You share as much code as you want in the shared/base project and are able to create "contracts" for each UI platform that are enforced by the compiler. If you

You share as much code as you want in the shared/base project and are able to create "contracts" for each UI platform that are enforced by the compiler. If you're business logic needs these platform specific functionalities (which will definitely be the case as you've stated) then you can abstract out interfaces and let each platform handle them natively.