func gestureRecognizer(_ gestureRecognizer: UIGestureRecognizer, 
         shouldRequireFailureOf otherGestureRecognizer: UIGestureRecognizer) -> Bool {
   // Don't recognize a single tap until a double-tap fails.
   if gestureRecognizer == self.tapGesture && 
          otherGestureRecognizer == self.doubleTapGesture {
      return true
   }
   return false
}