i can check my sample view on the X-code. can i know my fault in my code?

class MyViewController: UIViewController {

    let sampleView = UIView()

    override func viewDidLoad() {
        super.viewDidLoad()
        view.addSubview(sampleView)
    }

    override func viewWillLayoutSubviews() {
        super.viewWillLayoutSubviews()
        sampleView.translatesAutoresizingMaskIntoConstraints = false

        // Add constraints below
        let sampleViewCenterX = NSLayoutConstraint(item: sampleView, attribute: .CenterX, relatedBy: .Equal, toItem: view, attribute: .CenterX, multiplier: 1.0, constant: 0.0)
        let sampleViewCenterY = NSLayoutConstraint(item: sampleView, attribute: .CenterY, relatedBy: .Equal, toItem: view, attribute: .CenterY, multiplier: 1.0, constant: 0.0)
        let sampleViewHeight = NSLayoutConstraint(item: sampleView, attribute: .Height, relatedBy: .Equal, toItem: nil, attribute: .Height, multiplier: 1.0, constant: 50.0)
        let sampleViewWidth = NSLayoutConstraint(item: sampleView, attribute: .Width, relatedBy: .Equal, toItem: nil, attribute: .Width, multiplier: 1.0, constant: 50.0)

        view.addConstraints([sampleViewCenterX, sampleViewCenterY, sampleViewHeight, sampleViewWidth])

    }
}