Collision detection of two UIViews in swift

I have two UIViews on my ViewController. I added a panGesture for the first view, and when I start moving this view, the second view will move to the first view. I want to detect an event when these two kinds collide. Here is my code.

@IBAction func dragFirstView(sender: UIPanGestureRecognizer) {

        let translation = sender.translationInView(self.view)

        dispatch_async(dispatch_get_main_queue()) { () -> Void in
            UIView.animateWithDuration(2.5, delay: 0.0, options: UIViewAnimationOptions.CurveEaseOut, animations: {
                self.secondView.frame = CGRectMake(sender.view!.center.x + translation.x, sender.view!.center.y + translation.y, self.secondView.frame.size.width, self.secondView.frame.size.height)
                }, completion: nil)
        }

        sender.view!.center = CGPoint(x: sender.view!.center.x + translation.x, y: sender.view!.center.y + translation.y)
        sender.setTranslation(CGPointZero, inView: self.view)
    }