[twobyfour]

Not an iOS developer, so please excuse any incorrect details, but:

My understanding is that Swift and Objective-C can each be compiled together with libraries that use the other language. So writing in one language doesn't preclude use by users writing in the other language.

BUT you need to include header files for both. My company actually has to ditch a third party SaaS last year because they'd written a new SDK and header files were available only for Obj-C.