图片不会变成Alpha = 1

Question

当用户在滚动视图中滑动时尝试将thumbImage设置为alpha = 1，但是现在这取决于我在屏幕上开始滑动的位置。如果我长扫，则Alpha值= 1，短扫会使它更透明。我希望总是在显示scrollview（collectionView）中的新页面时始终为alpha = 1。我该如何更改？

 func scrollViewDidScroll(_ scrollView: UIScrollView) {
    let currentX =  scrollView.panGestureRecognizer.translation(in: scrollView.superview).x
    let currentXSign = Int(currentX).signum()
    if currentXSign != lastXSign &&
        currentXSign != 0{
    }

    if lastXSign < 0 {
        thumbImage.isHidden = false
        thumbImage.alpha = abs(currentX) / view.center.x
        thumbImage.image = #imageLiteral(resourceName: "thumbsdown")
        print("SCROLLING RIGHT")


    } else if lastXSign > 0 {
        thumbImage.isHidden = false
        thumbImage.alpha = abs(currentX) / view.center.x
        thumbImage.image = #imageLiteral(resourceName: "thumbsup")
        print("SCROLLING LEFT")
    }
    lastXSign = currentXSign
}