NSLayoutConstraints programmatically

Question

I want to add constraints to a view programmatically.

This is what I did:

extension UIView {
  func bottomToTop(other: UIView) {
     self.translatesAutoresizingMaskIntoConstraints = false
     other.translatesAutoresizingMaskIntoConstraints = false
     let constraint = NSLayoutConstraint(
        item: self,
        attribute: .bottom,
        relatedBy: .equal
        toItem: other,
        attribute: .top,
        multiplier: 1.0,
        constant: 0.0
    )
    superview?.addConstraint(constraint)
    constraint.isActive = true
  }
}


let label = UILabel()
label.text = "Lenaaaaa"
label.sizeToFit()
label.backgroundColor = .green

let label1 = UILabel()
label1.text = "Lena 2"
label1.sizeToFit()
label1.backgroundColor = .green

let uiView = UIView(frame: frame) (not zero)
uiView.addSubview(label)
uiView.addSubview(label2)

label.bottomToTop(label2)

Why do I end up with this?

matt · Accepted Answer · 2021-03-20 21:01:07Z

1

Why do I end up with this?

Because your constraints are ambiguous. Once you add even one constraint that affects a view, you must describe that view's position and size in terms of autolayout completely. (And you must stop talking about .frame, as it is now effectively meaningless.)

Thus, you have said only

label.bottomToTop(label2)

But you have not said where the top of label is, where the left of label is, where the left of label2 is, and so on. Thus the autolayout engine throws up its hands in despair.

You could easily have discovered this just by running your app and using the view debugger. It puts up great big exclamation marks telling you what your autolayout issues are.

answered Mar 20, 2021 at 21:01

matt

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2 Comments

matt Over a year ago

Also your code is wrong in various other ways. For instance you should never say both addConstraint and constraint.isActive = true; they do the same thing, and the second one does it way better.

matt Over a year ago

Plus your code makes no sense to me because it talks about a uiView that never even goes into the interface, so I do not understand why you say anything. If this is playground code, stop it; use a real project.

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