UICollectionView layout change when frame size changes

Question

I'm working with a UICollectionView but I have a weird behaviour.

I want a horizontal scroll, small cell height ( smaller than the UICollectionView height). Tapping a button will increase even more the height of the collection.

The problem is that the layout of my cells is changing also. I want the collection view height to increase with the cells staying in the same position. Here attached 2 screen captures (before/after button touched). Here is my code:

var smallLayout:UICollectionViewFlowLayout = UICollectionViewFlowLayout();
smallLayout.scrollDirection = .Horizontal
var itemHeight = 100
smallLayout.itemSize = CGSizeMake(150, itemHeight);
var bottomDist:CGFloat = 1
var topDist = collHeight - itemHeight - bottomDist
smallLayout.sectionInset = UIEdgeInsets(top: topDist, left: 2, bottom: bottomDist, right: 2)
smallLayout.minimumInteritemSpacing = 15
smallLayout.minimumLineSpacing = 10

So, when touching the button I have the following code:

let viewHeight = CGRectGetHeight(self.view.frame)
let viewWidth = CGRectGetWidth(self.view.frame)
var collHeight:CGFloat = 600
var collYPosition = viewHeight-CGFloat(collHeight)-20
var rect = CGRectMake(0, CGFloat(collYPosition), CGFloat(viewWidth), CGFloat(collHeight))
dishCollectionView.frame = rect

So, I cannot understand why my layout is changing.

I want to have my cell at the bottom.

Answer 1

Perhaps if you reset the top of your sectionInset to accommodate for the additional height, i.e. an updated topDist calculation. Try adding something like this to you on touch code:

var bottomDist:CGFloat = 1
var topDist = collHeight - itemHeight - bottomDist
dishCollectionView.collectionViewLayout.sectionInset = UIEdgeInsets(top: topDist, left: 2, bottom: bottomDist, right: 2)
dishCollectionView.collectionViewLayout.invalidateLayout()