KVO not working for `UIView.bounds` keypath when view resizing happens due to `autoresizingmask`

Question

I have following view controller:

class ViewController: UIViewController {

  lazy var superView: UIView = {
    let view = UIView(frame: CGRect(x: 0, y: 0, width: 300, height: 300))
    view.backgroundColor = UIColor.white
    return view
  }()

  lazy var childView: UIView = {
    let view = UIView(frame: CGRect(x: 0, y: 0, width: 200, height: 200))
    view.backgroundColor = UIColor.black
    return view
  }()

  var boundObservation: NSKeyValueObservation?

  override func viewDidLoad() {
    super.viewDidLoad()
    self.view.backgroundColor = UIColor.orange

    self.view.addSubview(self.superView)
    self.superView.center = CGPoint(
      x: self.view.frame.width / 2.0,
      y: self.view.frame.height / 2.0
    )

    self.superView.addSubview(self.childView)
    self.childView.center = CGPoint(
      x: self.superView.frame.width / 2.0,
      y: self.superView.frame.height / 2.0
    )
    self.childView.autoresizingMask = .flexibleWidth

    let button = UIButton(type: .system)
    button.setTitle("Tap me!", for: .normal)
    button.addTarget(self, action: #selector(buttonTapped(_:)), for: .touchUpInside)
    self.view.addSubview(button)
    button.translatesAutoresizingMaskIntoConstraints = false
    button.centerXAnchor.constraint(equalTo: self.view.centerXAnchor).isActive = true
    button.bottomAnchor.constraint(equalTo: self.view.safeAreaLayoutGuide.bottomAnchor, constant: -24).isActive = true

    self.boundObservation = self.childView.observe(
      \UIView.bounds,
      options: .new,
      changeHandler: { view, change in
        print(change.newValue ?? "nil")
    })
  }

  @objc func buttonTapped(_ sender: UIButton) {
    let view = self.superView
    let previousSize = view.bounds.size
    view.bounds.size = CGSize(width: previousSize.width + 50, height: previousSize.height + 20)
  }
}

My childView get's automatically resized due to autoresizing mask when superView is resized. But KVO observation is not getting called. Why is KVO not working in this case?

Answer 1

My issue was solved by observing \.layer.bounds instead of \UIView.bounds because I learned that the layer is the “source of truth” for a view’s layout, so I suspected the autoresizing mask was directly modifying the layer bounds without going through UIView.bounds