updateConstraints()

Override this method to optimize changes to your constraints.

To schedule a change, set the view’s needsUpdateConstraints property to true. The system then calls your implementation of updateConstraints() before the layout occurs. This lets you verify that all necessary constraints for your content are in place at a time when your custom view’s 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 set the needsUpdateConstraints property inside your implementation. Setting needsUpdateConstraints to true schedules another update pass, creating a feedback loop.