Hello world GUI window application, updated to Swift 4 from: http://www.rockhoppertech.com/blog/swift-script-to-create-a-cocoa-window/

run.swift

#!/usr/bin/env swift

import Cocoa

// due to the new constraint syntax
@available(OSX 10.11, *)

class AppDelegate: NSObject, NSApplicationDelegate {
    let window = NSWindow()
    let windowDelegate = WindowDelegate()

    func applicationDidFinishLaunching(_ notification: Notification) {
        window.setContentSize(NSSize(width:600, height:600))
        window.styleMask.formUnion([.titled, .closable, .miniaturizable, .resizable])

        window.level = .normal
        window.delegate = windowDelegate
        window.isOpaque = false
        window.title = "Window with a button"
        let contentView = window.contentView!

        let button = NSButton(frame: NSRect(x:0, y:0, width:10, height:10))
        button.bezelStyle = .regularSquare
        button.title = "Click Me"
        button.action = #selector(AppDelegate.myAction(_:))
        contentView.addSubview(button)

        // autolayout
        button.translatesAutoresizingMaskIntoConstraints = false
        var c = button.centerYAnchor.constraint(equalTo: contentView.centerYAnchor)
        c.isActive = true
        c.identifier = "to center y"

        c = button.centerXAnchor.constraint(equalTo: contentView.centerXAnchor)
        c.isActive = true
        c.identifier = "to center x"

        c = button.widthAnchor.constraint(equalToConstant: 200)
        c.isActive = true
        c.identifier = "width"

        c = button.heightAnchor.constraint(equalToConstant: 50)
        c.isActive = true
        c.identifier = "height"

        window.center()
        window.makeKey()
        window.orderFrontRegardless()
    }

    @objc
    func myAction(_ sender: AnyObject) {
        print("my action")
    }
}

class WindowDelegate: NSObject, NSWindowDelegate {
    private func windowWillClose(notification: Notification) {
        NSApplication.shared.terminate(0)
    }
}

if #available(OSX 10.11, *)  {
    let app = NSApplication.shared
    let appDelegate = AppDelegate()
    app.delegate = appDelegate
    app.run()
}