This snipped shows a well structured example of a Networking Class, used in makeing request to a Weather API server

DataManager.swift

import Foundation

enum DataManagerError: Error {

    case Unknown
    case FailedRequest
    case InvalidResponse

}

final class DataManager {

    typealias WeatherDataCompletion = (Location?, DataManagerError?) -> ()

    private let baseURL: URL

    // MARK: - Initialization

    init(baseURL: URL) {
        self.baseURL = baseURL
    }

    // MARK: - Requesting Data

    func weatherDataForLocation(latitude: Double, longitude: Double, completion: @escaping WeatherDataCompletion) {
        // Create URL
        let URL = baseURL.appendingPathComponent("\(latitude),\(longitude)")

        // Create Data Task
        URLSession.shared.dataTask(with: URL) { (data, response, error) in
            self.didFetchWeatherData(data: data, response: response, error: error, completion: completion)
            }.resume()
    }

    // MARK: - Helper Methods

    private func didFetchWeatherData(data: Data?, response: URLResponse?, error: Error?, completion: WeatherDataCompletion) {
        if let _ = error {
            completion(nil, .FailedRequest)

        } else if let data = data, let response = response as? HTTPURLResponse {
            if response.statusCode == 200 {
                processWeatherData(data: data, completion: completion)
            } else {
                completion(nil, .FailedRequest)
            }

        } else {
            completion(nil, .Unknown)
        }
    }

    private func processWeatherData(data: Data, completion: WeatherDataCompletion) {
        if let JSON = try? JSONSerialization.jsonObject(with: data, options: []) {
            completion(Location(JSON: JSON), nil)
        } else {
            completion(nil, .InvalidResponse)
        }
    }

}