在测试目标中模拟服务时出错

如何解决在测试目标中模拟服务时出错

我正在尝试为测试目标编写测试用例。该应用程序运行良好，但测试用例因模拟而失败。 这是我的代码，如果有其他解决方案，请告诉我。我如何模拟服务？

还有其他方法可以为 ViewCoontroller 初始化程序编写测试用例吗？

    
class NavigationCodeTests: XCTestCase {
    var subject: ViewController?

    override func setUp() {
        self.subject = ViewController(service: MockUserService())
        _ = self.subject?.view
    }

    func test_user_service_not_nil() {
        XCTAssertNotNil(self.subject?.service,"User service can't be nil after initialization of ViewController")
    }

    func test_user_service_should_have_user() {
        self.subject?.userviewmodel?.user.observe(on: self,observerBlock: { (user) in
            XCTAssertNotNil(user?.name,"User service can't be nil after initialization of ViewController")
        })
    }
}

class MockUserService: UserService  {
    func fetch(_ completion: @escaping(_ user: User) -> Void) {
        completion(User(name: "abc",contact: "124"))
    }
}

class UserService: UserServiceDelegate  {
    func fetch(_ completion: @escaping(_ user: User) -> Void) {
        completion(User(name: "damu",contact: "12"))
    }
}

protocol UserServiceDelegate {
    func fetch(_ completion: @escaping(_ user: User) -> Void)
}