如何在swift 4中动态创建控件并动态对齐？

let label = UILabel()

label.text = "Hello World"
label.textColor = UIColor.black

self.view.addSubview(label)

label.translatesAutoresizingMaskIntoConstraints = false
NSLayoutConstraint.activate([
    label.centerXAnchor.constraint(equalTo: self.view.centerXAnchor),
    label.centerYAnchor.constraint(equalTo: self.view.centerYAnchor)
])

import UIKit

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        
        let label = UILabel()
        label.text = "Hello World"
        label.textColor = UIColor.black
        label.translatesAutoresizingMaskIntoConstraints = false
        self.view.addSubview(label)
        
        NSLayoutConstraint.activate([
            label.centerXAnchor.constraint(equalTo: self.view.centerXAnchor),
            label.centerYAnchor.constraint(equalTo: self.view.centerYAnchor)
        ])
    }
}