What happens with constraints when a view is removed

The constraints are removed. If you add A again, you will have to make new constraints for it, or if you save the constraints before you remove A, you can add them back. When I do something like this, I save the constraints like this for a view called view1:

self.portraitConstraints = [NSMutableArray new];
for (NSLayoutConstraint *con in self.view.constraints) {
    if (con.firstItem == self.view1 || con.secondItem == self.view1) {
       [self.portraitConstraints addObject:con];
    }
}