WWDC2014 session 226, "What's new in Table and Collection Views", talks about self-sizing cells. These are collection view cells that the collection view will correctly size based the cell's own intrinsicContentSize, rather than by a size determined by the layout object or its delegate. You trigger this behavior by setting an estimatedItemSize.

The speaker, Olivier, shows an example of collection view cells that self-size based on their own Auto Layout constraints.

This project reproduces that example.

This project also shows that as of Xcode 6.3.2 iOS SDK 8.3, there is a bug in this feature. For certain values of estimatedItemSize, the collection view will simply fail to present some items provided by the data source.

In included sample project, toggle the boolean useMagicallyBrokenHeight. When the height of the estimatedItemSize is 20 or less, the collection view never renders the last item.