I have the following protocol that a NetworkEngine conforms to: protocol NetworkEngineProtocol { func request<T: Decodable>(from endpoint: Endpoint) async throws -> T } This protocol is passed as a dependency into a custom ViewModel and I want to unit test the request method of the ViewModel, that itself calls the dependency request method. class ViewModel: ViewModelProtocol { var passthrough: PassthroughSubject<[SomeProperty], Never> = PassthroughSubject<[SomeProperty], Never>() private let networkEngine: NetworkEngineProtocol init(networkEngine: NetworkEngineProtocol) { self.networkEngine = networkEngine } func request() { Task { do { let model = try await networkEngine.request(from: .messages()) as CustomModel // ... do more stuff passthrough.send(someValue) // would want to test this } catch let err as NetworkEngineErrors { // ... error handling } } } } struct CustomModel: Decodable { let message: [String] let status: String } In order to Unit Test this method, I'd have to create a Mock class, that conforms to the NetworkEngineProtocol. Something like: class MockNetworkProtocol: NetworkEngineProtocol { init() {} func request<T>(from endpoint: Endpoint) async throws -> T where T : Decodable { // how to mock this? } } What's the best way to do this? How can I mock the request method of the NetworkEngine? Thanks