CODEWITHSUNDEEP
iosswiftautolayoutnslayoutconstraint
J. Doe
J. Doe

Reputation: 13103

Unable to get constraints of NSLayoutConstraint

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.

Upvotes: 0

Views: 77

Answers (2)

Sulthan
Sulthan

Reputation: 130191

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.

Upvotes: 1

DonMag
DonMag

Reputation: 77690

Use a class-level variable to hold the constraint you want to modify. If you are using storyboards / Interface Builder, you can also assign a constraint as an IBOutlet.

class ViewController: UIViewController {

    var constraintToAnimate: NSLayoutConstraint?

    override func viewDidLoad() {
        super.viewDidLoad()

        constraintToAnimate = NSLayoutConstraint(item: some, attribute: fromAttribute, relatedBy: NSLayoutRelation.equal, toItem: some2, attribute: toAttribute, multiplier: multiplier, constant: -5)
        constraintToAnimate.isActive = true

    }

    // later, perhaps in a button tap...

    func animateIt() {

        constraintToAnimate?.constant = 100

    }

}

Upvotes: 0

Related Questions