class APIClient {
    let urlSession = URLSession(configuration: .default)
    let notificationCenter = NotificationCenter.default

    func perform(_ urlRequest: URLRequest,
                 completion: @escaping (Data?, URLResponse?, Error?) -> Void) {
        var urlSessionTask: URLSessionTask?
        urlSessionTask =
            urlSession
                .dataTask(with: urlRequest) { [weak self] (data, response, error) in
                    completion(data, response, error)
                    self?.notificationCenter
                        .post(.makeURLSessionTaskNotification(urlSessionTask,
                                                              urlSession: self?.urlSession,
                                                              forName: .urlSessionTaskDidStart))
        }
        urlSessionTask?.resume()
        notificationCenter
            .post(.makeURLSessionTaskNotification(urlSessionTask,
                                                  urlSession: urlSession,
                                                  forName: .urlSessionTaskDidStart))
    }
}