How can I activate, previously deactivated constraint?

Question

I keep references to my NSLayoutConstraint

var flag = true
@IBOutlet weak var myConstraint: NSLayoutConstraint!

Then for some @IBAction I activate/deactivate depending on my flag variable:

@IBAction func tapped(sender: UIButton) {
    flag = !flag
    UIView.animateWithDuration(1.0) {
        if self.flag {
            NSLayoutConstraint.activateConstraints([self. myConstraint])
        } else {
            NSLayoutConstraint.deactivateConstraints([self. myConstraint])
        }
    }
}

But when I call my action once again, I have an error unexpectedly found nil while unwrapping an Optional value for myConstrain.

More over it doesn't animate. What am I doing wrong?

I follow tutorial from WWDC 2015:

enter image description here

keep a strong reference of object in IBOutlet declaration. `@IBOutlet strong var myConstraint: NSLayoutConstraint!` — Gandalf, Jul 11 '15 at 15:01
Gandalf's comment is correct. deactivating a constraint must cause the view hierarchy to release it's ownership of it. if you don't keep a strong reference, it will be deallocated. @Gandalf, you should make this an answer so the OP can accept it. — Duncan C, Jul 11 '15 at 15:41

score 21 · Accepted Answer · answered Jul 11 '15 at 17:13

21

Deactivating a constraint is same as calling removeConstraint: for a view. Refer the documentation. So when you remove an object which has weak reference will cause the object deallocation for it. Now this object is nil and activating it won't have any effect at all. To solve the issue you need to have a strong reference to constraint object.

@IBOutlet strong var myConstraint: NSLayoutConstraint!

answered Jul 11 '15 at 17:13

Gandalf

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19

Why there is no animation when I change this? – Bartłomiej Semańczyk Jul 11 '15 at 20:47
1

Oh, I know, I should add `self.view.layoutIfNeeded()` – Bartłomiej Semańczyk Jul 11 '15 at 20:50
1

IDE gives error if you try to make a weak IBOutlet strong. – SanjayPathak Jul 28 '17 at 12:30
2

I've used IB in Xcode 8.3.3 to define a strong outlet in the ViewController and the IDE just removes the weak attribute (does not add the strong attribute). So the above code should be written as `@IBOutlet var myConstraint: NSLayoutConstraint!` for it to be a strong reference. And I can confirm it seems to work. – RoberRM Aug 29 '18 at 20:07

How can I activate, previously deactivated constraint?

1 Answers1