CollectionView Cells fit across screen the easy way

extension.swift

// Use this one and it pulls flowLayout info, even from storyboard

extension UICollectionViewFlowLayout {
    func cellsFitAcrossScreen(numberOfCells: Int, labelHeight: CGFloat, cellShapeRatio: CGFloat) -> CGSize {
        //using information from flowLayout get proper spacing for cells across entire screen
        let insideMargin = self.minimumInteritemSpacing
        let outsideMargins = self.sectionInset.left + self.sectionInset.right
        let numberOfDivisions: Int = numberOfCells - 1
        let subtractionForMargins: CGFloat = insideMargin * CGFloat(numberOfDivisions) + outsideMargins
        
        let fittedWidth = (UIScreen.mainScreen().bounds.width - subtractionForMargins) / CGFloat(numberOfCells)
        let ratioHeight = fittedWidth * cellShapeRatio
        
        return CGSize(width: fittedWidth, height: ratioHeight + labelHeight)
    }
}

// use this one when you dont have flow layout for whatever reason

static func cellsFitAcrossScreen(numberOfCells: Int, labelHeight: CGFloat, itemSpacing: CGFloat, sectionInsetLeft: CGFloat, sectionInsetRight: CGFloat) -> CGSize {
        //using hardwired info get proper spacing for cells across entire screen
        let insideMargin = itemSpacing
        let outsideMargins = sectionInsetLeft + sectionInsetRight
        let numberOfDivisions: Int = numberOfCells - 1
        let subtractionForMargins: CGFloat = insideMargin * CGFloat(numberOfDivisions) + outsideMargins
        
        let fittedWidth = (UIScreen.mainScreen().bounds.width - subtractionForMargins) / CGFloat(numberOfCells)
        return CGSize(width: fittedWidth, height: fittedWidth + labelHeight)
    }