コマンドプロンプトで「$ swift ./minimal-app.swift」とすれば実行可能。パッケージ化の手順は後で別途記述。

minimal-app.swift

import Cocoa

// ApplicationDelegate
class ApplicationDelegate: NSObject, NSApplicationDelegate {
    var window: NSWindow!
    init(window: NSWindow) {
        self.window = window
    }
    func applicationDidFinishLaunching(aNotification: NSNotification) {
        // Insert code here to initialize your application
        NSApp.activateIgnoringOtherApps(true)
        window.center()
        window.makeKeyAndOrderFront(window)
    }
    func applicationWillTerminate(aNotification: NSNotification) {
        // Insert code here to tear down your application
    }
    func applicationShouldTerminateAfterLastWindowClosed(sender: NSApplication) -> Bool {
        return true
    }
}

// WindowDelegate
class WindowDelegate: NSObject, NSWindowDelegate {
    func windowWillClose(aNotification: NSNotification) {
        // Insert code here to close your application's window
    }
}

// Application Name
let appName = NSProcessInfo.processInfo().processName

//// Window
let windowContent  = NSMakeRect(0, 0, 480, 360)
let windowStyle    = NSTitledWindowMask | NSClosableWindowMask | NSMiniaturizableWindowMask | NSResizableWindowMask
let windowDelegate = WindowDelegate()
let window         = NSWindow(contentRect: windowContent, styleMask: windowStyle, backing: .Buffered, `defer`: false)
window.title    = appName
window.delegate = windowDelegate

// Menu
let menuBar  = NSMenu()
let menuItem = NSMenuItem()
let menuApp  = NSMenu()
let menuQuitTitle = "Quit \(appName)"
let menuQuitItem  = NSMenuItem(title:menuQuitTitle, action: "terminate:", keyEquivalent: "q")
menuApp.addItem(menuQuitItem)
menuItem.submenu = menuApp
menuBar.addItem(menuItem)

// Application
let application = NSApplication.sharedApplication()
let applicationDelegate = ApplicationDelegate(window: window)
application.setActivationPolicy(NSApplicationActivationPolicy.Regular)
application.mainMenu = menuBar
application.delegate = applicationDelegate
application.run()