VFL constraints in swift: Crashes due to no superview - ios

Question

I'm trying to do a simple layout programmatically and I'm missing something simple, or have something out of place, I think. The following ViewController should center the label in the super view. However, it crashes with this trimmed message: The view hierarchy is not prepared for the constraint ... 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... View not found in container hierarchy: ... That view's superview: NO SUPERVIEW The other SO questions with this error message are using nibs for the most part, and I'm tring to avoid that, or use Obj-C instead of swift. This question deals with the topic a bit but is a bit old.
Can anyone point me in the right direction?
class ViewController: UIViewController {
let label1 = UILabel() as UILabel
func layoutView(){
label1.text = "Click to see device configuration"
label1.setTranslatesAutoresizingMaskIntoConstraints(false)
view.addSubview(label1)
let viewsDictionary = ["label1":label1]
let label1_H:NSArray = NSLayoutConstraint.constraintsWithVisualFormat("H:|-[label1]-|",
options: NSLayoutFormatOptions(0),
metrics: nil,
views: viewsDictionary)
let label1_V:NSArray = NSLayoutConstraint.constraintsWithVisualFormat("V:|-[label1]-|",
options: NSLayoutFormatOptions(0),
metrics: nil, views:
viewsDictionary)
label1.addConstraints(label1_H) // Comment these 2 lines and it runs, but
label1.addConstraints(label1_V) // of course the label is upper left
}
override func viewDidLoad() {
super.viewDidLoad()
layoutView()
}
}

Answer 1

Those constraints are made between the label and its superview. The constraints should be added to that superview, not to the label.

Answer 2

You're almost there. Just replace the following lines...
label1.addConstraints(label1_H) // Comment these 2 lines and it runs, but
label1.addConstraints(label1_V) // of course the label is upper left
... with the following code:
view.addConstraints(label1_H) // Comment these 2 lines and it runs, but
view.addConstraints(label1_V) // of course the label is upper left
However, the constraints H:|-[label1]-|" and V:|-[label1]-|" are equivalent to H:|-8-[label1]-8-|" and V:|-8-[label1]-8-|" (see the iOS Developer Library for more details on default margins). Thus, those constraints are not meant to center your label. In fact, you will just have an enormous label that has 8 unit top, leading, trailing and bottom margins to the viewController's view.
Add the following line of code in your layoutView() method to see what I mean:
label1.backgroundColor = UIColor.orangeColor()
It can be OK but if you really want to center your label, you will have to use the following code:
func layoutView() {
label1.text = "Click to see device configuration"
label1.backgroundColor = UIColor.orangeColor()
//Set number of lines of label1 to more than one if necessary (prevent long text from being truncated)
label1.numberOfLines = 0
label1.setTranslatesAutoresizingMaskIntoConstraints(false)
view.addSubview(label1)
let xConstraint = NSLayoutConstraint(item: label1,
attribute: NSLayoutAttribute.CenterX,
relatedBy: NSLayoutRelation.Equal,
toItem: view,
attribute: NSLayoutAttribute.CenterX,
multiplier: 1.0,
constant: 0.0)
self.view.addConstraint(xConstraint)
let yConstraint = NSLayoutConstraint(item: label1,
attribute: NSLayoutAttribute.CenterY,
relatedBy: NSLayoutRelation.Equal,
toItem: view,
attribute: NSLayoutAttribute.CenterY,
multiplier: 1.0,
constant: 0)
self.view.addConstraint(yConstraint)
//Add leading and trailing margins if necessary (prevent long text content in label1 to be larger than screen)
let viewsDictionary = ["label1" : label1]
view.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:|-(>=10#750)-[label1]-(>=10#750)-|", options: NSLayoutFormatOptions(0), metrics: nil, views: viewsDictionary))
}