UICollectionView: must be initialized with a non-nil layout parameter

Question

I added UICollectionView by code.
Now, the app crash with message: UICollectionView must be initialized with a non-nil layout parameter.
Do

Answer 1

In Swift

var collectionView: UICollectionView!
let layout = UICollectionViewFlowLayout()
collectionView = UICollectionView(frame: .zero, collectionViewLayout: layout)
layout.scrollDirection = .vertical
view.addSubview(collectionView)