use slider to change nslayout constraint size - swift

Question

I want to use the slider slizer to change the size of the uiview pic. With Nslayout constraints it creates a fixed object so I added to constraints as vars. So I assume in the increase func the startCon will become deactivated and the changeCon will become activated. I have not used constraints as var in a while and dont know what to do. But I want to position pic using nslayout constraints and then change the width and height together using the slider.
import UIKit
class ViewController: UIViewController{
var pic = UIView();var slizer = UISlider()
var startCon: NSLayoutConstraint!
var changeCon: NSLayoutConstraint!
override func viewDidLoad() {
[pic,slizer].forEach{
$0.translatesAutoresizingMaskIntoConstraints = false
view.addSubview($0)
}
NSLayoutConstraint.activate([
pic.centerYAnchor.constraint(equalTo: view.centerYAnchor),
pic.centerXAnchor.constraint(equalTo: view.centerXAnchor),
pic.heightAnchor.constraint(equalTo: view.heightAnchor,multiplier: 0.3),
pic.widthAnchor.constraint(equalTo: view.widthAnchor,multiplier: 0.3),
slizer.bottomAnchor.constraint(equalTo: view.bottomAnchor),
slizer.leadingAnchor.constraint(equalTo: view.leadingAnchor),
slizer.heightAnchor.constraint(equalTo: view.heightAnchor,multiplier: 0.2),
slizer.widthAnchor.constraint(equalTo: view.widthAnchor,multiplier: 1),
])
pic.backgroundColor = .orange
slizer.addTarget(self, action: #selector(increase), for: .touchDown)
}
#objc func increase(){
//increase decrease size of pic
}
}

Answer 1

It's impossible to modify an activated constraints. You have to disable your contraints and set a new one.
var pic = UIView()
var slizer = UISlider()
var slidermultiplier: CGFloat = 0.3
var widthConstraints: NSLayoutConstraint?
override func viewDidLoad() {
super.viewDidLoad()
widthConstraints = pic.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: slidermultiplier)
[pic,slizer].forEach{
$0.translatesAutoresizingMaskIntoConstraints = false
view.addSubview($0)
}
NSLayoutConstraint.activate([
pic.centerYAnchor.constraint(equalTo: view.centerYAnchor),
pic.centerXAnchor.constraint(equalTo: view.centerXAnchor),
pic.heightAnchor.constraint(equalTo: view.heightAnchor,multiplier: 0.3),
widthConstraints!,
slizer.bottomAnchor.constraint(equalTo: view.bottomAnchor),
slizer.leadingAnchor.constraint(equalTo: view.leadingAnchor),
slizer.heightAnchor.constraint(equalTo: view.heightAnchor,multiplier: 0.2),
slizer.widthAnchor.constraint(equalTo: view.widthAnchor,multiplier: 1),
])
pic.backgroundColor = .orange
// changed to .valueChanged
slizer.addTarget(self, action: #selector(increase), for: .valueChanged)
}
#objc func increase() {
slidermultiplier = CGFloat(slizer.value)
widthConstraints?.isActive = false
widthConstraints = pic.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: slidermultiplier)
widthConstraints?.isActive = true
}
and you can do the same with the height.
You can also set min and max value to your UISlider to have a Range of multiplier.

Answer 2

You can hold properties that are responsible for image size, change those values in slider action. For example:
change value pic.heightAnchor.constraint's multiplier in method increase action.