Q:

default value for collectionView:layout:referenceSizeForFooterInSection

What value do I have to return from collectionView:layout:referenceSizeForFooterInSection if I want everything to work like before.
Background: I want to disable the footer for some sections, so I am returning CGSize.zero for those, but I don't know what to return for the ones that should be shown

A:

From function's documentation  

If you do not implement this method, the flow layout uses the value in
  its  footerReferenceSize property to set the size of the footer.

So simply return footerReferenceSize from you flow layout, like so  
if let collectionViewFlowLayout = collectionView.collectionViewLayout as? UICollectionViewFlowLayout {

    return collectionViewFlowLayout.footerReferenceSize

}

