UIScrollView custom paging size

Question

paging in UIScrollView is a great feature, what I need here is to set the paging to a smaller distance, for example I want my UIScrollView to page less size that the UIScrol

Answer 1

Swift 4.1 solution that simplifies reusing:

/// Protocol that simplifies custom page size configuration for UIScrollView.
/// Sadly, can not be done better due to protocol extensions limitations - https://stackoverflow.com/questions/39487168/non-objc-method-does-not-satisfy-optional-requirement-of-objc-protocol
/// - note: Set `.decelerationRate` to `UIScrollViewDecelerationRateFast` for a fancy scrolling animation.
protocol ScrollViewCustomHorizontalPageSize: UIScrollViewDelegate {
    /// Custom page size
    var pageSize: CGFloat { get }

    /// Helper method to get current page fraction
    func getCurrentPage(scrollView: UIScrollView) -> CGFloat

    /// Helper method to get targetContentOffset. Usage:
    ///
    ///     func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer) {
    ///         targetContentOffset.pointee.x = getTargetContentOffset(scrollView: scrollView, velocity: velocity)
    ///     }
    func getTargetContentOffset(scrollView: UIScrollView, velocity: CGPoint) -> CGFloat

    /// Must be implemented. See `getTargetContentOffset` for more info.
    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer)
}

extension ScrollViewCustomHorizontalPageSize {
    func getCurrentPage(scrollView: UIScrollView) -> CGFloat {
        return (scrollView.contentOffset.x + scrollView.contentInset.left) / pageSize
    }

    func getTargetContentOffset(scrollView: UIScrollView, velocity: CGPoint) -> CGFloat {
        let targetX: CGFloat = scrollView.contentOffset.x + velocity.x * 60.0

        var targetIndex = (targetX + scrollView.contentInset.left) / pageSize
        let maxOffsetX = scrollView.contentSize.width - scrollView.bounds.width + scrollView.contentInset.right
        let maxIndex = (maxOffsetX + scrollView.contentInset.left) / pageSize
        if velocity.x > 0 {
            targetIndex = ceil(targetIndex)
        } else if velocity.x < 0 {
            targetIndex = floor(targetIndex)
        } else {
            let (maxFloorIndex, lastInterval) = modf(maxIndex)
            if targetIndex > maxFloorIndex {
                if targetIndex >= lastInterval / 2 + maxFloorIndex {
                    targetIndex = maxIndex
                } else {
                    targetIndex = maxFloorIndex
                }
            } else {
                targetIndex = round(targetIndex)
            }
        }

        if targetIndex < 0 {
            targetIndex = 0
        }

        var offsetX = targetIndex * pageSize - scrollView.contentInset.left
        offsetX = min(offsetX, maxOffsetX)

        return offsetX
    }
}

Just conform to ScrollViewCustomPageSize protocol in your UIScrollView/UITableView/UICollectionView delegate and you are done, e.g.:

extension MyCollectionViewController: ScrollViewCustomPageSize {
    var pageSize: CGFloat {
        return 200
    }

    func scrollViewWillEndDragging(_ scrollView: UIScrollView, withVelocity velocity: CGPoint, targetContentOffset: UnsafeMutablePointer) {
        targetContentOffset.pointee.x = getTargetContentOffset(scrollView: scrollView, velocity: velocity)
    }
}

For a fancy scrolling I also recommend to set collectionView.decelerationRate = UIScrollViewDecelerationRateFast