constraint object by percentage in a stackview - swift

Question

I want to create a constraint that is 80 percent of the width of the uiview controller not a 100 percent which is what it is now. I tried using the var percent but its not working. The code should be centered on the y axis. So 10 percent on the left side and ride side are not covered by the constraint. I am trying to do this in a stack view.
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view.
//Image View
let percent = ((UIScreen.main.bounds.midY) + (UIScreen.main.bounds.maxX * 0.8))
//Text Label
let textLabel = UILabel()
textLabel.backgroundColor = UIColor.yellow
textLabel.widthAnchor.constraint(equalToConstant: self.view.frame.width * percent).isActive = true
textLabel.heightAnchor.constraint(equalToConstant: 20.0).isActive = true
textLabel.text = "Hi World"
textLabel.textAlignment = .center
//Stack View
let stackView = UIStackView()
stackView.axis = NSLayoutConstraint.Axis.vertical
stackView.distribution = UIStackView.Distribution.equalSpacing
stackView.alignment = UIStackView.Alignment.center
stackView.spacing = 16.0
stackView.addArrangedSubview(textLabel)
stackView.translatesAutoresizingMaskIntoConstraints = false
self.view.addSubview(stackView)
//Constraints
stackView.centerXAnchor.constraint(equalTo: self.view.centerXAnchor).isActive = true
stackView.centerYAnchor.constraint(equalTo: self.view.centerYAnchor).isActive = true
}
}

Answer 1

Set the widthAnchor of stackView with that of superView's width (in this case view controller's view) and set multiplier value as 0.80
stackView.widthAnchor.constraint(equalTo: self.view.widthAnchor, multiplier: 0.80).isActive = true

Answer 2

You can achieve this by setting 'widthAnchor' and 'centerXAnchor' like below. We need to always keep horizontal center of child view and super view same. Whereas width of child should be 80 percentage(here 0.8 indicates 80%) of super view.
stackView.widthAnchor.constraint(equalTo: self.view.widthAnchor, multiplier: 0.8, constant: 0).isActive = true
stackView.centerXAnchor.constraint(equalTo: self.view.centerXAnchor).isActive = true
stackView.centerYAnchor.constraint(equalTo: self.view.centerYAnchor).isActive = true