Aspect fit in cgsize programmatically in swift

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Need help fitting label so that the left and right side fills. Code is at the bottom.

  @IBOutlet weak var header: UILabel!

override func viewDidLoad() {
    header.adjustsFontSizeToFitWidth = true

    let rectShape = CAShapeLayer()
    rectShape.bounds = self.header.frame
    rectShape.position = self.header.center
    rectShape.path = UIBezierPath(roundedRect: self.header.bounds, byRoundingCorners: [.bottomLeft , .bottomRight], cornerRadii: CGSize(width:300, height: 200)).cgPath

    self.header.layer.backgroundColor = UIColor.green.cgColor
    //Here I'm masking the textView's layer with rectShape layer
    self.header.layer.mask = rectShape


    super.viewDidLoad()

You can solve this issue by put the code block into the viewDidAppear(_:). In this method, the size will be corrected.

@IBOutlet weak var header: UILabel!

override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)

    header.adjustsFontSizeToFitWidth = true

    let rectShape = CAShapeLayer()
    rectShape.bounds = self.header.frame
    rectShape.position = self.header.center
    rectShape.path = UIBezierPath(roundedRect: self.header.bounds, byRoundingCorners: [.bottomLeft , .bottomRight], cornerRadii: CGSize(width:300, height: 200)).cgPath

    self.header.layer.backgroundColor = UIColor.green.cgColor
    //Here I'm masking the textView's layer with rectShape layer
    self.header.layer.mask = rectShape
}