Playgroundで動作するミニマムなUIViewControllerのサンプルコード

import UIKit
import PlaygroundSupport

class MyViewController : UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()

        self.view.backgroundColor = .white

        let label = UILabel()
        label.text = "Hello World!"
        label.sizeToFit()
        label.center = self.view.center

        self.view.addSubview(label)
    }
}

PlaygroundPage.current.liveView = MyViewController()