All code has an entry point: the place where whoever calls that code actually calls. How does the whole program, comprising many Swift files, actually get started? We need an entry point for the whole program, which the runtime will call to launch us.
In Swift, this is the main.swift file. Its job is to call UIApplicationMain, which creates some instances including the app and the app delegate and gets the event loop running (and stays running for the rest of the time the app runs). A minimal main.swift file would have to look like this:
import UIKit
UIApplicationMain(
CommandLine.argc, CommandLine.unsafeArgv, nil,
NSStringFromClass(AppDelegate.self)
)
However, no one ever uses a main.swift file! It's boilerplate, so why bother? Instead, you say @main, and a main.swift file is generated for you behind the scenes. In particular, you put the attribute @main on your AppDelegate class, so the main.swift generator knows which class to instantiate as your application delegate.
Two more things to know:
Before Swift 5.3, @main was called @UIApplicationMain instead. From that point of view, they are identical, two names for the same thing.
New in Swift 5.3 and Xcode 12, you can designate one of your own types as @main and give it a static main function, where you do whatever you would have done in the main.swift file. That is something @UIApplicationMain cannot do:
@main
struct MyMain {
static func main() -> Void {
UIApplicationMain(
CommandLine.argc, CommandLine.unsafeArgv, nil, NSStringFromClass(AppDelegate.self)
)
}
}
