Using UIPanGestureRecognizer to change images in UIImageView Swift 4 - swift

Question

I have loaded 31 images into a UIImageView and would like to use the UIPanGestureRecognizer to have the user simply pan up and down to change the images. The only example I could find is in Obj-C and can seem to figure out how to make it work. So far I have try to used the following code:
#IBAction func panPerformed(_ sender: UIPanGestureRecognizer) {
if sender.state == .began || sender.state == .changed {
let translation = sender.translation(in: self.view).y
//print (translation)
if translation > 0 { //swipe up
} else { //swipe down
var pos = Int(abs(translation / panThreshold))
if pos < chartImageArray.count {
print (pos)
chartView.image = chartImageArray[pos]
}
}
This does scroll through, but when you release it obviously go back to 0. Is there some better way to achieve this functionality?
I was try to figure out a way to track the transition number and somehow increase or decrease an index number that would correlate to the image array.
Thanks
Craig
EDIT: Here is the code I used to get the functionally
#objc func panPerformed(_ sender: UIPanGestureRecognizer) {
if sender.state == .began || sender.state == .changed {
let velocity = sender.velocity(in: self.view).y
var count = 0
if velocity > 0 {
count -= 1
} else {
count += 1
}
movePictures(count)
} else if sender.state == .ended { //release swipe
}
}
func movePictures(_ ind:Int) {
// print (index)
if index >= 0 {
index = index + ind
} else {
index = arrayCount * panThreshold
}
if index <= arrayCount * panThreshold {
index = index + ind
} else {
index = 0
}
}