Code sample #3 from https://medium.com/@johnsundell/testing-swift-code-that-uses-system-singletons-in-3-easy-steps-89f4884cd325

Testing-Swift-code-that-uses-system-singletons-in-3-easy-steps-2.swift

class DataLoader {
    enum Result {
        case data(Data)
        case error(Error)
    }
    
    private let engine: NetworkEngine
    
    init(engine: NetworkEngine = URLSession.shared) {
        self.engine = engine
    }
    
    func load(from url: URL, completionHandler: @escaping (Result) -> Void) {
        engine.performRequest(for: url) { (data, response, error) in
            if let error = error {
                return completionHandler(.error(error))
            }
            
            completionHandler(.data(data ?? Data()))
        }
    }
}