let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
layout.sectionInset = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
layout.itemSize = CGSize(width: 60, height: 60)
layout.estimatedItemSize = CGSize(width: 1, height: 1)
collectionView = UICollectionView(frame: self.view.frame, collectionViewLayout: layout)
let nib = UINib(nibName: "SimpleStackCollectionViewCell", bundle: nil);
collectionView.register(nib, forCellWithReuseIdentifier: "MyCell")
collectionView?.dataSource = self
collectionView?.delegate = self

func collectionView(_ collectionView: UICollectionView, layout collectionViewLayout: UICollectionViewLayout, sizeForItemAt indexPath: IndexPath) -> CGSize {
        return CGSize(width: view.bounds.width, height: 500)
    }

open var estimatedItemSize: CGSize // defaults to CGSizeZero - setting a non-zero size enables cells that self-size via -preferredLayoutAttributesFittingAttributes:

let layout: UICollectionViewFlowLayout = UICollectionViewFlowLayout()
layout.sectionInset = UIEdgeInsets(top: 0, left: 0, bottom: 0, right: 0)
layout.itemSize = CGSize(width: 60, height: 60)
collectionView = UICollectionView(frame: self.view.frame, collectionViewLayout: layout)
let nib = UINib(nibName: "SimpleStackCollectionViewCell", bundle: nil);
collectionView.register(nib, forCellWithReuseIdentifier: "MyCell")
collectionView?.dataSource = self
collectionView?.delegate = self