Swift Update Constraint

Question

I added the constraint to the buttons created in my UIView

func CreateButtonWithIndex(index:Int) {

    newButton.setTranslatesAutoresizingMaskI         


        

Answer 1

The issue is that you're adding a new constraint that conflicts with the existing constraint.

You have a few options:

1. Effective in iOS 8, you can set the active property to false for a constraint before you add a new constraint.

2. In iOS versions prior to 8, you would want to remove the old constraints before adding new constraints.

3. Ideally, it's best to not have to activate/deactivate (or, worse, add and remove) constraints, but rather just modify the constant property of a single constraint. For example in Swift 3/4:

   class ViewController: UIViewController {
   
       private var xConstraint: NSLayoutConstraint!
       private var yConstraint: NSLayoutConstraint!
   
       override func viewDidLoad() {
           super.viewDidLoad()
   
           let label = UILabel()
           label.text = "x"
           label.translatesAutoresizingMaskIntoConstraints = false
           view.addSubview(label)
   
           // I don't really need to save references to these, so these are local variables
   
           let widthConstraint = label.widthAnchor.constraint(equalToConstant: 50)
           let heightConstraint = label.heightAnchor.constraint(equalToConstant: 50)
   
           // but since I'll be modifying these later, these are class properties
   
           xConstraint = label.centerXAnchor.constraint(equalTo: view.centerXAnchor)
           yConstraint = label.centerYAnchor.constraint(equalTo: view.centerYAnchor)
   
           NSLayoutConstraint.activate([widthConstraint, heightConstraint, xConstraint, yConstraint])
   
           let pan = UIPanGestureRecognizer(target: self, action: #selector(handlePan(_:)))
           view.addGestureRecognizer(pan)
       }
   
       private var originalCenter: CGPoint!
   
       @objc func handlePan(_ gesture: UIPanGestureRecognizer) {
           if gesture.state == .began {
               originalCenter = CGPoint(x: xConstraint.constant, y: yConstraint.constant)
           }
   
           let translation = gesture.translation(in: gesture.view!)
   
           xConstraint.constant = originalCenter.x + translation.x
           yConstraint.constant = originalCenter.y + translation.y
       }
   
   }
   

   When the desired effect can be achieved by modifying the constant of the constraint, that's generally best.

For Swift 2 syntax, see previous revision of this answer.