Called when the view controller's view needs to update its constraints.
Override this method to optimize changes to your constraints.
Note
It is almost always cleaner and easier to update a constraint immediately after the affecting change has occurred. For example, if you want to change a constraint in response to a button tap, make that change directly in the button’s action method.
You should only override this method when changing constraints in place is too slow, or when a view is producing a number of redundant changes.
To schedule a change, call setNeedsUpdateConstraints on the view. The system then calls your implementation of updateViewConstraintsbefore 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.
Do not call setNeedsUpdateConstraints inside your implementation. Calling setNeedsUpdateConstraints schedules another update pass, creating a feedback loop.
Important
Call [super updateViewConstraints] as the final step in your implementation.
