As I am trying to learn TDD with iOS and Swift Development, I am following Pauls book Testing Swift, and tI have reached the part where he mocks networking and starts to implement test. I along side also am trying to do so but I am failing in the first test, could anybody point out why is my test failing.

The sample code in the book is testing dataTask for URLSession , but I am testing downloadTask . Here goes the sample code

The source which is responsible for downloading a file from network.

import Foundation

public enum DownloadError: Error {
    case responseCodeError
    case fileError
}

public class NetworkInteractor {
    var session: MockSession
    var url: URL

    public typealias DownloadCompletion = Result<URL, DownloadError>

    public convenience init? (urlString: String) {
        self.init(url: URL(string: urlString)!)
    }

    init(session: MockSession = URLSession.shared, url: URL) {
        self.session = session
        self.url = url
    }

    open func downloadFile(completion: @escaping(DownloadCompletion) -> Void) {

        session.downloadTask(with: url) { downloadeURL, response, error in
            if let _ = error {
                completion(.failure(.responseCodeError)) // replace with appropriate error
            }

            if let response = response as? HTTPURLResponse, response.statusCode != 200 {
                completion(.failure(.responseCodeError))
            }

            if let url = downloadeURL {
                completion(.success(url))
            }
        }.resume()
    }

}

The mock class,

  import Foundation

// MARK:- Mock to help download
protocol MockSession {
    func downloadTask(with url: URL, completionHandler: @escaping (URL?, URLResponse?, Error?) -> Void) -> URLSessionDownloadTask
}

class DataTaskMock: URLSessionDownloadTask {

    var completionHandler: (URL?, URLResponse?, Error?) -> Void?

    var resumeWasCalled: Bool = false

    init(completionHandler: @escaping(URL?, URLResponse?, Error?)-> Void) {
        self.completionHandler = completionHandler
    }

    override func resume() {
        resumeWasCalled = true
        completionHandler(nil, nil, nil)
    }
}

extension URLSession: MockSession {}

class URLSessionMock: MockSession {

    var lastURL: URL?
    var downloadTask: DataTaskMock?

    func downloadTask(with url: URL, completionHandler: @escaping (URL?, URLResponse?, Error?) -> Void) -> URLSessionDownloadTask {
        let newDataTask = DataTaskMock(completionHandler: completionHandler)
        lastURL = url
        downloadTask = newDataTask
        return newDataTask
    }

}

The test class,

class StorableTests: XCTestCase {

    public func testItLoadsURLCorrectly() {
        let url = URL(string: "https://a-url.com")!

        let session = URLSessionMock()

        let interactor = NetworkInteractor(session: session, url: url)

        let expectation = expectation(description: "Network was called")

        interactor.downloadFile { _ in
            XCTAssertEqual(URL(string: "https://a-url.com")!, session.lastURL)
            expectation.fulfill()
        }

        wait(for: [expectation], timeout: 5)
    }

}

The error I am receiving is Asynchronous wait failed: Exceeded timeout of 5 seconds, with unfulfilled expectations: "Network was called".. Please point me to the right direction.