How does AppDelegate.swift replace AppDelegate.h and AppDelegate.m in Xcode 6.3

Question

According to the iOS Developer Library

The app delegate is where you write your custom app-level code. Like all classes, the AppDelegate class is defined in two source code files in your app: in the interface file, AppDelegate.h, and in the implementation file, AppDelegate.m.

However, in Xcode 6.3 it appears that there is only AppDelegate.swift and no longer the .h and .m extensions. I would like to understand how the .swift replaced both the .h and .m extensions.

Answer 1

The simple answer is that AppDelegate.swift is just the translation from Objective-C AppDelegate.h and AppDelegate.m, as Swift does not require separate headers and implementations, but rather a single .swift file.

However, under the hood, there are other key differences between the two.

In Objective-C, there exists a main.m file that's sole purpose is instantiating UIApplicationMain

In Swift, the @UIApplicationMain annotation tag found at the top of AppDelegate.swift replaces the need for any main function that existed in the main.m file in Objective-C. If this tag is omitted, you can use a main.swift file to instantiate your UIApplication using the specified App Delegate.

A main.swift implementation looks like this:

import UIKit

autoreleasepool {
    UIApplicationMain(Process.argc, Process.unsafeArgv, nil, NSStringFromClass(AppDelegate.self))
}