Sticky Header CollectionViewFlowLayout

Sticky Header CollectionViewFlowLayout

class NonBlinkyCollectionViewFlowLayout: UICollectionViewFlowLayout {
    
    override init() {
        super.init()
        minimumInteritemSpacing = 0.0
        minimumLineSpacing = 0.0
        sectionInset = UIEdgeInsetsZero
        scrollDirection = .Vertical
    }
    
    var finalAttributes = [UICollectionViewLayoutAttributes]()
    
    required init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    override func initialLayoutAttributesForAppearingItemAtIndexPath(itemIndexPath: NSIndexPath) -> UICollectionViewLayoutAttributes? {
        let attributes = super.initialLayoutAttributesForAppearingItemAtIndexPath(itemIndexPath)
        attributes?.alpha = 1.0
        return attributes
    }
    
    override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        
        let superAttributes:NSMutableArray = NSMutableArray(array: super.layoutAttributesForElementsInRect(rect)!) as NSMutableArray
        
        let contentOffset = collectionView!.contentOffset
        let missingSections = NSMutableIndexSet()
        
        for layoutAttributes in superAttributes {
            if (layoutAttributes.representedElementCategory == .Cell) && layoutAttributes.indexPath != nil  {
                missingSections.addIndex(layoutAttributes.indexPath!.section)
            }
        }
        
        for layoutAttributes in superAttributes {
            if layoutAttributes.representedElementKind == UICollectionElementKindSectionHeader {
                if let indexPath = layoutAttributes.indexPath {
                    missingSections.removeIndex(indexPath.section)
                }
            }
        }
        
        missingSections.enumerateIndexesUsingBlock { idx, stop in
            let indexPath = NSIndexPath(forItem: 0, inSection: idx)
            let layoutAttributes = self.layoutAttributesForSupplementaryViewOfKind(UICollectionElementKindSectionHeader, atIndexPath: indexPath)
            superAttributes.addObject(layoutAttributes!)
        }
        
        for la in superAttributes {
            
            let layoutAttributes = la as! UICollectionViewLayoutAttributes;
            
            if let representedElementKind = layoutAttributes.representedElementKind {
                if representedElementKind == UICollectionElementKindSectionHeader {
                    let section = layoutAttributes.indexPath.section
                    let numberOfItemsInSection = collectionView!.numberOfItemsInSection(section)
                    
                    let firstCellIndexPath = NSIndexPath(forItem: 0, inSection: section)
                    let lastCellIndexPath = NSIndexPath(forItem: max(0, (numberOfItemsInSection - 1)), inSection: section)
                    
                    var firstCellAttributes:UICollectionViewLayoutAttributes
                    var lastCellAttributes:UICollectionViewLayoutAttributes
                    
                    if (self.collectionView!.numberOfItemsInSection(section) > 0) {
                        firstCellAttributes = self.layoutAttributesForItemAtIndexPath(firstCellIndexPath)!
                        lastCellAttributes = self.layoutAttributesForItemAtIndexPath(lastCellIndexPath)!
                    } else {
                        firstCellAttributes = self.layoutAttributesForSupplementaryViewOfKind(UICollectionElementKindSectionHeader, atIndexPath: firstCellIndexPath)!
                        if let  lastAttributes = self.layoutAttributesForSupplementaryViewOfKind(UICollectionElementKindSectionFooter, atIndexPath: lastCellIndexPath) {
                            lastCellAttributes = lastAttributes
                        } else {
                            lastCellAttributes = firstCellAttributes
                        }
                    }
                    
                    let headerHeight = CGRectGetHeight(layoutAttributes.frame)
                    var origin = layoutAttributes.frame.origin
                    
                    origin.y = min(max(contentOffset.y, (CGRectGetMinY(firstCellAttributes.frame) - headerHeight)), (CGRectGetMaxY(lastCellAttributes.frame) - headerHeight))
                    ;
                    
                    layoutAttributes.zIndex = 1024;
                    layoutAttributes.frame = CGRect(origin: origin, size: layoutAttributes.frame.size)
                }
            }
        }
        
        return NSArray(array: superAttributes) as? [UICollectionViewLayoutAttributes]
    }
    
    override func shouldInvalidateLayoutForBoundsChange(newBounds: CGRect) -> Bool {
        return true
    }
}