swift - What does "@UIApplicationMain" mean?

Question

I just created my first Swift project, in the AppDelegate.swift there is a line above a class declaration - why is it there?!

...
import UIKit
import CoreData

@UIApplicationMain // <- WHY IS IT HERE?
class AppDelegate: UIResponder, UIApplicationDelegate {
... 

Answer 1

The @UIApplicationMain attribute in Swift replaces the trivial main.m file found in Objective-C projects (whose purpose is to implement the main function that's the entry point for all C programs and call UIApplicationMain to kick off the Cocoa Touch run loop and app infrastructure).

In Objective-C, the main (heh) bit of per-app configuration that the UIApplicationMain function provides is designating one of your app's custom classes as the delegate of the shared UIApplication object. In Swift, you can easily designate this class by adding the @UIApplicationMain attribute to that class' declaration. (You can also still invoke the UIApplicationMain function directly if you have reason to. In Swift you put that call in top-level code in a main.swift file.)

@UIApplicationMain is for iOS only. In OS X, the app delegate is traditionally set in the main nib file designated by the Info.plist (the same for Swift as for ObjC) — but with OS X storyboards there's no main nib file, so @NSApplicationMain does the same thing there.