How to use custom Key for NSCache

Issue #766

Need to use a class, best is to subclass from NSObject

let cache = NSCache<Key, UIImage>()

final class Key: NSObject {
    override func isEqual(_ object: Any?) -> Bool {
        guard let other = object as? Key else {
            return false
        }
        return url == other.url
            && size == other.size
    }

    override var hash: Int {
        return url.hashValue ^ Int(size.width) ^ Int(size.height)
    }

    let url: URL
    let size: CGSize

    init(url: URL, size: CGSize) {
        self.url = url
        self.size = size
    }
}