iOS 13.0, *) {
            return DefaultThemeProvider.shared
        } else {
            return LegacyThemeProvider.shared
        }
    }
}

sgl0v

extension UIImage {

    func decodedImage() -> UIImage {
        guard let cgImage = cgImage else { return self }
        let size = CGSize(width: cgImage.width, height: cgImage.height)
        let colorSpace = CGColorSpaceCreateDeviceRGB()
        let context = CGContext(data: nil, width: Int(size.width), height: Int(size.height), bitsPerComponent: 8, bytesPerRow: cgImage.bytesPerRow, space: colorSpace, bitmapInfo: CGImageAlphaInfo.noneSkipFirst.rawValue)
        context?.draw(cgImage, in: CGRect(origin: .zero, size: size))
        guard let decodedImage = context?.makeImage() else { return self }
        return UIImage(cgImage: decodedImage)

sgl0v

// Declares in-memory image cache
protocol ImageCacheType: class {
    // Returns the image associated with a given url
    func image(for url: URL) -> UIImage?
    // Inserts the image of the specified url in the cache
    func insertImage(_ image: UIImage?, for url: URL)
    // Removes the image of the specified url in the cache
    func removeImage(for url: URL)
    // Removes all images from the cache
    func removeAllImages()

sgl0v

final class ImageCache {

    // 1st level cache, that contains encoded images
    private lazy var imageCache: NSCache<AnyObject, AnyObject> = {
        let cache = NSCache<AnyObject, AnyObject>()
        cache.countLimit = config.countLimit
        return cache
    }()
    // 2nd level cache, that contains decoded images
    private lazy var decodedImageCache: NSCache<AnyObject, AnyObject> = {

sgl0v

extension ImageCache: ImageCacheType {
    func insertImage(_ image: UIImage?, for url: URL) {
        guard let image = image else { return removeImage(for: url) }
        let decodedImage = image.decodedImage()

        lock.lock(); defer { lock.unlock() }
        imageCache.setObject(decodedImage, forKey: url as AnyObject)
        decodedImageCache.setObject(image as AnyObject, forKey: url as AnyObject, cost: decodedImage.diskSize)
    }

sgl0v

extension ImageCache {
    func image(for url: URL) -> UIImage? {
        lock.lock(); defer { lock.unlock() }
        // the best case scenario -> there is a decoded image
        if let decodedImage = decodedImageCache.object(forKey: url as AnyObject) as? UIImage {
            return decodedImage
        }
        // search for image data
        if let image = imageCache.object(forKey: url as AnyObject) as? UIImage {
            let decodedImage = image.decodedImage()

sgl0v

extension ImageCache {
    subscript(_ key: URL) -> UIImage? {
        get {
            return image(for: key)
        }
        set {
            return insertImage(newValue, for: key)
        }
    }
}

sgl0v

final class ImageLoader {

    func loadImage(from url: URL) -> AnyPublisher<UIImage?, Never> {
        return URLSession.shared.dataTaskPublisher(for: url) ➊
            .map { (data, _) -> UIImage? in return UIImage(data: data) } ➋
            .catch { error in return Just(nil) } ➌
            .subscribe(on: backgroundQueue) ➍
            .receive(on: RunLoop.main) ➎
            .eraseToAnyPublisher() ➏
    }