presenting a modal in viewdidappear with swift

Question

I'm new to swift and ios programming in general. I'm trying to display a modal view when my app first loads which it does. The problem I'm running into is that my modal keeps appearing over and over and over. Not sure where I'm going wrong.

BONUS QUESTION: Ultimately I'd like this to only happen the first time the user opens the app.

class ViewController: UIViewController {

    var introModalDidDisplay = false

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view, typically from a nib.
    }

    override func viewDidAppear(animated: Bool) {
        super.viewDidAppear(animated)
        showIntroModal()
    }

    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }

    func showIntroModal() {
        if (!introModalDidDisplay) {
            println(introModalDidDisplay)
            introModalDidDisplay = true
            let intro = self.storyboard?.instantiateViewControllerWithIdentifier("introModal") as IntroModalViewController
            intro.modalPresentationStyle = UIModalPresentationStyle.FormSheet
            self.presentViewController(intro, animated: true, completion: nil)
        }
    }
}

Answer 1

Found it. My "intro" class was extending ViewController rather than UIViewController...apparently that's bad. Thanks for the help! Sorry for the wild goose chase.