How to update swift Layout Anchors?

You can iterate over a view's constraints and check for matching items and anchors. Remember that the constraint will be on the view's superview unless it is a dimension constraint. I wrote some helper code that will allow you to find all constraints on one anchor of a view.

import UIKit

class ViewController: UIViewController {

    let label = UILabel()
    let imageView = UIImageView()

    override func viewDidLoad() {
        super.viewDidLoad()

        label.text = "Constraint finder"

        label.translatesAutoresizingMaskIntoConstraints = false
        imageView.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(label)
        view.addSubview(imageView)

        label.topAnchor.constraint(equalTo: view.topAnchor, constant: 30).isActive = true
        label.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 20).isActive = true
        label.widthAnchor.constraint(greaterThanOrEqualToConstant: 50).isActive = true

        imageView.topAnchor.constraint(equalTo: label.bottomAnchor).isActive = true
        imageView.leftAnchor.constraint(equalTo: view.leftAnchor, constant: 60).isActive = true
        imageView.widthAnchor.constraint(equalTo: label.widthAnchor).isActive = true
        imageView.heightAnchor.constraint(equalToConstant: 70).isActive = true

        print("Label's top achor constraints: \(label.constraints(on: label.topAnchor))")
        print("Label's width achor constraints: \(label.constraints(on: label.widthAnchor))")
        print("ImageView's width achor constraints: \(imageView.constraints(on: imageView.widthAnchor))")
    }

}

public extension UIView {

    public func constraints(on anchor: NSLayoutYAxisAnchor) -> [NSLayoutConstraint] {
        guard let superview = superview else { return [] }
        return superview.constraints.filtered(view: self, anchor: anchor)
    }

    public func constraints(on anchor: NSLayoutXAxisAnchor) -> [NSLayoutConstraint] {
        guard let superview = superview else { return [] }
        return superview.constraints.filtered(view: self, anchor: anchor)
    }

    public func constraints(on anchor: NSLayoutDimension) -> [NSLayoutConstraint] {
        guard let superview = superview else { return [] }
        return constraints.filtered(view: self, anchor: anchor) + superview.constraints.filtered(view: self, anchor: anchor)
    }

}

extension NSLayoutConstraint {

    func matches(view: UIView, anchor: NSLayoutYAxisAnchor) -> Bool {
        if let firstView = firstItem as? UIView, firstView == view && firstAnchor == anchor {
            return true
        }
        if let secondView = secondItem as? UIView, secondView == view && secondAnchor == anchor {
            return true
        }
        return false
    }

    func matches(view: UIView, anchor: NSLayoutXAxisAnchor) -> Bool {
        if let firstView = firstItem as? UIView, firstView == view && firstAnchor == anchor {
            return true
        }
        if let secondView = secondItem as? UIView, secondView == view && secondAnchor == anchor {
            return true
        }
        return false
    }

    func matches(view: UIView, anchor: NSLayoutDimension) -> Bool {
        if let firstView = firstItem as? UIView, firstView == view && firstAnchor == anchor {
            return true
        }
        if let secondView = secondItem as? UIView, secondView == view && secondAnchor == anchor {
            return true
        }
        return false
    }
}

extension Array where Element == NSLayoutConstraint {

    func filtered(view: UIView, anchor: NSLayoutYAxisAnchor) -> [NSLayoutConstraint] {
        return filter { constraint in
            constraint.matches(view: view, anchor: anchor)
        }
    }
    func filtered(view: UIView, anchor: NSLayoutXAxisAnchor) -> [NSLayoutConstraint] {
        return filter { constraint in
            constraint.matches(view: view, anchor: anchor)
        }
    }
    func filtered(view: UIView, anchor: NSLayoutDimension) -> [NSLayoutConstraint] {
        return filter { constraint in
            constraint.matches(view: view, anchor: anchor)
        }
    }

}

Have you tried saving the relevant constraints you created using the layout anchors to properties, and then just changing the constant? E.g.

var ticketTop : NSLayoutConstraint?

func setup() {
    ticketTop = ticketContainer.topAnchor.constraintEqualToAnchor(self.topAnchor, constant:100)
    ticketTop.active = true
}

func update() {
    ticketTop?.constant = 25
}

Possibly More Elegant

Depending on your taste for writing extensions, here is a possibly more elegant approach that doesn't use properties, but instead creates extension methods on NSLayoutAnchor and UIView to aid in more succinct usage.

First you would write an extension on NSLayoutAnchor like this:

extension NSLayoutAnchor {
    func constraintEqualToAnchor(anchor: NSLayoutAnchor!, constant:CGFloat, identifier:String) -> NSLayoutConstraint! {
        let constraint = self.constraintEqualToAnchor(anchor, constant:constant)
        constraint.identifier = identifier
        return constraint
    }
}

This extension allows you to set an identifier on the constraint in the same method call that creates it from an anchor. Note that Apple documentation implies that XAxis anchors (left, right, leading, etc.) won't let you create a constraint with YAxis anchors (top, bottom, etc.), but I don't observe this to actually be true. If you did want that type of compiler checking, you would need to write separate extensions for NSLayoutXAxisAnchor, NSLayoutYAxisAnchor, and NSLayoutDimension (for width and height constraints) that enforce the same-axis anchor type requirement.

Next you would write an extension on UIView to get a constraint by identifier:

extension UIView {
    func constraint(withIdentifier:String) -> NSLayoutConstraint? {
        return self.constraints.filter{ $0.identifier == withIdentifier }.first
    }
}

With these extensions in place, your code becomes:

func setup() {
    ticketContainer.topAnchor.constraintEqualToAnchor(anchor: self.topAnchor, constant:100, identifier:"ticketTop").active = true
}

func update() {
    self.constraint(withIdentifier:"ticketTop")?.constant = 25
}

Note that using constants or an enum instead of magic string names for the identifiers would be an improvement over the above, but I'm keeping this answer brief and focused.