3grid.swift

    // MARK: UICollectionViewFlowDelegate methods
    
    func collectionView(collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAtIndexPath indexPath: NSIndexPath) -> CGSize {
        var borderWidth = COLUMN_NUMBER * BORDER_WIDTH
        var width:CGFloat = CGFloat((collectionView.frame.width - borderWidth) / COLUMN_NUMBER)
        return CGSize(width: width, height: width) // square
    }