How to mock URLSession.DataTaskPublisher - ios

Question

How can I mock URLSession.DataTaskPublisher? I have a class Proxy that require to inject a URLSessionProtocol
protocol URLSessionProtocol {
func loadData(from url: URL) -> URLSession.DataTaskPublisher
}
class Proxy {
private let urlSession: URLSessionProtocol
init(urlSession: URLSessionProtocol) {
self.urlSession = urlSession
}
func get(url: URL) -> AnyPublisher<Data, ProxyError> {
// Using urlSession.loadData(from: url)
}
}
This code was originally used with the traditional version of URLSession with the completion handler. It was perfect since I could easily mock URLSession for testing like Sundell's solution here: Mocking in Swift.
Is it possible to do the same with the Combine Framework?

Answer 1

In the same way that you can inject a URLSessionProtocol to mock a concrete session, you can also inject a mocked Publisher. For example:
let mockPublisher = Just(MockData()).eraseToAnyPublisher()
However, depending on what you do with this publisher you might have to address some weirdnesses with Combine async publishers, see this post for additional discussion:
Why does Combine's receive(on:) operator swallow errors?

Answer 2

The best way to test your client is to use URLProtocol.
https://developer.apple.com/documentation/foundation/urlprotocol
You can intercept all your request before she performs the real request on the cloud, and so creates your expectation. Once you have done your expectation, she will be destroyed, so you never make real requests.
Tests more reliable, faster, and you got the control!
You got a little example here: https://www.hackingwithswift.com/articles/153/how-to-test-ios-networking-code-the-easy-way
But it's more powerful than just this, you can do everything you want like: Check your Events/Analytics...
I hope it'll help you!

Answer 3

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).