Sample iOS Unit Tests: Working with a ViewController that presents a UIAlertController

Sample iOS Unit Tests: Working with a ViewController that presents a UIAlertController

func testControllerShowsAlertIfUserLogsInWithEmptyUsernameAndPasswordTextField() {
        
        // mock of LoginViewController
        class MockLoginViewController: LoginViewController {
            var presentViewControllerTarget: UIViewController?
            override func presentViewController(viewControllerToPresent: UIViewController, animated flag: Bool, completion: (() -> Void)?) {
                presentViewControllerTarget = viewControllerToPresent
            }
        }
    
        // instantiate MockLoginViewController and create TextFields
        let mockLoginViewController = MockLoginViewController()
        mockLoginViewController.userNameTextField = UITextField()
        mockLoginViewController.passwordTextField = UITextField()
        
        // TextFields have an empty state
        mockLoginViewController.userNameTextField.text = String()
        mockLoginViewController.passwordTextField.text = String()
        
        // Tap login button when TextFields have empty state
        mockLoginViewController.handleLogInTap(UIButton())
        
        // Assertions
        let alertController = mockLoginViewController.presentViewControllerTarget as! UIAlertController
        
        XCTAssertNotNil(alertController, "UIAlertController was not presented")
        
        let expectedAlertTitle = "Oops!"
        let actualAlertTitle = alertController.title
        XCTAssertEqual(expectedAlertTitle, actualAlertTitle)
        
        let expectedAlertMessage = "Please enter username and password."
        let actualAlertMessage = alertController.message
        XCTAssertEqual(expectedAlertMessage, actualAlertMessage)
        
        let expectedActionNumber = 1
        let actualActionNumber = alertController.actions.count
        XCTAssertEqual(expectedActionNumber, actualActionNumber)
        
        let expectedActionTitle = "Ok"
        let actualActionTitle = alertController.actions[0].title
        XCTAssertEqual(expectedActionTitle, actualActionTitle)
    }