Why is superview returning nil? [duplicate] - swift

Question

This question already has answers here:
Displaying instances of view controllers within Xcode without disturbing the current hierarchy
(1 answer)
Adding a view controller as a subview in another view controller
(8 answers)
Closed 1 year ago.
At the very bottom is my code where MainController initiates a subview called setController. However, within setController's code I get back a nil when I try to the superview:
override func viewDidLoad() {
super.viewDidLoad()
print(self.view.superview ?? "no parent")
}
I am assigning the second view as a subview, but obviously I am missing something. Have I misunderstood how UIView hierarchy works?
class MainController: UIViewController {
private lazy var setController = SetController()
var invButton : MyButton!
override func viewDidLoad() {
super.viewDidLoad()
view.backgroundColor = .black
invButton = makeButton(vControl: self, btype: ButtType.inv, action: #selector(self.buttonAction(sender:)))
invButton.frame.origin.x = self.view.frame.width * 0.1
invButton.frame.origin.y = self.view.frame.height * 0.1
invButton.setTitle("Settings", for: .normal)
}
override var prefersStatusBarHidden: Bool {
return false
}
#objc func buttonAction(sender: UIButton!) {
guard let theButton = sender as? MyButton else { return}
UIView.transition(with: self.view, duration: 0.5, options: .transitionCurlDown, animations: { [self] in
self.view.addSubview(setController.view)
}, completion: nil)
}
}

Answer 1

For anyone coming across this problem as well, I found the answer here:
https://guides.codepath.com/ios/Adding-and-Removing-Child-View-Controllers
The error was mine in that I didn't realize that adding a subview was a multiple line process.
So self.view.addSubview(setController.view) became:
addChild(setController);
view.addSubview(setController.view);
setController.didMove(toParent: self);