flip.swift

class ViewController: NSViewController {

  @IBOutlet weak var button: NSButton!
  
  override func viewDidLoad() {
    super.viewDidLoad()
  }
  
  @IBAction func buttonClicked(_ sender: NSButton) {
    let centerX = sender.frame.origin.x + (sender.frame.size.width / 2)
    let centerY = sender.frame.origin.y + (sender.frame.size.height / 2)
    
    var transform = sender.layer?.affineTransform()
    transform = transform?.translatedBy(x: -centerX, y: -centerY)
    transform = transform?.rotated(by: 180.0 * (CGFloat.pi / 180))
    transform = transform?.translatedBy(x: centerX, y: centerY)
    
    sender.layer?.setAffineTransform(transform!)
  }
}