Called when the view controller's view needs to update its constraints.
- iOS 6.0+
- tvOS 9.0+
Override this method to optimize changes to your constraints.
To schedule a change, call
set on the view. The system then calls your implementation of
update before the layout occurs. This lets you verify that all necessary constraints for your content are in place at a time when your properties are not changing.
Your implementation must be as efficient as possible. Do not deactivate all your constraints, then reactivate the ones you need. Instead, your app must have some way of tracking your constraints, and validating them during each update pass. Only change items that need to be changed. During each update pass, you must ensure that you have the appropriate constraints for the app’s current state.