gistfile1.txt

class ViewController: UIViewController {
    
    class TestVC: UIViewController {
        override func viewDidLoad() {
            super.viewDidLoad()
            view.backgroundColor = .green
        }
    }
    
    let tvc = TestVC()

    override func viewDidLoad() {
        super.viewDidLoad()
    
        addChild(tvc)
        
        view.addSubview(tvc.view)
        
        tvc.view.translatesAutoresizingMaskIntoConstraints = false
        tvc.view.centerXAnchor.constraint(equalTo: view.centerXAnchor).isActive = true
        tvc.view.centerYAnchor.constraint(equalTo: view.centerYAnchor).isActive = true
        tvc.view.widthAnchor.constraint(equalToConstant: 200).isActive = true
        tvc.view.heightAnchor.constraint(equalToConstant: 200).isActive = true
        
        tvc.didMove(toParent: self)
    }
}