Snaping CollectionViewFlowLayout like AppStore

SnapingCollectionViewFlowLayout.swift

/**
 Support snaping to the left as AppStore layout
 */
class SnapingCollectionViewFlowLayout: UICollectionViewFlowLayout {
    override func targetContentOffset(forProposedContentOffset proposedContentOffset: CGPoint,
                                      withScrollingVelocity velocity: CGPoint) -> CGPoint {
        guard let collectionView = collectionView else {
            return super.targetContentOffset(forProposedContentOffset: proposedContentOffset,
                                             withScrollingVelocity: velocity)
        }

        var offsetAdjustment = CGFloat.greatestFiniteMagnitude
        let horizontalOffset = proposedContentOffset.x + collectionView.contentInset.left

        let targetRect = CGRect(x: proposedContentOffset.x, y: 0, width: collectionView.bounds.size.width,
                                height: collectionView.bounds.size.height)

        let layoutAttributesArray = super.layoutAttributesForElements(in: targetRect)

        // Looking for the item that has most visible area (percentage) and scroll to that item
        let contentOffset = collectionView.contentOffset
        var selectedAttributes: UICollectionViewLayoutAttributes?
        var selectedAttributesPct: CGFloat?
        layoutAttributesArray?.forEach({ (layoutAttributes) in
            // Avoid zero divine
            if targetRect.width > 0 && layoutAttributes.frame.width > 0 {

                // Calculate the percentage of visible area (width)
                let intersectPct = layoutAttributes.frame.intersection(targetRect).width / layoutAttributes.frame.width

                // Then find the item that hast most visible area
                if selectedAttributesPct == nil || selectedAttributesPct! < intersectPct {

                    // We need to care about the velocity, if user is scrolling to the right, we will not want to snap
                    // to the left item, and vice versa.
                    if (velocity.x > 0 && layoutAttributes.frame.minX > contentOffset.x)
                        || (velocity.x < 0 && layoutAttributes.frame.minX < contentOffset.x)
                        || velocity.x == 0 {
                        selectedAttributes = layoutAttributes
                        selectedAttributesPct = intersectPct
                    }
                }
            }
        })

        // Snap to that item if found
        if let attributes = selectedAttributes {
            return CGPoint(x: attributes.frame.minX - sectionInset.left, y: proposedContentOffset.y)
        }

        // Fallback. In case above logic not cover enough
        return super.targetContentOffset(forProposedContentOffset: proposedContentOffset,
                                             withScrollingVelocity: velocity)
    }
}