Using size classes programmatically
I (hopefully) watched all the relevant WWDC2014 session videos and read the docs, so this question is mostly to confirm my suspicions, but please educate me.
What I want to do is animate views using Auto Layout. That in itself is not a problem. But these animations' endpoints change with different orientations. I thought I might be able to use size classes to move the views automatically on rotation, but Apple's developer guide says that animations have to be done programmatically, and from what I can gather, size classes are an Interface-Builder-only thing.
Another idea I had was using custom layout guides like the top/bottom ones IB provides, but those seem to be hardcoded.
The last thing I could do is update constraints by hand after listening to rotation events, but that is nothing new, and I feel like size classes should be useable for more than just static interfaces. Am I overestimating their purpose?
TLDR: Given two points A and B that a view can have its origin at (due to animations), how can I move both points using size classes or something similar?
Answer
After some more digging in the docs I have finally found something useable. The UIContentContainer protocol defines willTransitionToTraitCollection(:withTransitionCoordinator:), and that method's first parameter (a UITraitCollection) contains horizontal and vertical size classes as well as a UIUserInterfaceIdiom (that can be used to know whether the app is running on a iPhone or iPad, although size classes should be used for most things).
Additionally, since iOS 8 hides the status bar in landscape view, traitCollectionDidChange(previousTraitCollection:) is the corresponding method that gets called after the change happened, so the value of UIApplication.sharedApplication().statusBarHidden has changed when this method is called. Can be useful for UIScrollView's contentInset for example.
Lastly, if you need the exact screen sizes (in points, of course, but the above mentioned trait collection also knows about pixel density), there is viewWillTransitionToSize(:withTransitionCoordinator:).
Hope this helps someone else as well.