Honestly there are no perfect solutions to this. There are cross platform frameworks (React Native, Xamarin etc) that you can try but you'll definitely hit the wall when trying to access low level native APIs. Kotlin Multiplatform seems to be the step in the right direction. It has first party support on Android and works seamlessly for Android apps, but it's too early to tell for the other platforms. In an ideal scenario, you can share the core business logic using Kotlin Multiplatform and then implement UI and platform specific stuff using native APIs (or something like Flutter (again, not production ready)).
I’ve heard of people running all the logic in a self-contained local http server (developped in whatever language) then rely on local socket connection + protobuf for communicating between the native and the cross-platform codes.
i had hoped some people here would chime in with this trick, but it doesn’t seem like it
That's interesting, because i've been looking for a way to share core logic code across platform (mobile & desktop at least), and still haven't found something really user friendly. From my brief lookup (rust & gomobile are the ones i've looked at), it seems that most dev environment seem to support some kind of C-style interfacing, but it becomes much more clumsy as soon as you're trying to have it run on java.
Has anyone found a solution that he's comfortable with and would recommend ?