NSLayoutConstraint IBOutlet does not respond to setActive: - ios

Question

I have a UIViewController and I want to dynamically modify some of its layout according to, say, the click of a button.
As a simplified example, let's say I have created UIView in IB (self.box). In IB, set a constraint that makes the view's width equal to 0.9 of the VC's view's width.
Then I programmatically add another constraint and save it as a strong property. It sets the view's width to 0.7 of the VC's view's width, and I remove the previous constraint.
This leads to conflicting constraint errors and to a completely broken view. Whether I add or remove first or use the .active property instead of adding / removing, the same happens.
Any idea on how to make this work? My actual case is more complicated, but I have checked and only modifying a view's width constraint in my UIViewController leads to the error, even though setting it to different multipliers in IB does not lead to any conflicts.
- (void) viewDidLoad
{
NSLayoutConstraint* lowWidth = [NSLayoutConstraint constraintWithItem:self.box attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeWidth multiplier:0.7 constant:0.f];
self.lowWidth = lowWidth;
[self.view removeConstraint:self.highWidth];
[self.view addConstraint:self.lowWidth];
}
The constraint conflict includes the following:
Probably at least one of the constraints in the following list is one you don't want.
Try this:
<NSLayoutConstraint:0x145efb0f0 UIView:0x145d8d330.width == 0.7*UIView:0x14755e890.width>,
<NSLayoutConstraint:0x1474589b0 UIView:0x145d8d330.width == 0.9*UIView:0x14755e890.width>
EDIT: calling setActive: on my IBOutlet constraint did not seem to work, so I removed the outlet and programmatically created the constraint instead. This time it works as intended, but I have no explanation as to why the outlet did not work. Note that setting to weak / strong does not change anything.
#property (weak, nonatomic) IBOutlet NSLayoutConstraint *highWidth;

Answer 1

I'd use only one NSLayoutConstraint and modifying that constraint size.
self.highWidth?.contant = size
[self.view layoutIfNeeded]
or activating/deactivting constraints... NSLayoutConstraint class