UICollectionViewCell全屏显示时，分页很简单，只要一句代码collectView?.isPagingEnabled = true就搞定了。

但如果cell之间有间隙，例如：10pixel，上面的代码就会导致cell滑动后不居中，此时通过scrollViewWillEndDragging回调来处理比较好。直接贴上Swift代码，如下：

override func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer) {

var item = ceil(scrollView.contentOffset.x/kScreenHeight)

print("scrollViewWillEndDragging  \(velocity)  \(targetContentOffset.pointee)")

if abs(velocity.x) > 1.0 || scrollView.contentOffset.x.truncatingRemainder(dividingBy: kScreenHeight) >= kScreenHeight/2 {

targetContentOffset.deinitialize()

var index = scrollView.contentOffset.x/kScreenHeight + 1

if velocity.x < 0 {

index = scrollView.contentOffset.x/kScreenWidth

item = floor(scrollView.contentOffset.x/kScreenWidth)

}

targetContentOffset.pointee = CGPoint(x:floor(index) * kScreenWidth + 5 * item, y:0)

}else {

targetContentOffset.deinitialize()

targetContentOffset.pointee = CGPoint(x:floor(scrollView.contentOffset.x/kScreenWidth)*kScreenWidth + 5 * item, y:0)

}

}