How to mock DataTaskPublisher? - swift

Question

I'm trying to write some unit tests for my API using URLSession.DataTaskPublisher. I've found an already existing question on Stackoverflow for the same but I'm struggling to implement a working class using the proposed solution.
Here's the existing question: How to mock URLSession.DataTaskPublisher
protocol APIDataTaskPublisher {
func dataTaskPublisher(for request: URLRequest) -> URLSession.DataTaskPublisher
}
class APISessionDataTaskPublisher: APIDataTaskPublisher {
func dataTaskPublisher(for request: URLRequest) -> URLSession.DataTaskPublisher {
return session.dataTaskPublisher(for: request)
}
var session: URLSession
init(session: URLSession = URLSession.shared) {
self.session = session
}
}
class URLSessionMock: APIDataTaskPublisher {
func dataTaskPublisher(for request: URLRequest) -> URLSession.DataTaskPublisher {
// How can I return a mocked URLSession.DataTaskPublisher here?
}
}
My API then uses the above like this:
class MyAPI {
/// Shared URL session
private let urlSession: APIDataTaskPublisher
init(urlSession: APIDataTaskPublisher = APISessionDataTaskPublisher(session: URLSession.shared)) {
self.urlSession = urlSession
}
}
What I don't know is how to implement URLSessionMock.dataTaskPublisher().

Answer 1

It would probably be simpler not to mock DataTaskPublisher. Do you really care if the publisher is a DataTaskPublisher? Probably not. What you probably care about is getting the same Output and Failure types as DataTaskPublisher. So change your API to only specify that:
protocol APIProvider {
typealias APIResponse = URLSession.DataTaskPublisher.Output
func apiResponse(for request: URLRequest) -> AnyPublisher<APIResponse, URLError>
}
Conform URLSession to it for production use:
extension URLSession: APIProvider {
func apiResponse(for request: URLRequest) -> AnyPublisher<APIResponse, URLError> {
return dataTaskPublisher(for: request).eraseToAnyPublisher()
}
}
And then your mock can create the publisher in any way that's convenient. For example:
struct MockAPIProvider: APIProvider {
func apiResponse(for request: URLRequest) -> AnyPublisher<APIResponse, URLError> {
let response = HTTPURLResponse(url: request.url!, statusCode: 200, httpVersion: "HTTP/1.1", headerFields: nil)!
let data = "Hello, world!".data(using: .utf8)!
return Just((data: data, response: response))
.setFailureType(to: URLError.self)
.eraseToAnyPublisher()
}
}

Answer 2

If you store in UT bundle stub JSON (XML, or something) for every API call that you want to test then the simplest mocking code might look as following
class URLSessionMock: APIDataTaskPublisher {
func dataTaskPublisher(for request: URLRequest) -> URLSession.DataTaskPublisher {
// here might be created a map of API URLs to cached stub replies
let stubReply = request.url?.lastPathComponent ?? "stub_error"
return URLSession.shared.dataTaskPublisher(for: Bundle(for: type(of: self)).url(forResource: stubReply, withExtension: "json")!)
}
}
so instead call to network server your publisher is created with URL of locally stored resource with known data, so you can verify all your workflow.

Answer 3

I will develop the step from having a simple Get request, to mocking .dataTaskPublisher for Combine and for the last part, testing the call. It is a ready to use code, for everyone in case someone else would need it.
Follow the comment to add your model or anything that depends on your project data.
So this is the protocol that give the rules to my NetworkRequest class:
protocol NetworkRequestProtocol {
associatedtype Resource
var resourceURL: NetworkEndpoint { get set }
var resourceSession: URLSession { get set }
func download() -> AnyPublisher<Resource, NetworkError>
}
There is some custom class, NetworkEndpoint and NetworkError, you can add your own here if you want or use URL and URLError instead:
enum NetworkEndpoint {
static let baseURL = URL(string: "API_BASE_URL")! // Add your api base url here
case live
var url: URL {
switch self {
case .live:
return NetworkEndpoint.baseURL!.appendingPathComponent("END_OR_YOUR_API_URL") // Add the end of your API url here
}
}
}
enum NetworkError: LocalizedError {
case addressUnreachable(URL)
case invalidResponse
var errorDescription: String? {
switch self {
case .invalidResponse:
return "The server response is invalid."
case .addressUnreachable(let url):
return "\(url.absoluteString) is unreachable."
}
}
}
Now, I am creating the NetworkRequest class to handle the API call. RessourceSession initializer is used for the UnitTest part only:
final class NetworkRequest<Resource> where Resource: Codable {
var resourceURL: NetworkEndpoint
var resourceSession: URLSession
init(_ resourceURL: NetworkEndpoint,
resourceSession: URLSession = URLSession(configuration: .default)) {
self.resourceURL = resourceURL
self.resourceSession = resourceSession
}
// MARK: - Dispatch Queues
let downloadQueue = DispatchQueue(
label: "downloadQueue", qos: .userInitiated,
attributes: .concurrent, autoreleaseFrequency: .inherit, target: .main)
}
// MARK: - Network Requests
extension NetworkRequest: NetworkRequestProtocol {
func download() -> AnyPublisher<Resource, NetworkError> {
resourceSession
.dataTaskPublisher(for: resourceURL.url)
.receive(on: downloadQueue)
.map(\.data)
.decode(type: Resource.self, decoder: JSONDecoder())
.mapError { error -> NetworkError in
switch error {
case is URLError:
return .addressUnreachable(self.resourceURL.url)
default:
return .invalidResponse }}
.eraseToAnyPublisher()
}
}
For the production code, this is an example of use of the NetworkRequest class, and of course, your model must be Codable:
var subscriptions = Set<AnyCancellable>()
func downloadData() {
NetworkRequest<YOUR_MODEL_NAME>(.live).download() // Add your model name inside the brackets
.sink(
receiveCompletion: { completion in
switch completion {
case .failure(let error):
print(error)
case .finished:
break }},
receiveValue: { data in
print(data) })
.store(in: &subscriptions)
}
So now that all the code is setup in the project, we can pass to the UnitTest part and start mocking URLSession:
class MockURLSession: URLSession {
var data: Data?
var response: URLResponse?
var error: Error?
init(data: Data?, response: URLResponse?, error: Error?) {
self.data = data
self.response = response
self.error = error
}
override func dataTask(with request: URLRequest,
completionHandler: #escaping (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTask {
let data = self.data
let response = self.response
let error = self.error
return MockURLSessionDataTask {
completionHandler(data, response, error)
}
}
}
Now, we mock URLSessionDataTask that we return when overriding dataTask in MockURLSession, and it will work for .dataTaskPublisher:
class MockURLSessionDataTask: URLSessionDataTask {
private let closure: () -> Void
init(closure: #escaping () -> Void) {
self.closure = closure
}
override func resume() {
closure()
}
}
We create fake response data to pass into our tests, but you must create a .json file with your data in it to fetch them in the tests:
class FakeResponseData {
static let response200OK = HTTPURLResponse(url: URL(string: "https://test.com")!,
statusCode: 200,
httpVersion: nil,
headerFields: nil)!
static let responseKO = HTTPURLResponse(url: URL(string: "https://test.com")!,
statusCode: 500,
httpVersion: nil,
headerFields: nil)!
class RessourceError: Error {}
static let error = RessourceError()
static var correctData: Data {
let bundle = Bundle(for: FakeResponseData.self)
let fakeJsonURL = bundle.url(forResource: "FAKE_JSON_FILE_NAME", withExtension: "json") // Add your fake json file name in here
let fakeJsonData = try! Data(contentsOf: fakeJsonURL!)
return fakeJsonData
}
static let incorrectData = "error".data(using: .utf8)!
}
And to finish, this is the part where you test your NetworkRequest, with the fake data coming from the .json file, or the error. You use resourceSession initializer to add your MockURLSession here and avoid making real network call:
class NetworkRequestTests: XCTestCase {
var expectation: XCTestExpectation!
var subscriptions: Set<AnyCancellable>!
override func setUpWithError() throws {
try super.setUpWithError()
expectation = XCTestExpectation(description: "wait for queue change")
subscriptions = Set<AnyCancellable>()
}
override func tearDownWithError() throws {
subscriptions = nil
expectation = nil
try super.tearDownWithError()
}
func testNetworkRequest_mockURLSessionAddCorrectDataResponse_returnRatesDataModelValues() throws {
let expectedTestValue = "test" // This value is set in your .json fake data for testing
// This is where you use resourceSession to pass your fake data
let networkRequest = NetworkRequest<RatesData>(.live, resourceSession:
MockURLSession(data: FakeResponseData.correctData,
response: FakeResponseData.response200OK,
error: nil))
networkRequest.download()
.sink(
receiveCompletion: { completion in
self.expectation.fulfill() },
receiveValue: { value in
XCTAssertEqual(expectedTimestamp, value.InFakeJson) // Compare with your fake json file
})
.store(in: &subscriptions)
wait(for: [expectation], timeout: 0.1)
}
func testNetworkRequest_mockURLSessionAddServerErrorAsResponse_returnNetworkErrorInvalidResponse() throws {
let expectedNetworkError = NetworkError.invalidResponse.localizedDescription
// This is where you use resourceSession to pass your fake data
let networkRequest = NetworkRequest<RatesData>(.live, resourceSession:
MockURLSession(data: nil,
response: FakeResponseData.responseKO,
error: nil))
networkRequest.download()
.sink(
receiveCompletion: { completion in
switch completion {
case .failure(let error):
XCTAssertEqual(expectedNetworkError, error.localizedDescription)
case .finished:
break
}
self.expectation.fulfill() },
receiveValue: { value in
XCTAssertNil(value)
})
.store(in: &subscriptions)
wait(for: [expectation], timeout: 0.1)
}
}

Answer 4

Answered on original question, but will repost here:
Since DataTaskPublisher uses the URLSession it is created from, you can just mock that. I ended up creating a URLSession subclass, overriding dataTask(...) to return a URLSessionDataTask subclass, which I fed with the data/response/error I needed...
class URLSessionDataTaskMock: URLSessionDataTask {
private let closure: () -> Void
init(closure: #escaping () -> Void) {
self.closure = closure
}
override func resume() {
closure()
}
}
class URLSessionMock: URLSession {
var data: Data?
var response: URLResponse?
var error: Error?
override func dataTask(with request: URLRequest, completionHandler: #escaping (Data?, URLResponse?, Error?) -> Void) -> URLSessionDataTask {
let data = self.data
let response = self.response
let error = self.error
return URLSessionDataTaskMock {
completionHandler(data, response, error)
}
}
}
Then obviously you just want your networking layer using this URLSession, I went with a factory to do this:
protocol DataTaskPublisherFactory {
func make(for request: URLRequest) -> URLSession.DataTaskPublisher
}
Then in your network layer:
func performRequest<ResponseType>(_ request: URLRequest) -> AnyPublisher<ResponseType, APIError> where ResponseType : Decodable {
Just(request)
.flatMap {
self.dataTaskPublisherFactory.make(for: $0)
.mapError { APIError.urlError($0)} } }
.eraseToAnyPublisher()
}
Now you can just pass a mock factory in the test using the URLSession subclass (this one asserts URLErrors are mapped to a custom error, but you could also assert some other condition given data/response):
func test_performRequest_URLSessionDataTaskThrowsError_throwsAPIError() {
let session = URLSessionMock()
session.error = TestError.test
let dataTaskPublisherFactory = mock(DataTaskPublisherFactory.self)
given(dataTaskPublisherFactory.make(for: any())) ~> {
session.dataTaskPublisher(for: $0)
}
let api = API(dataTaskPublisherFactory: dataTaskPublisherFactory)
let publisher: AnyPublisher<TestCodable, APIError> =
api.performRequest(URLRequest(url: URL(string: "www.someURL.com")!))
let _ = publisher.sink(receiveCompletion: {
switch $0 {
case .failure(let error):
XCTAssertEqual(error, APIError.urlError(URLError(_nsError: NSError(domain: "NSURLErrorDomain", code: -1, userInfo: nil))))
case .finished:
XCTFail()
}
}) { _ in }
}
The one issue with this is that URLSession init() is deprecated from iOS 13, so you have to live with a warning in your test. If anyone can see a way around that I'd greatly appreciate it.
(Note: I'm using Mockingbird for mocks).