Styjun

I am writing some code to be able to make testing easier. After researching, I found a good way of making URLSession testable is to make it conform to a protocol and use that protocol in the class I need to test. I applied the same method to URL. However, I now need to downcast the url parameter as a URL type. This seems a bit "dangerous" to me. What is the proper way to make URL testable? How can I also mock the URLSessionDataTask type returned by dataTask?

import Foundation

protocol URLSessionForApiRequest 
 func dataTask(with url: URL, completionHandler: @escaping (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTask


protocol URLForApiRequest 
 init?(string: String)


extension URLSession: URLSessionForApiRequest 

extension URL: URLForApiRequest 

class ApiRequest 
 class func makeRequest(url: URLForApiRequest, urlSession: URLSessionForApiRequest) 
 guard let url = url as? URL else return 
 let task = urlSession.dataTask(with: url) _, _, _ in print("DONE") 
 task.resume()
 

You want to make a protocol that wraps the function that you are going to call, then make a concrete implementation and mock implementation that returns whatever you initialize it with. Here is an example:

import UIKit
import PlaygroundSupport

protocol RequestProvider 
 func request(from: URL, completion: @escaping (Data?, URLResponse?, Error?) -> Void)


class ApiRequest: RequestProvider 
 func request(from url: URL, completion: @escaping (Data?, URLResponse?, Error?) -> Void) 
 URLSession.shared.dataTask(with: url, completionHandler: completion).resume
 


class MockApiRequest: RequestProvider 
 enum Response 
 case value(Data, URLResponse), error(Error)
 
 private let response: Response
 init(response: Response) 
 self.response = response
 
 func request(from url: URL, completion: @escaping (Data?, URLResponse?, Error?) -> Void) 
 switch response 
 case .value(let data, let response):
 completion(data, response, nil)
 case .error(let error):
 completion(nil, nil, error)
 
 


class SomeClassThatMakesAnAPIRequest 
 private let requestProvider: RequestProvider
 init(requestProvider: RequestProvider) 
 self.requestProvider = requestProvider
 
 //Use the requestProvider here and it uses either the Mock or the "real" API provider based don what you injected