a.swift

class ViewController: UIViewController {
    
    let box = UIView()
    override func viewDidLoad() {
        super.viewDidLoad()
        
        box.backgroundColor = UIColor.red
        view.addSubview(box)
    }
    
    override func viewDidLayoutSubviews() {
        super.viewDidLayoutSubviews()
        
        box.frame = CGRect(x: 20, y: 50, width: view.frame.size.width - 40, height: 100)
    }
}