a.swift

    func animateLabels() {
        self.view.layoutIfNeeded()
        UIView.animateWithDuration(3.0, delay: 1, options: .CurveEaseIn,
            animations: {
                println(self.prevConstraint.constant)
                self.prevConstraint.constant += 50.0;
                self.view.layoutIfNeeded()
        }, completion: nil)
    }