Owner of NSLayoutConstraint

When working with Autolayout the identifier property of a NSLayoutConstraint can be helpful to re-identify a specific constraint of a UIView. This might for example come in handy when you want to modify the constant of a constraint dynamically at runtime.

Side node: It’s quite fun that a property with the name constant is the only property of a NSLayoutConstraint that is readwrite and allowed to be modified after the constant is activated. 😜👍

Using the identifier to identify a specific constraint might help if you don’t want/ can’t store a reference to a certain constraint. In order to obtain the reference to the constraint you simply filter the list of constraints of the view.

aView.constraints.filter { $0.identifier == "my constraint identifier"}.first

But this might turn out more tricky than expected bc. a constraint might not be found in the constraints of the view it constrains. 😱

For instance the leading/ trailing/ top/ bottom constraints of a view are to be found in it’s superview’s list of constraints. That’s still true if a view is constrained to a sibling view and not to its superview:

// view1 and view2 are both subviews of a common superview
// constraint view2.leadingAnchor to view1.trailingAnchor
view2.leadingAnchor.constraint(equalTo: view1.trailingAnchor).active = true

// the created constraint will be found in their common superview
// neither in view1.constraints nor in view2.constraints

If you want to take a look for yourself you might find this playground helpful.

So the next time you’re looking to obtain a reference to a layoutConstraint you might want to look in the constraints of the view’s superview! This might save you some time 😎⏲