iOS change auto layout constraints when device rotates

Question

I want to modify the layout constraints when the device rotates. My UIViewController is composed of 2 UIViews, in landscape they are horizontally aligned, and in portrait they are vertically aligned.

It does work actually, in willAnimateRotationToInterfaceOrientation, I remove the desired constraints and replaced them with others to have the right layout...

But there are problems, during rotation auto layout starts breaking constraints before willAnimateRotationToInterfaceOrientation is called, so where are we meant to replace our constraints when the device reorientation occurs ?

Another issue is performance, after a few rotations the system doesn't break anymore constraints, but I have a huge performance drop, especially in portrait mode...

Answer 1

In willRotateToInterfaceOrientation:duration:, send setNeedsUpdateConstraints to any view that needs its constraints modified.

Alternatively, make a UIView subclass. In your subclass, register to receive UIApplicationWillChangeStatusBarOrientationNotification. When you receive the notification, send yourself setNeedsUpdateConstraints.

This sets the needsUpdateConstraints flag on the view. Before the system performs layout (by sending layoutSubviews messages), it sends an updateConstraints message to any view that has the needsUpdateConstraints flag set. This is where you should modify your constraints. Make a UIView subclass and override updateConstraints to update your constraints.