How can I center rows in UICollectionView?

Question

I have a UICollectionView with random cells. Is there any method that allows me to center rows?

This is how it looks by default:

[ x x x x         


        

Answer 1

This can be achieved with a (relatively) simple custom layout, subclassed from UICollectionViewFlowLayout. Here's an example in Swift:

/**
 * A simple `UICollectionViewFlowLayout` subclass that would make sure the items are center-aligned in the collection view, when scrolling vertically.
 */
class UICollectionViewFlowCenterLayout: UICollectionViewFlowLayout {

    override func layoutAttributesForElementsInRect(rect: CGRect) -> [UICollectionViewLayoutAttributes]? {
        guard let suggestedAttributes = super.layoutAttributesForElementsInRect(rect) else { return nil }

        guard scrollDirection == .Vertical else { return suggestedAttributes }

        var newAttributes: [UICollectionViewLayoutAttributes] = []

        /// We will collect items for each row in this array
        var currentRowAttributes: [UICollectionViewLayoutAttributes] = []
        /// We will use this variable to detect new rows when iterating over items
        var yOffset:CGFloat = sectionInset.top
        for attributes in suggestedAttributes {
            /// If we happen to run into a new row...
            if attributes.frame.origin.y != yOffset {
                /*
                 * Update layout of all items in the previous row and add them to the resulting array
                 */
                centerSingleRowWithItemsAttributes(¤tRowAttributes, rect: rect)
                newAttributes += currentRowAttributes
                /*
                 * Reset the accumulated values for the new row
                 */
                currentRowAttributes = []
                yOffset = attributes.frame.origin.y
            }
            currentRowAttributes += [attributes]
        }
        /*
         * Update the layout of the last row.
         */
        centerSingleRowWithItemsAttributes(¤tRowAttributes, rect: rect)
        newAttributes += currentRowAttributes

        return newAttributes
    }

    /**
     Updates the attributes for items, so that they are center-aligned in the given rect.

     - parameter attributes: Attributes of the items
     - parameter rect:       Bounding rect
     */
    private func centerSingleRowWithItemsAttributes(inout attributes: [UICollectionViewLayoutAttributes], rect: CGRect) {
        guard let item = attributes.last else { return }

        let itemsCount = CGFloat(attributes.count)
        let sideInsets = rect.width - (item.frame.width * itemsCount) - (minimumInteritemSpacing * (itemsCount - 1))
        var leftOffset = sideInsets / 2

        for attribute in attributes {
            attribute.frame.origin.x = leftOffset
            leftOffset += attribute.frame.width + minimumInteritemSpacing
        }
    }
}