After further experimentation I found the order in which the underling methods are called. It seems that preferredLayoutAttributesFitting() has the final and definitive say about cell size. It knows the current cell layout attributes and the layout attributes the cell wants to have after resizing itself which then gets sent to the cells apply() method. How would one animate the difference between those two layout attributes though?

I have improved my question and since you can't edit or delete I reposted it. You can find it here: Improved Question