Simple service class template for URLCache example

APIService-URLCache.swift

class APIService: ServiceProtocol {
    func get( url: URL, callback: @escaping (_ data: Data?, _ error: Error?) -> Void ) {
        let request = URLRequest(url: url, cachePolicy: .returnCacheDataElseLoad, timeoutInterval: 30)
        URLSession.shared.dataTask(with: request) { (data, _, error) in
            callback(data, error)
            }.resume()
    }
}