How to use constraint Identifiers in autolayout and how to change constrain using identifiers? [Swift]

Question

While I was editing my constraint in Xcode 7 I found identifier field in Interface Builder. What is an identifier for the constraint how do I use it? using identifier can I access constraint programmatically and change the constant? my question is why and how is that identifier is helpful?

Is there any way to access the constraint with an identifier with looping in subviews and again looping constrain subviews. I mean is there any way we can access constraint directly without a loop.

UPDATE

I Tryed this code but only width and height constrain is accessed

    for subview in view.subviews {
        for constraint in subview.constraints() {
           if constraint.identifier == "identifier" {
                return constraint
           }
        }
    }

Answered here: http://stackoverflow.com/questions/27791597/is-there-a-way-to-add-an-identifier-to-auto-layout-constraints-in-interface-buil/31831576#31831576 — oyalhi, Apr 27 '16 at 04:46
http://stackoverflow.com/questions/33038451/get-reference-to-nslayoutconstraint-using-identifier-set-in-storyboard — Bhavin Bhadani, Apr 27 '16 at 04:50
if you have constraint error , you can debug your constraint using identifier — Bhavin Bhadani, Apr 27 '16 at 05:06
@O-mkar please check this out this one works for me. https://stackoverflow.com/a/44558288/6897825 — Azharhussain Shaikh, Jan 20 '19 at 14:03

Lawrence413 · Accepted Answer · 2016-04-27T10:11:37.163

HOW: The identifier is useful when debugging (e.g. constraints not matching and one of them gets broken at run time; the constraint identifiers are being shown in log so you can see which one might cause problems)

WHY: Constraint identifiers make logs easier to read, more accurate and they save you a lot of time.

Constraint editing: If you want to change constraints programmatically you will have to declare them as outlets (like a label or button), then remove them from the view (NOT the object itself) and then set them again to the view. From my knowledge you can't just "edit" the constraints programmatically.

Your code gives you only width and height because you access the view's constraints which only contains the objects' widths and heights.

It is actually possible to modify a constraint after you have declared it as an outlet, like this: self.myConstraint.constant = 50.0 — Sjors, Apr 12 '17 at 20:24

score 5 · Answer 2 · edited Jun 20 '20 at 09:12

5

In Swift 4 it's possible:

With this extension:

extension UIView{
    func constraintWith(identifier: String) -> NSLayoutConstraint?{
        return self.constraints.first(where: {$0.identifier == identifier})
    }
}

And you can use't this way:

 if let myConstraint = myView.constraintWith(identifier: "myConstraintID"){
     myConstraint.constant = 8
 }

Ps: Be sure to call constraintWith(identifier: String) on the view where the constraint was added, normally on the superView of the related view you are changing, or on itself if the constraint is for size (width/height)

edited Jun 20 '20 at 09:12

Community

1
1

answered Jul 22 '19 at 17:31

Romulo BM

671
8
16

1

More accurately, if you want to access the height, width, or aspect ratio of the view, call this method on that `view` itself. On the other hand, to access other constraints such as leading or trailing, call this method on the `SuperView` that contains the view. – rule_it_subir Jan 25 '22 at 12:28
@rule_it_subir it all depends the way you setup your constraints – Romulo BM Jan 27 '22 at 14:49

score 1 · Answer 3 · answered Oct 21 '20 at 13:58

To supplement the answer answer, we are also able to set constraints programmatically, and it was easier than I thought:

        let yConstraint = imageView.centerYAnchor.constraint(equalTo: layout.centerYAnchor)
        yConstraint.identifier = "yConstraint"
        let xConstraint = imageView.centerXAnchor.constraint(equalTo: layout.centerXAnchor)
            xConstraint.identifier = "xConstraint"

        NSLayoutConstraint.activate([
            yConstraint,
            xConstraint
        ])

Then, it showed up in the console, albeit difficult to read still...

<NSLayoutConstraint:0x6000025fe490 'xConstraint' UIImageView:0x7f8f41107fa0.centerX == UILayoutGuide:0x600003fcc1c0'UIViewLayoutMarginsGuide'.centerX   (active)>

How to use constraint Identifiers in autolayout and how to change constrain using identifiers? [Swift]

3 Answers3

In Swift 4 it's possible:

Linked