Display phases in iOs (Constraints)

updateConstraints()

Override this method when you want a UIView subclass to specify constraints for your subviews.

class AView: UIView {
    var yaAgregoConstraints = false
    override func updateConstraints() {
        print(#function)
        if yaAgregoConstraints == false {
            yaAgregoConstraints = true
            addConstraint(NSLayoutConstraint(item: self, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 60))
            addConstraint(NSLayoutConstraint(item: self, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .NotAnAttribute, multiplier: 1, constant: 60))
        }
        super.updateConstraints()
    }
}

override func viewDidLoad() {
    super.viewDidLoad()

    let aView = AView()
    aView.backgroundColor = UIColor.redColor()
    aView.translatesAutoresizingMaskIntoConstraints = false
    view.addSubview(aView)
}

setNeedsLayout() and layoutIfNeeded()

Continuing the previous example, let's say we want to know the view size% co_of% after creating it. We add this to end of aView :

override func viewDidLoad() {
    super.viewDidLoad()

    ...

    print(aView.frame)
}

Print:

(0.0, 0.0, 0.0, 0.0)  // esto es incorrecto!
updateConstraints()   // lo que hace print(#function)

Print a size of zero because after creating the view viewDidLoad() , it has constraints, but they have not yet been calculated so that aView has a size (a layout pass, which gives size to aView , a a little later). If we already want its size to be calculated, we invoke these methods before requesting the size at aView :

override func viewDidLoad() {
    super.viewDidLoad()

    ...

    aView.setNeedsLayout()
    aView.layoutIfNeeded()
    print(aView.frame) 
}

Print:

updateConstraints()    // ahora updateConstraints se ejecuta primero
(0.0, 0.0, 60.0, 60.0)  // y el tamaño es correcto

aView and setNeedsUpdateConstraints()

The system calls the updateConstraintsIfNeeded() method in each layout-pass (every certain amount of milli-seconds). Call the default implementation of updateConstraints() or the implementation that we specify. To cut this wait from the layout-pass and have the system call updateConstraints() , invoke the methods like this:

setNeedsUpdateConstraints()
updateConstraintsIfNeeded()

updateConstraints()

updateViewConstraints() is equivalent to updateViewConstraints() , but is a method of setNeedsUpdateConstraints() , not UIViewController , how the previous methods.