// Timer function
    func timerTextToggle(timer: NSTimer) {
    if self.animationOngoing == false {
        self.flipAnimation()
    }
}

// Animation function
func flipAnimation() {
// important note: it's UIViewAnimationOptions,
// not UIViewAnimationTransition
    self.animationOngoing = true
    if self.animationToggle == false {
        UIView.transitionFromView(self.singleTapLabel!,
            toView: self.doubleTapLabel!,
            duration: animDuration,
            options: UIViewAnimationOptions.TransitionFlipFromBottom,
            completion: {
                (value: Bool) in
                self.animationOngoing = false
        })
    } else {
        UIView.transitionFromView(self.doubleTapLabel!,
            toView: self.singleTapLabel!,
            duration: animDuration,
            options: UIViewAnimationOptions.TransitionFlipFromTop,
            completion: {
                (value: Bool) in
                self.animationOngoing = false
        })
    }
    self.animationToggle = !self.animationToggle
}