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Type-based program entry points

Available from Swift 5.3

Paul Hudson      @twostraws

SE-0281 introduced a new @main attribute to allow us to declare where the entry point for a program is. This allows us to control exactly which part of our code should start running, which is particularly useful for command-line programs.

For example, when creating a terminal app previously we needed to create a file called main.swift that was able to bootstrap our code:

struct OldApp {
    func run() {
        print("Running!")
    }
}

let app = OldApp()
app.run()

Swift automatically considered code in main.swift to be top-level code, so it would create the App instance and run it. That is still the case even after SE-0281, but now if you want to you can remove main.swift and instead use the @main attribute to mark a struct or base class that contains a static main() method to be used as the program’s entry point:

@main
struct NewApp {
    static func main() {
        print("Running!")
    }
}

When that runs, Swift will automatically call NewApp.main() to start your code.

The new @main attribute will be familiar to UIKit and AppKit developers, where we use @UIApplicationMain and @NSApplicationMain to mark our app delegates.

However, there are some provisos you should be aware of when using @main:

  • You may not use this attribute in an app that already has a main.swift file.
  • You may not have more than one @main attribute
  • The @main attribute can be applied only to a base class – it will not be inherited by any subclasses.

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