UICollectionView sticky header in swift

The final solution I found:

Using this custom flow layout it was possible to fix this sticky header:

class StickyHeaderCollectionViewFlowLayout: UICollectionViewFlowLayout {

    override func layoutAttributesForElementsInRect(rect: CGRect) -> [AnyObject]? {

        var superAttributes: [UICollectionViewLayoutAttributes]? = super.layoutAttributesForElementsInRect(rect) as? [UICollectionViewLayoutAttributes]

        if superAttributes == nil {
            // If superAttributes couldn't cast, return
            return super.layoutAttributesForElementsInRect(rect)
        }

        let contentOffset = collectionView!.contentOffset
        var missingSections = NSMutableIndexSet()

        for layoutAttributes in superAttributes! {
            if (layoutAttributes.representedElementCategory == .Cell) {
                if let indexPath = layoutAttributes.indexPath {
                    missingSections.addIndex(layoutAttributes.indexPath.section)
                }
            }
        }

        for layoutAttributes in superAttributes! {
            if let representedElementKind = layoutAttributes.representedElementKind {
                if representedElementKind == UICollectionElementKindSectionHeader {
                    if let indexPath = layoutAttributes.indexPath {
                        missingSections.removeIndex(indexPath.section)
                    }
                }
            }
        }

        missingSections.enumerateIndexesUsingBlock { idx, stop in
            let indexPath = NSIndexPath(forItem: 0, inSection: idx)
            if let layoutAttributes = self.layoutAttributesForSupplementaryViewOfKind(UICollectionElementKindSectionHeader, atIndexPath: indexPath) {
                superAttributes!.append(layoutAttributes)
            }
        }

        for layoutAttributes in superAttributes! {
            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)!


                    let (firstCellAttributes: UICollectionViewLayoutAttributes, lastCellAttributes: UICollectionViewLayoutAttributes) = {
                        if (self.collectionView!.numberOfItemsInSection(section) > 0) {
                            return (
                                self.layoutAttributesForItemAtIndexPath(firstCellIndexPath),
                                self.layoutAttributesForItemAtIndexPath(lastCellIndexPath))
                        } else {
                            return (
                                self.layoutAttributesForSupplementaryViewOfKind(UICollectionElementKindSectionHeader, atIndexPath: firstCellIndexPath),
                                self.layoutAttributesForSupplementaryViewOfKind(UICollectionElementKindSectionFooter, atIndexPath: lastCellIndexPath))
                        }
                        }()

                    let headerHeight = CGRectGetHeight(layoutAttributes.frame)
                    var origin = layoutAttributes.frame.origin

                    origin.y = min(contentOffset.y, (CGRectGetMaxY(lastCellAttributes.frame) - headerHeight))
                    // Uncomment this line for normal behaviour:
                    // 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 superAttributes
    }

    override func shouldInvalidateLayoutForBoundsChange(newBounds: CGRect) -> Bool {
        return true
    }

}

To create a layout where the headers are sticky like traditional, change this line:

origin.y = min(contentOffset.y, (CGRectGetMaxY(lastCellAttrs.frame) - headerHeight))

to this line:

origin.y = min(max(contentOffset.y, (CGRectGetMinY(firstCellAttrs.frame) - headerHeight)), (CGRectGetMaxY(lastCellAttrs.frame) - headerHeight))

Hoping this is useful for others!

Update

Updated to fix a crash (thanks to Robert Atkins!) and some updates to Swift 1.2

tvOS & iOS 9

tvOS and iOS 9 introduced the property sectionHeadersPinToVisibleBounds which can be used

Simplest solution for iOS 9 + as it doesn't need of writing subclass of UICollectionViewFlowLayout.

In viewDidLoad of viewController with collectionView use following code:

let layout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout // casting is required because UICollectionViewLayout doesn't offer header pin. Its feature of UICollectionViewFlowLayout
layout?.sectionHeadersPinToVisibleBounds = true

It is hinted by @Antoine also.