Swift Playgorund window

app.swift

import AppKit
import PlaygroundSupport
import SwiftUI

@objc protocol _NSWindow {
    var title: String? { get set }
    var styleMask: UInt { get set }
    func setFrame(_ frameRect: NSRect, display flag: Bool)
    func center()
    func makeKeyAndOrderFront(_ sender: Any?)
}

let _NSWindowStyleMaskClosable: UInt = 1 << 1

// A bit more roundabout than it needs to be: see https://bugs.swift.org/browse/SR-4243
let window = unsafeBitCast((NSClassFromString("NSWindow")! as! NSObject.Type).init(), to: _NSWindow.self)
window.styleMask |= _NSWindowStyleMaskClosable
window.title = "Hello from Swift Playgrounds!"
window.setFrame(CGRect(x: 0, y: 0, width: 300, height: 300), display: true)
window.center()
window.makeKeyAndOrderFront(nil)

PlaygroundPage.current.needsIndefiniteExecution = true