NSLayoutConstraint constant not updating after setting

You will need to call setNeedsUpdateConstraints method of corresponding UIView(control) of which your NSLayoutConstraint is present to update constraint.

For example of UIButton

self.buttonConstraint.constant = 55;[self.btnTest setNeedsUpdateConstraints];

In your case

[self setNeedsUpdateConstraints];

I ran into a problem where the first time I set the constraint constant in code the constant would be set but then after calling layoutIfNeeded(), such as this:

myConstraint.constant = newValue;[view layoutIfNeeded]

the constant would go back to the original value! I could watch the value change back in the debugger.

I finally figured out that it was because, in the storyboard, I had set a variation on the constant. For example:

As soon as I removed the variation, the constraint constant value did not change back to the original value with layoutIfNeeded call.

Are you sure this constraint is not de-activated somewhere?Setting the "active" property of a constraint to false causes the view hierarchy to remove the constraint. If you don’t also have a reference to it, the constraint object will be deleted from memory.

I had the same issue as you, and removed the "weak" so the constraint is now a strong property. Consequently, it's not set to nil when de-activated (because my view controller always has a strong pointer to it), and I can re-activate it and re-set its constant.

// NB: don't create these contraints outlets as "weak" if you intend to de-activate them, otherwise they would be set to nil when deactivated!@IBOutlet private var captionViewHeightConstraint: NSLayoutConstraint!