Load UIViewController from the separate nib file in swift?

I had take one ViewController with separate nib file. and my initial root viewcontroller is set in the storyBoard. Now the problem is that when I push to this controller the View hireachy methods are not being called (ViewDidLoad , ViewWillApper , etc).. Code (View is loaded but methods are not calling) ``` var viewController = UIViewController(nibName: "OfferDetailViewController", bundle: nil) as OfferDetailViewController self.navigationController?.pushViewController(viewController, animated: true); ``` The same thing if i do with the storyboard its working fine. ``` let mainStoryboard: UIStoryboard = UIStoryboard(name: "Main", bundle: nil) var viewController = mainStoryboard.instantiateViewControllerWithIdentifier("offer") as OfferDetailViewController self.navigationController?.pushViewController(viewController, animated: true); ``` Problem : With storyboard View hierarchy methods are calling but not with the separate nib file?