why.swift

func gestureRecognizer(gestureRecognizer: UIGestureRecognizer, shouldBeRequiredToFailByGestureRecognizer otherGestureRecognizer: UIGestureRecognizer) -> Bool {
  if let swipeRec = gestureRecognizer as? UISwipeGestureRecognizer {
    if let panRec = otherGestureRecognizer as? UIPanGestureRecognizer {
      return true
    }
  } else if let panRec = gestureRecognizer as? UIPanGestureRecognizer {
    if let tapRec = otherGestureRecognizer as? UITapGestureRecognizer {
      return true
    }
  }
  return false
}