Expressions are not allowed at the top level - swift

Question

I am a beginning Swift programmer.
The following code seems to compile fine in Xcode 7.0 Playground (no visible errors):
//: Playground - noun: a place where people can play
//#!/usr/bin/env xcrun swift
import WebKit
let application = NSApplication.sharedApplication()
application.setActivationPolicy(NSApplicationActivationPolicy.Regular)
let window = NSWindow()
window.setContentSize(NSSize(width:800, height:600))
window.styleMask = NSTitledWindowMask | NSClosableWindowMask | NSMiniaturizableWindowMask
window.center()
window.title = "Minimal Swift WebKit Browser"
window.makeKeyAndOrderFront(window)
class WindowDelegate: NSObject, NSWindowDelegate {
func windowWillClose(notification: NSNotification) {
NSApplication.sharedApplication().terminate(0)
}
}
let windowDelegate = WindowDelegate()
window.delegate = windowDelegate
class ApplicationDelegate: NSObject, NSApplicationDelegate {
var _window: NSWindow
init(window: NSWindow) {
self._window = window
}
func applicationDidFinishLaunching(notification: NSNotification) {
let webView = WebView(frame: self._window.contentView!.frame)
self._window.contentView!.addSubview(webView)
webView.mainFrame.loadRequest(NSURLRequest(URL: NSURL(string: "http://www.apple.com")!))
}
}
When pasting that exact same code into the "AppDelegate.swift" file of a new Cocoa application for OSX, I get 7 errors, all exactly the same: "Expressions are not allowed at the top level".
Through searching I've deduced that the Playground allows things that normal projects do not and the errors are occurring because the expressions are "outside of a class or instance method".
However I'm not sure how the program could be modified in order to build correctly.

Answer 1

Yes, normal projects do not allow code at the top level, because there is no obvious time for it to run. You need to decide when your activation policy and window/delegate code should run (that is, move that code inside of a method). I suggest applicationDidFinishLaunching(_:), as it is called when your app is finished launching and is a common place to do this kind of setup. The finished code would read:
import WebKit
class WindowDelegate: NSObject, NSWindowDelegate {
func windowWillClose(notification: NSNotification) {
NSApplication.sharedApplication().terminate(0)
}
}
class ApplicationDelegate: NSObject, NSApplicationDelegate {
var _window: NSWindow
init(window: NSWindow) {
self._window = window
}
func applicationDidFinishLaunching(notification: NSNotification) {
let webView = WebView(frame: self._window.contentView!.frame)
self._window.contentView!.addSubview(webView)
webView.mainFrame.loadRequest(NSURLRequest(URL: NSURL(string: "http://www.apple.com")!))
let application = NSApplication.sharedApplication()
application.setActivationPolicy(NSApplicationActivationPolicy.Regular)
let window = NSWindow()
window.setContentSize(NSSize(width:800, height:600))
window.styleMask = NSTitledWindowMask | NSClosableWindowMask | NSMiniaturizableWindowMask
window.center()
window.title = "Minimal Swift WebKit Browser"
window.makeKeyAndOrderFront(window)
let windowDelegate = WindowDelegate()
window.delegate = windowDelegate
}
}