▶ 运行代码import UIKit
class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        let label = UILabel(frame: CGRect(x: 0, y: 0, width: 200, height: 50))
        label.text = "Hello, iOS!"
        label.center = view.center
        view.addSubview(label)
    }
}