Center different height CollectionView items - ios

Question

I followed this tutorial: https://www.raywenderlich.com/164608/uicollectionview-custom-layout-tutorial-pinterest-2
and came up with the following code.
My question is now, how I can center the items of my collection view. Every item has a different height. (I have vertical scrolling enabled.)
class DynamicLayout: UICollectionViewFlowLayout {
var delegate: DynamicLayoutDelegate?
var numberOfColumns = 2
fileprivate var cellPadding: CGFloat = 6
fileprivate var contentHeight: CGFloat = 0
var xOffset = 0
fileprivate var contentWidth: CGFloat {
guard let c = collectionView else { return 0 }
return c.bounds.width - (c.contentInset.left + c.contentInset.right)
}
fileprivate var cache = [UICollectionViewLayoutAttributes]()
override func shouldInvalidateLayout(forBoundsChange newBounds: CGRect) -> Bool {
return true
}
override func prepare() {
guard cache.isEmpty == true, let collectionView = collectionView else { return }
// let columnWidth = contentWidth / CGFloat(numberOfColumns)
let columnWidth = CGFloat(310)
var xOffset = [CGFloat]()
for column in 0 ..< numberOfColumns {
xOffset.append(CGFloat(column) * columnWidth)
}
var column = 0
var yOffset = [CGFloat](repeating: 0, count: numberOfColumns)
// 3. Iterates through the list of items in the first section
for item in 0 ..< collectionView.numberOfItems(inSection: 0) {
let indexPath = IndexPath(item: item, section: 0)
// 4. Asks the delegate for the height of the picture and the annotation and calculates the cell frame.
let cellHeight = delegate?.collectionView(collectionView, heightFor: indexPath)
let height = cellPadding * 2 + cellHeight!
let frame = CGRect(x: xOffset[column], y: yOffset[column], width: columnWidth, height: height)
// let totalCellWidth = CGFloat(310 * numberOfColumns)
// let totalSpacingWidth = cellPadding * CGFloat(numberOfColumns - 1)
// let leftInset = (collectionView.frame.width - CGFloat(totalCellWidth + totalSpacingWidth)) / 2
// let leftInset = collectionView.frame.width - 310 / 2
let insetFrame = frame.insetBy(dx: cellPadding, dy: cellPadding)
// 5. Creates an UICollectionViewLayoutItem with the frame and add it to the cache
let attributes = UICollectionViewLayoutAttributes(forCellWith: indexPath)
attributes.frame = insetFrame
cache.append(attributes)
// 6. Updates the collection view content height
contentHeight = max(contentHeight, frame.maxY)
yOffset[column] = yOffset[column] + height
column = column < (numberOfColumns - 1) ? (column + 1) : 0
}
}
override func layoutAttributesForElements(in rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
var visibleLayoutAttributes = [UICollectionViewLayoutAttributes]()
// Loop through the cache and look for items in the rect
for attributes in cache {
if attributes.frame.intersects(rect) {
visibleLayoutAttributes.append(attributes)
}
}
return visibleLayoutAttributes
}
override func layoutAttributesForItem(at indexPath: IndexPath) -> UICollectionViewLayoutAttributes? {
return cache[indexPath.item] }
}
protocol DynamicLayoutDelegate: class {
func collectionView(_ collectionView:UICollectionView, heightFor indexPath:IndexPath) -> CGFloat
}
The problem is, that I don't want to have empty space below and above smaller cells.
Screenshot of the problem: