Why do I get an autolayout error on a constraint that should not be installed for the size class?

Question

I'm using iOS 8. My nib has autolayout and size classes enabled. I've made two layouts in IB, one for Any Width/Any Height, and one for Any Width/Compact Height. I've tested different orientations and device sizes in IB and I don't have constraint problems in IB.

Now when I run the app on my phone. The view lays out correctly in portrait. Then I rotate my phone into landscape and I get a "Unable to simultaneously satisfy constraints." error. When I look at the list of conflicting constraints, I see a constraint that should have been uninstalled for landscape layouts. When rotation completes the landscape layout appears correctly.

It's as if iOS is trying to lay out the view with the new bounds before the old constraints are uninstalled. Is this a known problem with iOS? Is there a step I need to implement to correctly support size classes?

Answer 1

I ran into this same issue. Very frustrating! At first I added code to willTransitionToTraitCollection:withTransitionCoordinator: in order to detect if the newCollection was a size class that didn't need the constraint. If so, then I set the constraint.active = NO.

It did the trick, but I hated the code. I was just like, "Why should I have to do this? The constraint is not installed for the new size class!"

Then I figured out that if I lowered the priority of the offending constraint to 999 in interface builder I no longer got the constraint error in the console when I rotated. :-) And all is right with the world again.