How to mock URL struct?

Question

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()
    }

}

Answer 1

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
}