Restrict callers of method to a class

Question

In Kotlin, I frequently come across the following pattern:

class DlgManager {
   fun dismiss(dlg: Dialogue) {
      // ...
      dlg.onDismiss()
      // ...
   }
}
class Dialogue {
   // Never call this directly, always go through DlgManager!
   fun onDismiss() {
      // ...
   }
}

I would like to make it impossible for arbitrary callers to call onDismiss except if the caller is DlgManager. Is this possible with an annotation?

@Target(AnnotationTarget.FUNCTION)
@Retention(AnnotationRetention.RUNTIME)
annotation class RestrictCallers(val caller: KClass<out Any>)

class DlgManager {
   fun dismiss(dlg: Dialogue) {
      // ...
      dlg.onDismiss()
      // ..
   }
}
class Dialogue {
   @RestrictCallers(DlgManager::class)
   fun onDismiss() {
      // ...
   }
}

How would such an implementation of @RestrictCallers look like?

Answer 1

To do precisely what you want to do would require going outside normal language rules and writing a compiler plugin.

Opt-in annotations

However you can get pretty close using an opt-in annotation if you don't mind annotating acceptable callers as well.

To do this, define your opt-in annotation:

@RequiresOptIn(message = "Only to be called by annotated callers")
@Retention(AnnotationRetention.BINARY)
annotation class RestrictOnDismissCallers

Annotate onDismiss:

class Dialogue {
   @RestrictOnDismissCallers
   fun onDismiss() {
      // ...
   }
}

Then an error will be generated by any function trying to call onDismiss except for those annotated with the opt-in annotation, such as:

@OptIn(RestrictOnDismissCallers::class)
fun dismiss(dlg: Dialogue)