How to change Auto Layout constraints after they are set when using constraintEqualToAnchor()?

Question

I try to set up a view with AutoLayout constraints by using constraintEqualToAnchor():

override func viewDidLoad() {     super.viewDidLoad()      let myView = UIView()     myView.backgroundColor = UIColor.orangeColor()     myView.translatesAutoresizingMaskIntoConstraints = false     view.addSubview(myView)      myView.leftAnchor.constraintEqualToAnchor(view.leftAnchor).active = true     myView.trailingAnchor.constraintEqualToAnchor(view.trailingAnchor).active = true     myView.topAnchor.constraintEqualToAnchor(view.topAnchor).active = true     myView.bottomAnchor.constraintEqualToAnchor(view.bottomAnchor).active = true      /******************************************/     /* I try to change one of the constraints */     /******************************************/     myView.leftAnchor.constraintEqualToAnchor(view.rightAnchor, constant: -100).active = true   } 

In the last line of code, I try to change one of the constraints. I thought it would work but it gives some error in the console log

"<NSLayoutConstraint:0x7fb53a5180d0 H:|-(0)-[UIView:0x7fb53a5190b0](LTR)   (Names: '|':UIView:0x7fb53a519240 )>", "<NSLayoutConstraint:0x7fb53a51f660 H:[UIView:0x7fb53a519240]-(-100)-[UIView:0x7fb53a5190b0](LTR)>", "<NSLayoutConstraint:0x7fb53a711ee0 'UIView-Encapsulated-Layout-Width' H:[UIView:0x7fb53a519240(414)]>" 

When using constraintEqualToAnchor()?, what's the right way to change the constraint later after I have set them?

Answer 1

You need to deactivate the previous constraint when activating a new one so that you don't end up over constraining your view. To do that, store a reference to each of the constraints as a property in your ViewController and then set the active property of the old constraint to false before creating and activating the new constraint:

Swift 2.x:

class ViewController: UIViewController {     var leftConstraint: NSLayoutConstraint?     var trailingConstraint: NSLayoutConstraint?     var topConstraint: NSLayoutConstraint?     var bottomConstraint: NSLayoutConstraint?      override func viewDidLoad() {         super.viewDidLoad()          let myView = UIView()         myView.backgroundColor = UIColor.orangeColor()         myView.translatesAutoresizingMaskIntoConstraints = false         view.addSubview(myView)          leftConstraint = myView.leftAnchor.constraintEqualToAnchor(view.leftAnchor)         leftConstraint?.active = true          trailingConstraint = myView.trailingAnchor.constraintEqualToAnchor(view.trailingAnchor)         trailingConstraint?.active = true          topConstraint = myView.topAnchor.constraintEqualToAnchor(view.topAnchor)         topConstraint?.active = true          bottomConstraint = myView.bottomAnchor.constraintEqualToAnchor(view.bottomAnchor)         bottomConstraint?.active = true          /******************************************/         /* I try to change one of the constraints */         /******************************************/         leftConstraint?.active = false         leftConstraint = myView.leftAnchor.constraintEqualToAnchor(view.rightAnchor, constant: -100)         leftConstraint?.active = true     } } 

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Update for Swift 3 syntax:

class ViewController: UIViewController {     var leftConstraint: NSLayoutConstraint?     var trailingConstraint: NSLayoutConstraint?     var topConstraint: NSLayoutConstraint?     var bottomConstraint: NSLayoutConstraint?      override func viewDidLoad() {         super.viewDidLoad()          let myView = UIView()         myView.backgroundColor = UIColor.orange         myView.translatesAutoresizingMaskIntoConstraints = false         view.addSubview(myView)          leftConstraint = myView.leftAnchor.constraint(equalTo: view.leftAnchor)         leftConstraint?.isActive = true          trailingConstraint = myView.trailingAnchor.constraint(equalTo: view.trailingAnchor)         trailingConstraint?.isActive = true          topConstraint = myView.topAnchor.constraint(equalTo: view.topAnchor)         topConstraint?.isActive = true          bottomConstraint = myView.bottomAnchor.constraint(equalTo: view.bottomAnchor)         bottomConstraint?.isActive = true          /******************************************/         /* I try to change one of the constraints */         /******************************************/         leftConstraint?.isActive = false         leftConstraint = myView.leftAnchor.constraint(equalTo: view.rightAnchor, constant: -100)         leftConstraint?.isActive = true     } } 

Answer 2

Here is an example declaring a constraint c which will be referenced later in time. We set a new constant value and then call layout on the superview.

myView.translatesAutoresizingMaskIntoConstraints = false  var constraints: [NSLayoutConstraint] = [     myView.topAnchor.constraintEqualToAnchor(view.topAnchor),     myView.leftAnchor.constraintEqualToAnchor(view.leftAnchor),     myView.bottomAnchor.constraintEqualToAnchor(view.bottomAnchor) ]          let c = myView.rightAnchor.constraintEqualToAnchor(view.rightAnchor)        constraints.append(c)     view.addSubview(myView) NSLayoutConstraint.activateConstraints(constraints)  // Some time later c.constant = -100 view.setNeedsLayout()