Showing any NSWindowController as an app modal

ModalDialogController.swift

class ModalDialogController: NSWindowController {

    convenience init() {

        self.init(windowNibName: "ModalDialogController")
    }

    @IBOutlet var closeButton: NSButton!

    override func windowDidLoad() {

        super.windowDidLoad()

        self.window?.level = Int(CGWindowLevelForKey(CGWindowLevelKey.ModalPanelWindowLevelKey))
    }

    @IBAction func close(sender: AnyObject!) {

        self.close()
    }
}