swift 3: How to center align (vertically & horizontally) UIButton on UIView?

Question

I'm trying to create an image UIButton and set its position programmatically vs using Interface Builder. The reason being I'll be showing/hiding the button based on some logic in ViewController. However, i've spent the past ~4hrs searching to no avail. I have tried following approaches:

1. Constraints: seemingly the recommended method
2. Setting UIButton.center property to CGPoint(x: self.view.frame.width / 2, y: self.view.frame.height / 2)

Here's the full code:

    class ViewController: UIViewController{
    //this is here so i can access playButton across different functions that i'll define in this ViewController
    let playButton:UIButton = {
      let play_btn = UIButton()
        play_btn.setImage(UIImage(named:"my_button_image.png"), for: UIControlState.normal)
        play_btn.translatesAutoresizingMaskIntoConstraints = false
        return play_btn
    }()

    override func viewDidLoad() {

       super.viewDidLoad()


       playButton.addTarget(self, action:#selector(self.toggle), for: .touchDown)
       let centerX = NSLayoutConstraint(item: self.playButton, attribute: NSLayoutAttribute.centerX, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.centerX, multiplier: 1, constant: 0)
       let centerY = NSLayoutConstraint(item: self.playButton, attribute: NSLayoutAttribute.centerY, relatedBy: NSLayoutRelation.equal, toItem: self.view, attribute: NSLayoutAttribute.centerY, multiplier: 1, constant: 0)

       self.view.addConstraints([centerX, centerY])

       view.addSubview(playButton)

    }
}

I've also tried doing: playButton.center = CGPoint(x: self.view.frame.width / 2, y: self.view.frame.height / 2)

as well as:

playButton.center.x = self.view.center.x playButton.center.y = self.view.center.y

None of the above works :(

Any help/advice/suggestions are much appreciated!

Answer 1

When I tried your code, it crashed. The problem was you were trying to add constraints before the view is initialized, adding constraints should be inside viewWillLayoutSubviews()

In any case I would suggest to create the button simpler, i.e. To make the button centering the view

let btn = UIButton(frame: CGRect(x:0 , y:0, width:100, heigth: 60))
btn.center = self.view.center
self.view.addSubview(btn)

The above will automatically constraints the button to the center of the view

Hope it helps!