Switch root view controller

[Swift] Switch-RootViewController

func switchRootViewController(rootViewController: UIViewController, animated: Bool, completion: (() -> Void)?) {
        if animated {
            UIView.transitionWithView(window, duration: 0.5, options: .TransitionCrossDissolve, animations: {
                let oldState: Bool = UIView.areAnimationsEnabled()
                UIView.setAnimationsEnabled(false)
                self.window!.rootViewController = rootViewController
                UIView.setAnimationsEnabled(oldState)
            }, completion: { (finished: Bool) -> () in
                if completion {
                    completion!()
                }
            })
        } else {
            window!.rootViewController = rootViewController
        }
    }