Swift5-SampleOldStyleFetchData.swift

func fetchDataFromServer(_ urlString: String, completion: @escaping (Data?, String?) -> ()) {
    // Check if URL valid
    guard let url = URL(string: urlString) else {
        completion(nil, "Invalid URL")
        return
    }

    let defaultSession = URLSession(configuration: URLSessionConfiguration.default)
    let dataTask : URLSessionTask = defaultSession.dataTask(with: url) { data, response, error in
        guard error == nil else {
            completion(nil, error!.localizedDescription)
            return
        }

        guard let data = data else {
            completion(nil, "Get data failed")
            return
        }

        completion(data, nil)
    }

    dataTask.resume()
}

fetchDataFromServer("https://samples.openweathermap.org/data/2.5/weather?q=London,uk&appid=b6907d289e10d714a6e88b30761fae22") { (data, error) in
    if let error = error {
        print("Get data failed: \(error)")
        return
    }

    if let data = data {
        print("Data from server = \(String(data: data, encoding: .utf8) ?? "Convert data failed")")
    }
}