viewWillTransitionToSize: vs willTransitionToTraitCollection:

Question

Can anyone explain the differences between these 2 methods? The docs for UIViewController explicitly state that viewWillTransitionToSize should be used for managing rotations, but clicking through to the UIContentContainer page, the willTransitionToTraitCollection method makes a confusing entrance.

I think I understand the conceptual difference between a size class change ( trait collection change ) and a size change, but I'm not sure which method to implement in which circumstances. Clarification from a UIKit wizard would be helpful!

Answer 1

Whenever you want to do something aa a response to a user rotating their device you should use viewWillTransitionToSize, if you do that you know for sure that your actions are executed as this is called every time your app's window changes size.

If you only want to take action when the trait collection changes, for instance if you have a certain collectionViewLayout set for a Compact size class and another you want to use for Regular you use willTransitionToTraitCollection.

If the trait collection changes then the size also changes. But it doesn't work the other way around. A portrait iPad and a landscape iPad have the same traits but are different sizes. Add multitasking to the mix and you have a whole variety of sizes that would map to just two traitCollection size classes.