Swift Update Constraint

Question

I added the constraint to the buttons created in my UIView

func CreateButtonWithIndex(index:Int) {

    newButton.setTranslatesAutoresizingMaskIntoConstraints(false)

    self.view.addSubview(newButton)

    let newButtonConstraintL = NSLayoutConstraint(item: newButton, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50)
    let newButtonConstraintH = NSLayoutConstraint(item: newButton, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50)
    var newButtonConstraintX = NSLayoutConstraint(item: newButton, attribute: .CenterX, relatedBy: .Equal, toItem: view, attribute: .CenterX, multiplier: 1, constant: CGFloat(riga))
    var newButtonConstraintY = NSLayoutConstraint(item: newButton, attribute: .CenterY, relatedBy: .Equal, toItem: view, attribute: .CenterY, multiplier: 1, constant: CGFloat(colonna))

    view.addConstraints([newButtonConstraintX,newButtonConstraintY,newButtonConstraintH,newButtonConstraintL])

    var pan = UIPanGestureRecognizer(target:self, action:"pan:")
    pan.maximumNumberOfTouches = 2
    pan.minimumNumberOfTouches = 1
    newButton.addGestureRecognizer(pan)
}

func pan(rec:UIPanGestureRecognizer) {
  case .Ended: 
        if let subview = selectedView {
            if let button = rec.view as? UIButton {
                if let title = button.titleForState(.Normal){
                    button.setTranslatesAutoresizingMaskIntoConstraints(false)
                    let line = Float(p.x % snapX)
                    let column = Float(p.x % snapY)
                    let constraintL = NSLayoutConstraint(item: button, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50)
                    let constraintH = NSLayoutConstraint(item: button, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1.0, constant: 50)
                    var constraint2 = NSLayoutConstraint(item: button, attribute: .CenterX, relatedBy: .Equal, toItem: view, attribute: .CenterX, multiplier: 1, constant: CGFloat(line))
                    var constraint3 = NSLayoutConstraint(item: button, attribute: .CenterY, relatedBy: .Equal, toItem: view, attribute: .CenterY, multiplier: 1, constant: CGFloat(column))

                    view.addConstraints([constraint2,constraint3,constraintL.constraintH])
                }
            }
        }
}

In My Project my users can move these buttons and then I tried to add more constraints to the buttons recognized but I just get an endless error constraint

  "<NSLayoutConstraint:0x7f8e5a4e9450 UIButton:0x7f8e5a644f60'Button6'.centerX == UIView:0x7f8e5a648bc0.centerX - 120>",
  "<NSLayoutConstraint:0x7f8e5a5be040 'UIView-Encapsulated-Layout-Width' H:[UIView:0x7f8e5a648bc0(736)]>",
  "<NSAutoresizingMaskLayoutConstraint:0x7f8e5a5be1f0 h=-&- v=-&- 'UIView-Encapsulated-Layout-Left' H:|-(0)-[UIView:0x7f8e5a648bc0]   (Names: '|':UITransitionView:0x7f8e5a5a8190 )>",
  "<NSAutoresizingMaskLayoutConstraint:0x7f8e5a5b53b0 h=--& v=--& UIButton:0x7f8e5a644f60'Button6'.midX == + 200>"

Will attempt to recover by breaking constraint

"<NSLayoutConstraint:0x7f8e5a4e9450 UIButton:0x7f8e5a644f60'Button6'.centerX == UIView:0x7f8e5a648bc0.centerX - 120>"

"<NSLayoutConstraint:0x7f8e5a4e9510 UIButton:0x7f8e5a644f60'Button6'.centerY == UIView:0x7f8e5a648bc0.centerY - 40>",
"<NSLayoutConstraint:0x7f8e5a5be1a0 'UIView-Encapsulated-Layout-Height' V:[UIView:0x7f8e5a648bc0(414)]>",
"<NSAutoresizingMaskLayoutConstraint:0x7f8e5a5be240 h=-&- v=-&- 'UIView-Encapsulated-Layout-Top' V:|-(0)-[UIView:0x7f8e5a648bc0]   (Names: '|':UITransitionView:0x7f8e5a5a8190 )>",
"<NSAutoresizingMaskLayoutConstraint:0x7f8e5a5b82d0 h=--& v=--& UIButton:0x7f8e5a644f60'Button6'.midY == + 189>"

Will attempt to recover by breaking constraint

<NSLayoutConstraint:0x7f8e5a4e9510 UIButton:0x7f8e5a644f60'Button6'.centerY == UIView:0x7f8e5a648bc0.centerY - 40>

... should I update newButtonConstraintX / Y but I'm not understanding how I can do this ?

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.