Mocking a class with generic parameter function inside Swift

Question

I am trying to unit test simple HttpClient behaviour. For that I have created an GenericHttpClientInterface protocol and concrete class GenericHttpClient that implements this protocol.

protocol GenericHttpClientInterface {
    func makeRequest(request: URLRequest) -> Observable
}

class GenericHttpClient: GenericHttpClientInterface {
    func makeRequest(request: URLRequest) -> Observable {
        return URLSession.shared.rx.data(request: request).jsonDecode(to: T.self)
    }
}

What I wanted to achieve is to mock that class :

class MockHttpClient: GenericHttpClientInterface {
    var invokedMakeRequestCount = 0
    var invokedMakeRequestParameters: (request: URLRequest, Void)?
    var stubbedMakeRequestResult: Observable!
    func makeRequest(request: URLRequest) -> Observable {
        invokedMakeRequestCount += 1
        invokedMakeRequestParameters = (request, ())
        return stubbedMakeRequestResult as! Observable;
    }
}

What gives ma a problem is that the method I am mocking has generic parameter T inside which is a class that the request will be decoded to. I don't know this parameter until I call this function so basically in MockHttpClient class for a property that stores stub data for makeRequest I've created:

stubbedMakeRequestResult: Observable

and after returning it I'm trying to cast that to result type Observable. This gives me an warning

Cast from 'Observable?' to unrelated type 'Observable' always fails

and in consequence

Thread 1: signal SIGABRT.

Any idea how to stub that data?

Example test that creates SIGABRT:

class GenericHttpTest: XCTestCase {
    
    var sut: Repository!
    var mockHttpClient: MockHttpClient!
    
    override func setUp() {
        mockHttpClient = MockHttpClient()
        sut = Repository(httpClient: mockHttpClient)
    }
        
    let test_mocked_data_stub = DataModelStruct(args: DataModelStruct.InsideModelStruct(foo1: "bar"))

    func test_should_return_mocked_data_from_mock_http_client() {
        mockHttpClient.stubbedMakeRequestResult = Observable.just(test_mocked_data_stub)
        
        let response = try! sut.getFooBar().toBlocking().first()

        XCTAssertEqual(response, test_mocked_data_stub)
    }
}

user_1974224 · Answer

Due to the fact that generics are invariant in Swift, Observable will never ever be convertible to Observable, unless T is Any. This is what cause your crash, as when you assign the value to stubbedMakeRequestResult, the concrete Observable is converted to Observable, and there's no turning point from here.

What you can do to avoid this is to make stubbedMakeRequestResult an Any, as this will not make any conversions behind the scenes. A small problem is that you loose the type safety and inference, but you can fix this stubbing via a function:

class MockHttpClient: GenericHttpClientInterface {
    ...
    var stubbedMakeRequestResult: Any!
    
    func stubMakeRequestResult(_ result: Observable) {
        stubbedMakeRequestResult = result
    }
    ...
}

Mocking a class with generic parameter function inside Swift

1 Answers