What is the entry point of swift code execution?

There is no main() method in swift. The program must start the execution from somewhere. So what is the entry point of swift code execution and how is it decided?

The entry point in a plain Swift module is the file in the module called main.swift. main.swift is the only file which is allowed to have expressions and statements at the top level (all other Swift files in the module can only contain declarations).

Cocoa Touch uses the @UIApplicationMain attribute on an implementation of UIApplicationDelegate instead of a main.swift file to mark the entry point. Cocoa used to use a minimal main.swift file which simply called NSApplicationMain, but as of Xcode 6.1 uses the @NSApplicationMain attribute on an implementation of NSApplicationDelegate.

In the AppDelegate.swift file you can see @UIApplicationMain.
The AppDelegate is the initial entry file.

Basically: main.m and AppDelegate.m are kinda merged in Swift to just AppDelegate.swift

You may want to read Files and Initialization

> The exception is a special file named “main.swift”, which behaves much > like a playground file, but is built with your app’s source code. The > “main.swift” file can contain top-level code, and the order-dependent > rules apply as well. In effect, the first line of code to run in > “main.swift” is implicitly defined as the main entrypoint for the > program. This allows the minimal Swift program to be a single line — > as long as that line is in “main.swift”. > > In Xcode, Mac templates default to including a “main.swift” file, but > for iOS apps the default for new iOS project templates is to add > @UIApplicationMain to a regular Swift file. This causes the compiler > to synthesize a main entry point for your iOS app, and eliminates the > need for a “main.swift” file. > > Alternatively, you can link in an implementation of main written in > Objective-C, common when incrementally migrating projects from > Objective-C to Swift.

In Swift 5.3 there is a new @main attribute which lets you control where your entry point is in your project rather than just main.swift. There can only be one main entry and you can't have a main.swift file and a an attribute @main. See https://github.com/apple/swift-evolution/blob/master/proposals/0281-main-attribute.md for more details.

@main
struct App {
    static func main() {
        print("Starting.")
    }
}

In Swift apps there are attributes:

• @UIApplicationMain (Cocoa Touch)
• @NSApplicationMain (Cocoa)

that tell the swift compiler where is the entry point of the application.

What swift compiler does under the hood is that it creates a main function, which basically looks the same as in Objective-C apps and treats this method as the app's entry point (a first method that is called when the application process is started).

If you want to read more about what swift compiler does with Main attributes, how the OS knows where is the entry point of the application, I encourage you to read this article: Understanding iOS app entry point