- data structures for your domain objects
- network code (just plug in an appropriate platform-specific driver)
- like you said, business logic - especially if the app has to work offline
- if you're using something like https://realm.io/, you could avoid a lot of duplication
Anything to do with the UI then has to be platform-specific of course.
not sure there are a problem since you have data classes in kotlin. I bet swift provides something similar.
> - network code
In Android you must use okhttp, and I guess there are similar powerful libraries in swift for this.
> - if you're using something like https://realm.io/
Last time I checked it I decided to keep using SQLite as it allows me to do everything I want. Realm doesn't support "complex" SQL queries.
Again, just business logic. But there is no too much apart from field validation or simplex operations to show the data (or it should be much more). Complex things must be in the server, that is the best way to share business logic among all your apps in all the platforms.