Demo of a button that changes a constraint

Autolayout Animation

//: A UIKit based Playground for presenting user interface
  
import UIKit
import PlaygroundSupport

class MyViewController : UIViewController {
    var innerViewLeadingConstraint: NSLayoutConstraint?
    var innerViewCenterXConstraint: NSLayoutConstraint?
    var innerView: UIView?
    
    @objc
    func animate(sender: AnyObject)
    {
        guard let innerView = self.innerView,
            let innerViewLeadingConstraint = self.innerViewLeadingConstraint,
            let innerViewCenterXConstraint = self.innerViewCenterXConstraint else {
            return
        }
        
        self.view.layoutIfNeeded()
        UIView.animate(withDuration: 1.8, animations: {
            if self.view.constraints.contains(innerViewCenterXConstraint) {
                self.view.removeConstraint(innerViewCenterXConstraint)
                self.view.addConstraint(innerViewLeadingConstraint)
            } else {
                self.view.removeConstraint(innerViewLeadingConstraint)
                self.view.addConstraint(innerViewCenterXConstraint)
            }
            self.view.layoutIfNeeded()
        })
    }
    
    override func loadView() {
        let view = UIView()
        view.translatesAutoresizingMaskIntoConstraints = false
        view.backgroundColor = .gray
        
        self.view = view
    }
    
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let button = UIButton()
        button.translatesAutoresizingMaskIntoConstraints = false
        button.setTitle("Click", for: .normal)
        button.addTarget(self, action: #selector(animate(sender:)), for: .touchUpInside)
        self.view.addSubview(button)
        
        let innerView = UIView()
        innerView.translatesAutoresizingMaskIntoConstraints = false
        innerView.backgroundColor = .red
        self.view.addSubview(innerView)
        self.innerView = innerView
        
        let views:[String: Any] = ["view": view,
                     "innerView": innerView,
                     "button": button]

        let buttonCenterX = NSLayoutConstraint(item: button, attribute: .centerX, relatedBy: .equal, toItem: self.view, attribute: .centerX, multiplier: 1.0, constant: 0.0)
        self.view.addConstraint(buttonCenterX)
        
        let buttonVConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:|-16-[button]", metrics: nil, views: views)
        self.view.addConstraints(buttonVConstraints)
        
        innerView.addConstraint(NSLayoutConstraint(item: innerView, attribute: .width, relatedBy: .equal, toItem: nil, attribute: .notAnAttribute, multiplier: 1.0, constant: 150.0))
        
        let innerViewLeadingConstraint = NSLayoutConstraint(item: innerView, attribute: .leading, relatedBy: .equal, toItem: self.view, attribute: .leading, multiplier: 1.0, constant: 0.0)
        self.innerViewLeadingConstraint = innerViewLeadingConstraint
        
        let innerViewCenterXConstraint = NSLayoutConstraint(item: innerView, attribute: .centerX, relatedBy: .equal, toItem: self.view, attribute: .centerX, multiplier: 1.0, constant: 0.0)
        self.view.addConstraint(innerViewCenterXConstraint)
        self.innerViewCenterXConstraint = innerViewCenterXConstraint
        
        let innerViewVConstraints = NSLayoutConstraint.constraints(withVisualFormat: "V:|-100-[innerView]-50-|", metrics: nil, views: views)
        self.view.addConstraints(innerViewVConstraints)
    }
}
// Present the view controller in the Live View window
PlaygroundPage.current.needsIndefiniteExecution = true
PlaygroundPage.current.liveView = MyViewController()