var constraints not appearing when assigned to a function that creates a view

Question

I want my swift code to use var constraints to let the objects added to the code. I have added a gif below of what I am looking for. Right now the code below compiles but the black view you can see in the gif below is not appearing in the code below. In my code below the black box is not appearing at all.

import UIKit
public class ViewController : UIViewController {
    var slider = UISlider()
    var image1Width2: NSLayoutConstraint!
    var iHieght: NSLayoutConstraint!
    public override func viewDidLoad() {
        super.viewDidLoad()
        slider.value = 0.5
        slider.translatesAutoresizingMaskIntoConstraints = false
        view.addSubview(slider)
        slider.isUserInteractionEnabled = true
        slider.bottomAnchor.constraint(equalTo: view.bottomAnchor).isActive = true
        slider.leadingAnchor.constraint(equalTo: view.leadingAnchor).isActive = true
        slider.heightAnchor.constraint(equalTo: view.heightAnchor,multiplier: 0.1).isActive = true
        slider.widthAnchor.constraint(equalTo: view.widthAnchor,multiplier: 1).isActive = true
        
        button.frame = .init(x: self.view.bounds.midX,
                             y: 0,
                             width: 100,
                             height: 100)
        self.view.addSubview(button)
        image1Width2 =  view.widthAnchor.constraint(equalTo:  view.widthAnchor ,multiplier:  0.06)
        iHieght =  view.widthAnchor.constraint(equalTo:  view.heightAnchor ,multiplier:  0.06)
        slider.addTarget(self, action: #selector(increase), for: .allEvents)
    }
    
    private lazy var button: UIButton = {
        let button = UIButton()
        button.backgroundColor = .blue
        button.setTitleColor(.white, for: .normal)
        button.setTitle("add", for: .normal)
        button.addTarget(self,
                         action: #selector(addBlackView),
                         for: .touchUpInside)
        return button
    }()
    
    private func getBlackView() -> UIView {
        let view = UIView()
        view.backgroundColor = .black
     
    
        image1Width2 =  view.widthAnchor.constraint(equalTo:  view.widthAnchor ,multiplier:  0.06)
        iHieght =  view.widthAnchor.constraint(equalTo:  view.heightAnchor ,multiplier:  0.06)
    
        let recognizer = UIPanGestureRecognizer(target: self, action: #selector(moveView(_:)))
        view.addGestureRecognizer(recognizer)
        return view
    }
    
    @objc
    private func addBlackView() {
        let view = getBlackView()
        self.view.addSubview(view)
    }
    
    @objc
    private func moveView(_ recognizer: UIPanGestureRecognizer) {
        switch recognizer.state {
        case .began:
            print("gesture began")
        case .changed:
            let translation = recognizer.translation(in: self.view)

            recognizer.view!.center = .init(x: recognizer.view!.center.x + translation.x,
                                            y: recognizer.view!.center.y + translation.y)
            recognizer.setTranslation(.zero, in: self.view)
        default:
            break
        }
    }



@objc func increase() {


        image1Width2.constant = CGFloat(slider.value) * view.frame.size.width * 0.10
    iHieght.constant = CGFloat(slider.value) * view.frame.size.width * 0.10
    






}


}

Answer 1

You are doing couple of mistakes when writing this code.Mainly iOS basic principles Please refer this modified code, modify it and get your result.

class sampleViewController: UIViewController {

var image1Width2: NSLayoutConstraint!
var iHieght: NSLayoutConstraint!

override func viewDidLoad() {
    super.viewDidLoad()
    view.backgroundColor = .white

    view.addSubview(slider)
    slider.translatesAutoresizingMaskIntoConstraints = false
    slider.value = 0.5
    slider.isUserInteractionEnabled = true
    NSLayoutConstraint.activate([
        slider.bottomAnchor.constraint(equalTo: view.safeAreaLayoutGuide.bottomAnchor),
        slider.leadingAnchor.constraint(equalTo: view.leadingAnchor),
        slider.heightAnchor.constraint(equalToConstant: 100),
        slider.widthAnchor.constraint(equalTo: view.widthAnchor,multiplier: 1),
    ])

    view.addSubview(button)
    button.translatesAutoresizingMaskIntoConstraints = false
    NSLayoutConstraint.activate([
        button.trailingAnchor.constraint(equalTo: view.trailingAnchor, constant: -16),
        button.topAnchor.constraint(equalTo: view.safeAreaLayoutGuide.topAnchor, constant: 16),
        button.widthAnchor.constraint(equalToConstant: 100),
        button.heightAnchor.constraint(equalToConstant: 80),
    ])
    button.addTarget(self,action: #selector(addBlackView),for: .touchUpInside)
    slider.addTarget(self, action: #selector(increase), for: .allEvents)

}

let slider:UISlider = {
    let slider = UISlider(frame: .zero)
    return slider
}()

private lazy var button: UIButton = {
    let button = UIButton()
    button.backgroundColor = .blue
    button.setTitleColor(.white, for: .normal)
    button.setTitle("add", for: .normal)
    return button
}()

let blackView: UIView = {
    let view = UIView()
    view.backgroundColor = .black
    return view
}()

@objc
private func addBlackView() {
    self.view.addSubview(blackView)
    blackView.translatesAutoresizingMaskIntoConstraints = false
    blackView.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
    blackView.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
    image1Width2 = blackView.widthAnchor.constraint(equalTo: view.widthAnchor, multiplier: 0.1)
    image1Width2.isActive = true
    iHieght = blackView.heightAnchor.constraint(equalTo: view.heightAnchor, multiplier: 0.1)
    iHieght.isActive = true
    view.layoutIfNeeded()

    let recognizer = UIPanGestureRecognizer(target: self, action: #selector(moveView(_:)))
    blackView.addGestureRecognizer(recognizer)
}

@objc private func moveView(_ recognizer: UIPanGestureRecognizer) {
    switch recognizer.state {
    case .began:
        print("gesture began")
    case .changed:
        let translation = recognizer.translation(in: self.view)

        recognizer.view!.center = .init(x: recognizer.view!.center.x + translation.x,
                                        y: recognizer.view!.center.y + translation.y)
        recognizer.setTranslation(.zero, in: self.view)
    default:
        break
    }
}

@objc func increase() {
    image1Width2.constant = CGFloat(slider.value) * view.frame.size.width * 0.10
    iHieght.constant = CGFloat(slider.value) * view.frame.size.width * 0.10
}}