HeaderPagingLayout

HeaderPagingLayout.swift

class HeaderPagingLayout : UICollectionViewFlowLayout{
    let velocityThreshold : CGFloat = 0.1
    func proposedAttributes() -> [UICollectionViewLayoutAttributes]{
        if let proposedRect = self.collectionView?.bounds{
            return self.layoutAttributesForElementsInRect(proposedRect) as? [UICollectionViewLayoutAttributes] ?? []
        }
        return []
    }
    
    func findProposedHeaderAttribute() -> UICollectionViewLayoutAttributes?{
        for at in proposedAttributes(){
            if (at.representedElementKind ?? "") == UICollectionElementKindSectionHeader{
                return at
            }
        }
        return nil
    }
    
    override func targetContentOffsetForProposedContentOffset(proposedContentOffset: CGPoint, withScrollingVelocity velocity: CGPoint) -> CGPoint {
        if let at = findProposedHeaderAttribute(){
            let scrollLeft = self.collectionView?.contentOffset.x
            if(velocity.x < 0){
                return CGPoint.zeroPoint
            }
            return (scrollLeft > at.center.x) || (velocity.x > velocityThreshold)
                ? CGPoint(x: at.size.width, y: 0)
                : CGPoint.zeroPoint
        }

        return proposedContentOffset
    }
}