Enable + Disable Auto-Layout Constraints

Question

I have a simple (I think) problem: I have a UIImageView, which I have set multiple constraints for in the storyboard.

At times, I'll need to disable the constraints, and set its frame manually, but later, I'll want to re-enable those constraints and have the view return to where it was as decided by the constraints.

I thought I could do this with something like:

@IBOutlet weak var imageView: UIImageView! @IBOutlet weak var constraint: NSLayoutConstraint!  func example() {       //to set its location and size manually     imageView.removeConstraint(constraint)      imageView.frame = CGRectMake(...)      //to cause it to return to its original position     imageView.addConstraint(constraint)   } 

With this, however, I get the error 2015-08-26 01:14:55.417 Constraints[18472:923024] The view hierarchy is not prepared for the constraint: <NSLayoutConstraint:0x7fbb72815f90 V:[_UILayoutGuide:0x7fbb72814c20]-(100)-[UIImageView:0x7fbb72814540]>. Does anyone know why this error occurs?

Adding the constraints instead to view by calling self.view.addConstraint(...) gets rid of the error, but still doesn't bring the image view back to where it should be.

I notice I don't even have to remove nor deactivate constraints to be able to set the frame of the imageView without problem, so long as I don't call setTranslatesAutoresizingMaskIntoConstraints(true).

I've already seen this question, but the answer seems unnecessarily complicated, perhaps outdated, and even inapplicable.

I also saw this question, but this doesn't cover constraints can be re-enabled—only how they can be disabled.

Trying to set the active property:

self.top.active = false self.right.active = false self.bottom.active = false self.left.active = false      imageView.frame = CGRectMake(100, 100, 100, 100) 

This actually just results in imageView being no longer visible. (If I don't set all the constraints' active property to false, however, imageView moves to the proper position.

The real problem, though, is that when I try to have the image view return to where it was, by setting all the constraints' active property to true, nothing happens—the imageView stays where it was (in this case as specified by imageView.frame = CGRectMake(100, 100, 100, 100).

Also, it seems, according to my testing and Joachim Bøggild's answer here, that when a constraint's active property is set to false, that constraint becomes nil, and thus cannot be re-activated.

Adding constraints to an array and activating/deactivating them:

Interestingly, this causes almost exactly the same issues as setting the active property. Placing all the constraints in an array array, then calling NSLayoutConstraint.deactivateConstraints(array) makes the image view disappear. Also, re-activating the constraints later with NSLayoutConstraint.activateConstraints(array) doesn't have the image view return to where it should—it stays where it was.

Answer 1

Aha!

Constraints become nil (and thus can't be reactivated), once their active property is set to false. Making them strong references (thanks, Caleb, for clearing up the nomenclature) preserves them so they can be activated and deactivated as desired.