How do I set collection view's cell size via the auto layout

Question

Hi I\'m trying to resize the cell via the auto layout.

I want display the cell by the 3 by 3.

First Cell\'s margin left=0

Last Cell\'s m

Answer 1

Great answer by vacawama but I had 2 issues. I wanted the spacing that I defined in storyboard to automatically be used.

And secondly, I could not get this function to invoke and no one mentioned how? In order to invoke the collectionView's sizeForItemAt you need to extend UICollectionViewDelegateFlowLayout instead of extending UICollectionViewDelegate. I hope this saves someone else some time.

extension MyViewController: UICollectionViewDelegateFlowLayout, UICollectionViewDataSource {

    func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        let cellsAcross: CGFloat = 3
        var widthRemainingForCellContent = collectionView.bounds.width
        if let flowLayout = collectionViewLayout as? UICollectionViewFlowLayout {
            let borderSize: CGFloat = flowLayout.sectionInset.left + flowLayout.sectionInset.right
            widthRemainingForCellContent -= borderSize + ((cellsAcross - 1) * flowLayout.minimumInteritemSpacing)
        }
        let cellWidth = widthRemainingForCellContent / cellsAcross
        return CGSize(width: cellWidth, height: cellWidth)
    }

}