swift 5 autoresizingMask not working at all

Question

I'm familiar with Autolayout programmingly but I also want to try autoresizingMask programmingly. I have a green box created and want center it horizontally, but fixed distance for top and bottom, however whatever I tried any combination of autoresizingMask it never work.

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

    let myView = UIView(frame: CGRect(x: 175, y: 100, width: 30, height: 30))
    myView.backgroundColor = .green
    view.addSubview(myView)
    myView.autoresizingMask = [.flexibleRightMargin, .flexibleLeftMargin]
}

I tried it on iphone SE2 and iphone 11 Pro Max, the green block is centered on SE but never on Pro Max, anyone know what is going on?

Answer 1

What autoresizing does is to maintain the view according to its frame and the autoresizing mask. So the first step, if you want this view centered, is to center it:

myView.center = CGPoint(x:view.bounds.midX, y:view.bounds.midY)

Now make all four margins flexible, and the view will stay where you put it in relation to any screen size.

Answer 2

By default the view is rigid and has constant distance to the top left corner. You are looking for a flexibleWidth and Height

autoresizingMask = [.flexibleHeight, .flexibleWidth]