How to use constraints using code on UITextField in swift

Question

I've done this by xcode without code, just grab it with control key and select add constraint, but now i need to do this using code, so the code looks like this:

class RegistrationViewController: UIViewController, UITextFieldDelegate {

    // UserInputs extends UITextField
    var firstName: UserInputs!
    var lastName: UserInputs!

    override func viewDidLoad() {
        super.viewDidLoad()
        firstName = UserInputs(frame: CGRectMake(75, 150, 230, 40))
        lastName = UserInputs(frame: CGRectMake(75, 200, 230, 40))
        addPlaceHolder(firstName, name: "Firstname")
        addPlaceHolder(lastName, name: "Lastname")
        view.addSubview(firstName!)
        view.addSubview(lastName!)
    }
    func addPlaceHolder(textField: UserInputs, name: String){
        textField.attributedPlaceholder = NSAttributedString(string: name,
            attributes:[NSForegroundColorAttributeName: UIColor.whiteColor()])
    }
}

i want to make my textfields to go center horizontally i'm reading a lot about it but couldn't find anything yet

i've read something and solved it by doing this:

func doThis(textField: UserInputs){
    textField.translatesAutoresizingMaskIntoConstraints = false
    let bounds = UIScreen.mainScreen().bounds
    let width = bounds.size.width
    //let height = bounds.size.height
    //230 because width of my textField is 230
    let left = (width - 230)/2
    let right = -((width - 230)/2)
    textField.addConstraint(
        NSLayoutConstraint(
            item: textField,
            attribute: NSLayoutAttribute.Height,
            relatedBy: NSLayoutRelation.Equal,
            toItem: nil,
            attribute: NSLayoutAttribute.NotAnAttribute,
            multiplier: 1.0,
            constant: 40
        ))

    //left
    self.view.addConstraint(
        NSLayoutConstraint(
            item: textField,
            attribute: NSLayoutAttribute.Leading,
            relatedBy: NSLayoutRelation.Equal,
            toItem: self.view,
            attribute: NSLayoutAttribute.Leading,
            multiplier: 1.0,
            constant: left
        ))

    //right
    self.view.addConstraint(
        NSLayoutConstraint(
            item: textField,
            attribute: NSLayoutAttribute.Trailing,
            relatedBy: NSLayoutRelation.Equal,
            toItem: self.view,
            attribute: NSLayoutAttribute.Trailing,
            multiplier: 1.0,
            constant: right
        ))

    //top
    self.view.addConstraint(
        NSLayoutConstraint(
            item: textField,
            attribute: NSLayoutAttribute.Top,
            relatedBy: NSLayoutRelation.Equal,
            toItem: self.view,
            attribute: NSLayoutAttribute.Top,
            multiplier: 1.0,
            constant: 300
        ))
}