added it to the parent view, doesn't seem to work?

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 sampleViewHeightConstraint = NSLayoutConstraint(item: sampleView, attribute: .Height, relatedBy: .Equal, 
        toItem: nil, attribute: .CenterX, multiplier: 1.0, constant: 50.0)

        let sampleViewWidthConstraint = NSLayoutConstraint(item: sampleView, attribute: .Width, relatedBy: .Equal, 
        toItem: nil, attribute: .CenterX, multiplier: 1.0, constant: 50.0)

        view.addConstraints([sampleViewHeightConstraint,sampleViewWidthConstraint])
    }
}