如何在一个函数中使用UISwipeGestureRecognizer的不同滑动手势方向？

Question

    func swipeForMouth(_ recognizer:UISwipeGestureRecognizer) {
    if recognizer.direction == .up{
        expression.mouth = expression.mouth.happierMouth()
    }
    else if recognizer.direction == .down{
        expression.mouth = expression.mouth.sadderMouth()
    }
}//I wanna use this func to draw a MouthView BUT it won't work

//One Practical way to achieve that:
   let happierGesture = UISwipeGestureRecognizer(target: self, action: #selector(FaceViewController.increaseHappiness))
             //There is a "increaseHappiness" func to do such stuff like "expression.mouth = expression.mouth.sadderMouth()"
   happierGesture.direction = .up
   faceView.addGestureRecognizer(happierGesture)
   let sadderGesture = UISwipeGestureRecognizer(target: self, action: #selector(self.decreaseHappiness))
   sadderGesture.direction = .down
   faceView.addGestureRecognizer(sadderGesture)

我可能会省略代码的一些细节，但我想知道为什么前一个代码不起作用？显然后者需要更多的代码才能实现这一目标。如何在一个函数中使用UISwipeGestureRecognizer的不同滑动手势方向修复前一个？