-(void)testBadLogin {
    // enter username and password in UITextFields
    self.viewController.usernameField.text = @"test@test.com";
    self.viewController.passwordField.text = @"incorrect_pass";
    [loginButton sendActionsForControlEvents: UIControlEventTouchUpInside];

    // this isn't right
    XCTAssertNotNil([self.viewController alertController],@"alertController should appear"); 
}

如何检查UIAlertController是否已显示在当前视图之上？