add constraints programmatically swift

Question

I am trying to add constraints to a facebook sdk login button.

I have the button inside a scroll view and I am trying to add a top constraint to a label that is also in the scroll view. I am able to successfully add the height constraint with no run time errors but the actual constraint does not seem to be applied to the button.

@IBOutlet weak var orLbl: UILabel!
@IBOutlet weak var scrollView: UIScrollView!

override func viewDidLoad() {
    super.viewDidLoad()
    var loginFBButton = FBSDKLoginButton()
    loginFBButton.readPermissions = ["public_profile", "email"]

    let heightConstraint = NSLayoutConstraint(
        item: loginFBButton,
        attribute: NSLayoutAttribute.Height,
        relatedBy: NSLayoutRelation.Equal,
        toItem: nil,
        attribute: NSLayoutAttribute.NotAnAttribute,
        multiplier: 1,
        constant: 41)
    let topConstraint = NSLayoutConstraint(
        item: loginFBButton,
        attribute: NSLayoutAttribute.TopMargin,
        relatedBy: NSLayoutRelation.Equal,
        toItem: self.orLbl,
        attribute: NSLayoutAttribute.BottomMargin,
        multiplier: 1,
        constant: 31)
    let leadingConstraint = NSLayoutConstraint(
        item: loginFBButton,
        attribute: NSLayoutAttribute.Leading,
        relatedBy: NSLayoutRelation.Equal,
        toItem: self.registerButton,
        attribute: NSLayoutAttribute.Left,
        multiplier: 1,
        constant: 0)
    let trailingConstraint = NSLayoutConstraint(
        item: loginFBButton,
        attribute: NSLayoutAttribute.Leading,
        relatedBy: NSLayoutRelation.Equal,
        toItem: self.registerButton,
        attribute: NSLayoutAttribute.Right,
        multiplier: 1,
        constant: 0)

    self.scrollView.addSubview(loginFBButton)
    //loginFBButton.center = self.scrollView.center

    loginFBButton.addConstraints([heightConstraint, topConstraint])

}

Then when I include the addition of the top constraint, I am getting a runtime error:

When added to a view, the constraint's items must be descendants of that view (or the view itself). This will crash if the constraint needs to be resolved before the view hierarchy is assembled.

Terminating app due to uncaught exception 'NSGenericException', reason: 'Unable to install constraint on view. Does the constraint reference something from outside the subtree of the view? That's illegal.

Both the label and the facebook button are in my scroll view? I've even printed out orLbl.superview and loginFBButton.superview and I am getting optional uiscrollview for both

Answer 1

There is a new (iOS8, OS 10.10), easier way to activate constraints that doesn't involve figuring out which views to add them to. The constraints already know which views they belong to, so first make sure your views have been added as subViews, then call the class method activateConstraints on NSLayoutConstraint to activate them:

 NSLayoutConstraint.activateConstraints([heightConstraint, topConstraint])

When you create UI elements programmatically, you need to tell iOS not to turn its frame into constraints. To do that, just after creating loginFBButton do:

For Swift 1.2:

loginFBButton.setTranslatesAutoresizingMaskIntoConstraints(false)

For Swift 2.0 & 3.0:

loginFBButton.translatesAutoresizingMaskIntoConstraints = false

Finally, you are going to need more constraints. I suggest setting a width constraint for your loginFBButton and adding a constraint to position the button horizontally.