Center View in superview - ios

Question

I'm trying to center my subview with a button in its superview. So I want the center of the subview be the center of the superview. I'm trying that with following code:
override func viewDidLoad() {
self.view.backgroundColor = UIColor.redColor()
var menuView = UIView()
var newPlayButton = UIButton()
//var newPlayImage = UIImage(named: "new_game_button_5cs")
var newPlayImageView = UIImageView(image: UIImage(named: "new_game_button_5cs"))
newPlayButton.frame = CGRectMake(0, 0, newPlayImageView.frame.width, newPlayImageView.frame.height)
newPlayButton.setImage(newPlayImage, forState: .Normal)
newPlayButton.backgroundColor = UIColor.whiteColor()
menuView.backgroundColor = UIColor.whiteColor()
menuView.addSubview(newPlayButton)
menuView.addConstraint(
NSLayoutConstraint(item: self.view,
attribute: .CenterX,
relatedBy: .Equal,
toItem: menuView,
attribute: .CenterX,
multiplier: 1, constant: 0)
)
}
Unfortunately the program breaks when I try to run it.
(Thread 1: signal SIGABRT)

Answer 1

Your code triggers an assertion saying:
When added to a view, the constraint's items must be descendants of
that view (or the view itself).
This means you have to add menuView as a subview to self.view before adding constraints. You should also add the constraints to self.view, not the menuView. Last but not least, remove autoresizing masks constraints that were implicitly added to menuView by calling setTranslatesAutoresizingMaskIntoConstraints(false) or autolayout will complain about conflicting constraints.
menuView.addSubview(newPlayButton)
menuView.setTranslatesAutoresizingMaskIntoConstraints(false)
self.view.addSubview(menuView)
self.view.addConstraint(
NSLayoutConstraint(item: self.view,
attribute: .CenterX,
relatedBy: .Equal,
toItem: menuView,
attribute: .CenterX,
multiplier: 1, constant: 0)
)