What does the "optional" keyword mean in a function declaration?

Question

What does optional func myFunction() { .. } mean in the Apple API docs; for example this entry for UICollectionViewDelegate :

optional func collectionView(_ collectionView: UICollectionView,
didSelectItemAtIndexPath indexPath: NSIndexPath)

Answer 1

It means that classes adopting this protocol are not required to implement those methods in order to be conformant. Although not implementing any of these methods will not make for a good data source.