How to unit test Task in Swift? - ios

Question

I'm writing a unit test for my view model layer which calls an async function inside a Task block. I've noticed the Task is executed on the background so I cannot control this and force it to return early for my unit test. I'd like to know a way to force the Task to run in a unit test or mock the Task so I can manually invoke the closure in my unit test.
For clarity Task - https://developer.apple.com/documentation/swift/task
I've provided some sample code below
protocol NetworkSession {
func data(for request: URLRequest) async throws -> (Data, URLResponse)
}
extension URLSession: NetworkSession {
func data(for request: URLRequest) async throws -> (Data, URLResponse) {
try await data(for: request, delegate: nil)
}
}
class ViewModel: ObservableObject {
#Published var errorMessage: String? = "Loading"
private let networkSession: NetworkSession
init(networkSession: NetworkSession = URLSession.shared) {
self.networkSession = networkSession
}
func onAppear() {
Task {
errorMessage = try await fetchErrorMessage()
}
}
#MainActor
private func fetchErrorMessage() async throws -> String? {
let urlRequest = URLRequest(url: URL(string: "https://api.api-ninjas.com/v1/randomword")!)
let (data, _) = try await networkSession.data(for: urlRequest)
let json = try JSONSerialization.jsonObject(with: data) as? [String: String]
return json?["error"]
}
}
Unit Tests
class MockNetworkSession: NetworkSession {
func data(for request: URLRequest) async throws -> (Data, URLResponse) {
let data = "Hello".data(using: .utf8)!
let response = HTTPURLResponse(url: URL(string: "www.help.com")!, statusCode: 200, httpVersion: nil, headerFields: [:])!
return (data, response)
}
}
class TaskTests: XCTestCase {
func testExample() throws {
let mockNetworkSession = MockNetworkSession()
let viewModel = ViewModel(networkSession: mockNetworkSession)
viewModel.onAppear()
XCTAssertEqual(viewModel.errorMessage, "Hello")
}
}
The unit test fails with the error
XCTAssertEqual failed: ("Optional("Loading")") is not equal to ("Optional("Hello")")
This is because I cannot invoke the Task manually, can someone please provide a way to mock the Task or inject it into the unit test so I get the test passing!