Write unit tests for ObservableObject ViewModels with Published results - swift

Question

Today again one combine problem I currently run in and I hope that someone of you can help. How can normal unit tests be written for ObservableObjects classes which contain #Published attributes? How can I subscribe in my test to them to get the result object which I can assert?
The injected mock for the web service works correctly, loadProducts() function set exactly the same elements from the mock in the fetchedProducts array.
But I don't know currently how to access this array in my test after it is filled by the function because it seems that I cannot work with expectations here, loadProducts() has no completion block.
The code looks like this:
class ProductsListViewModel: ObservableObject {
let getRequests: GetRequests
let urlService: ApiUrls
private let networkUtils: NetworkRequestUtils
let productsWillChange = ObservableObjectPublisher()
#Published var fetchedProducts = [ProductDTO]()
#Published var errorCodeLoadProducts: Int?
init(getRequestsHelper: GetRequests, urlServiceClass: ApiUrls = ApiUrls(), utilsNetwork: NetworkRequestUtils = NetworkRequestUtils()) {
getRequests = getRequestsHelper
urlService = urlServiceClass
networkUtils = utilsNetwork
}
// nor completion block in the function used
func loadProducts() {
let urlForRequest = urlService.loadProductsUrl()
getRequests.getJsonData(url: urlForRequest) { [weak self] (result: Result<[ProductDTO], Error>) in
self?.isLoading = false
switch result {
case .success(let productsArray):
// the products filled async here
self?.fetchedProducts = productsArray
self?.errorCodeLoadProducts = nil
case .failure(let error):
let errorCode = self?.networkUtils.errorCodeFrom(error: error)
self?.errorCodeLoadProducts = errorCode
print("error: \(error)")
}
}
}
}
The test I try to write looks like this at the moment:
import XCTest
#testable import MyProject
class ProductsListViewModelTest: XCTestCase {
var getRequestMock: GetRequests!
let requestManagerMock = RequestManagerMockLoadProducts()
var productListViewModel: ProductsListViewModel!
override func setUp() {
super.setUp()
getRequestMock = GetRequests(networkHelper: requestManagerMock)
productListViewModel = ProductsListViewModel(getRequestsHelper: getRequestMock)
}
func test_successLoadProducts() {
let loginDto = LoginResponseDTO(token: "token-token")
UserDefaults.standard.save(loginDto, forKey: CommonConstants.persistedLoginObject)
productListViewModel.loadProducts()
// TODO access the fetchedProducts here somehow and assert them
}
}
The Mock looks like this:
class RequestManagerMockLoadProducts: NetworkRequestManagerProtocol {
var isSuccess = true
func makeNetworkRequest<T>(urlRequestObject: URLRequest, completion: #escaping (Result<T, Error>) -> Void) where T : Decodable {
if isSuccess {
let successResultDto = returnedProductedArray() as! T
completion(.success(successResultDto))
} else {
let errorString = "Cannot create request object here"
let error = NSError(domain: ErrorDomainDescription.networkRequestDomain.rawValue, code: ErrorDomainCode.unexpectedResponseFromAPI.rawValue, userInfo: [NSLocalizedDescriptionKey: errorString])
completion(.failure(error))
}
}
func returnedProductedArray() -> [ProductDTO] {
let product1 = ProductDTO(idFromBackend: "product-1", name: "product-1", description: "product-description", price: 3.55, photo: nil)
let product2 = ProductDTO(idFromBackend: "product-2", name: "product-2", description: "product-description-2", price: 5.55, photo: nil)
let product3 = ProductDTO(idFromBackend: "product-3", name: "product-3", description: "product-description-3", price: 8.55, photo: nil)
return [product1, product2, product3]
}
}

Answer 1

Maybe this article can help you
Testing your Combine Publishers
To solve your issue I will use code from my article
typealias CompetionResult = (expectation: XCTestExpectation,
cancellable: AnyCancellable)
func expectValue<T: Publisher>(of publisher: T,
timeout: TimeInterval = 2,
file: StaticString = #file,
line: UInt = #line,
equals: [(T.Output) -> Bool])
-> CompetionResult {
let exp = expectation(description: "Correct values of " + String(describing: publisher))
var mutableEquals = equals
let cancellable = publisher
.sink(receiveCompletion: { _ in },
receiveValue: { value in
if mutableEquals.first?(value) ?? false {
_ = mutableEquals.remove(at: 0)
if mutableEquals.isEmpty {
exp.fulfill()
}
}
})
return (exp, cancellable)
}
your test needs to use this function
func test_successLoadProducts() {
let loginDto = LoginResponseDTO(token: "token-token")
UserDefaults.standard.save(loginDto, forKey: CommonConstants.persistedLoginObject)
/// The expectation here can be extended as needed
let exp = expectValue(of: productListViewModel .$fetchedProducts.eraseToAnyPublisher(), equals: [{ $0[0].idFromBackend == "product-1" }])
productListViewModel.loadProducts()
wait(for: [exp.expectation], timeout: 1)
}

Answer 2

The easy and clearest way for me is simply to test #published var after X seconds. An example bellow :
func test_successLoadProducts() {
let loginDto = LoginResponseDTO(token: "token-token")
UserDefaults.standard.save(loginDto, forKey: CommonConstants.persistedLoginObject)
productListViewModel.loadProducts()
// TODO access the fetchedProducts here somehow and assert them
let expectation = XCTestExpectation()
DispatchQueue.main.asyncAfter(deadline: .now() + 5) {
XCTAssertEqual(self.productListViewModel.fetchedProducts, ["Awaited values"])
expectation.fulfill()
}
wait(for: [expectation], timeout: 5.0)
}
I hope that helps !