How can safari extension communicate with native app

Far as I understand (as always might be wrong), Apple does not support this direction at all (with some limitations, you can message your extension from your container app, though).

To send messages from the extension to the container, you need to open your own communication channel. Depending on your needs, the plain old Distributed Objects might work; or you might get down to sockets; or, alas, in a pretty limited circumstances I've just tried to describe in the thread https://forums.developer.apple.com/thread/62932 you can use XPC.

(Is there a way to get a link to a particular post, not to whole thread? Perhaps today's my dumb day, but I can't see anything in there...)

P.S. Or perhaps — to effectively send a message from the extension to the container — one might write to a shared user defaults group (as the documentation suggests), and observe defaults changes at the container side.

Sounds a bit at the crazy side, but possibly might be efficient enough for all practical purposes? No idea, I've never tried 🙂