Unable to get constraints of NSLayoutConstraint

Question

This is my code:

let b = NSLayoutConstraint(item: some, attribute: fromAttribute, relatedBy: NSLayoutRelation.equal, toItem: some2, attribute: toAttribute, multiplier: multiplier, constant: -5)
b.isActive = true
self.layoutIfNeeded()
print(b.constant)
print(some.constraints.first(where: {$0.constant == -5 }))

And this is my print:

-5.0
nil

How can I get that constraint back in code? Why does it print out nil? I want to animate the constraint's constant later on. Thanks.

Answer 1

Let's start with the core question:

How can I get that constraint back in code?

Ideally, you don't. You save it to a variable when you create it, e.g.:

var myConstraint: NSLayoutConstraint?

func x() {
   let b = NSLayoutConstraint(...)
   ...
   myConstraint = b
}

Why does it print out nil?

When setting isActive = true, the constraint is added to the closest common superview. For example, if A is a superview of B and you have a same-width constraint, then the constraint is added to A and it won't be present on B.

The constraint will be added to some only if some2 is a subview of some.