Animate NSLayoutConstraint created programmatically

Question

Something special has to be done to animate a constraint instantiated programmatically?

I instantiate a constraint programmatically as a lazy variable:

private lazy var topConstraint: NSLayoutConstraint = {
    let constraint = view.topAnchor.constraint(equalTo: otherView.topAnchor, constant: otherView)
    return constraint
}()

Then, I animate a change in the constant of this constraint:

topConstraint.constant = newValue
view.setNeedsLayout()
UIView.animate(withDuration: 1) {
    self.view.layoutIfNeeded()
}

It is not working... I also have tried putting the constant setting into the block:

UIView.animate(withDuration: 1) {
    self.topConstraint.constant = newValue
    self.view.setNeedsLayout()
    self.view.layoutIfNeeded()
}

But I achieved nothing at all, I've tried almost all the combinations. The constraint constant is properly changed, but always without animation.

PD: I also gave a shot to UIViewPropertyAnimator.

Possible duplicate of [How do I animate constraint changes?](https://stackoverflow.com/questions/12622424/how-do-i-animate-constraint-changes) — Shamas S, Sep 10 '19 at 09:48
No, this question asks if there is something special to do when the constraint is created by code... No storyboard. — Sergio, Sep 10 '19 at 09:51
First, you set `constant: otherView` ... that should be a `CGFloat`, not a reference to a view. Second, are you ever setting `topConstraint.isActive = true`? — DonMag, Sep 10 '19 at 12:01

PGDev · Answer 1 · 2019-09-10T10:05:18.940

1

No need for setNeedsLayout().

self.topConstraint.constant = 0.0
UIView.animate(withDuration: 1.0) {[weak self] in
    self?.topConstraint.constant = 10.0 
    self?.view.layoutIfNeeded()
}

edited Sep 10 '19 at 10:05

answered Sep 10 '19 at 09:58

PGDev

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I tried this, it's not working in this scenario neither. – Sergio Sep 10 '19 at 10:04
Try setting some hardcode values first and check if is working. Updated the answer. See if it works now. – PGDev Sep 10 '19 at 10:05
No, I tried all this things that should work... But the constraint change can't be animated – Sergio Sep 11 '19 at 07:15
Inside the handler (selector) of the pan gesture recognizer. – Sergio Sep 11 '19 at 08:29
Let us [continue this discussion in chat](https://chat.stackoverflow.com/rooms/199289/discussion-between-sergio-and-pgdev). – Sergio Sep 11 '19 at 08:31

Animate NSLayoutConstraint created programmatically

1 Answers1