// we will create a shared instance of the NetworkHelper

static let shared = NetworkHelper()

private var session: URLSession

// we will make the default initializer private

// required in order to be considered a singleton

// also forbids anyone from creating an instance of NetworkHelper

private init() {

session = URLSession(configuration: .default)

}

func performDataTask(with urlString: String,

completion: @escaping (Result<Data, AppError>) -> ()) {

// creating a URL from the given String

guard let url = URL(string: urlString) else {

// handle bad url error case

completion(.failure(.badURL(urlString)))

return

}

// two states on dataTask, resume() and suspended by default

// suspended simply won't perform network request

// this ultimately leads to debugging errors and time lost if

// you don't explicitly resume() request

let dataTask = session.dataTask(with: url) { (data, response, error) in

// 1. deal with error if any

// check for client network errors

if let error = error {

completion(.failure(.networkClientError(error)))

}

// 2. downcast URLResponse (response) to HTTPURLResponse to

// get access to the statusCode property on HTTPURLResponse

guard let urlResponse = response as? HTTPURLResponse else {

completion(.failure(.noResponse))

return

}

// 3. unwrap the data object

guard let data = data else {

completion(.failure(.noData))

return

}

// 4. validate that the status code is in the 200 range otherwise it's a

// bad status code

switch urlResponse.statusCode {

case 200...299: break // everything went well here

default:

completion(.failure(.badStatusCode(urlResponse.statusCode)))

return

}

// 5. capture data as success case

completion(.success(data))

}

dataTask.resume()

}