Martin Woolstenhulme Martin Woolstenhulme - 2 years ago 121
iOS Question

Is there a way to add an identifier to Auto Layout Constraints in Interface Builder?

After doing some visual layout in Interface Builder I've created some constraints that I want to access at runtime. Is there a way to label or identify constraints in Interface Builder so they can be looked-up later?

The reason I want to do this is I need to perform some calculation base upon the constraints that are visually specified. I am aware that Apple has provided the Visual Format Language and I could specify the constraints programmatically in a controller. I rather not use this approach so I don't lose the design time feedback of IB.


Making a referencing outlet did work, but the question still stands.

Answer Source

Yes, this can be done. NSLayoutConstraint has a property called identifier than can be exposed in Interface Builder and assigned. To demo this, I created a Single View Application that has a single subview that is a red box. This subview has 4 constraints: width, height, centered horizontally in container, centered vertically in container. I gave the width constraint the identifier redBoxWidth by doing the following:

  1. Click on the width constraint in the Document Layout View. Then in the Identity Inspector under User Defined Runtime Attributes, click on the + under Key Path. Change keyPath to identifier, change the Type Boolean to String, and set the Value to redBoxWidth.

    naming a constraint

  2. Then in ViewDidLoad it is possible to find the constraint by name and change its value:

    class ViewController: UIViewController {
        override func viewDidLoad() {
            for subview in view.subviews as [UIView] {
                for constraint in subview.constraints() as [NSLayoutConstraint] {
                    if constraint.identifier == "redBoxWidth" {
                        constraint.constant = 300
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