Type-based program entry points
Type-based program entry points 관련
Available from Swift 5.3
SE-0281 (apple/swift-evolution) 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.swiftfile. - You may not have more than one
@mainattribute - The
@mainattribute can be applied only to a base class – it will not be inherited by any subclasses.
Other Changes in Swift 5.3