what objects can you transfer back and fourth between kotlin and swift via the bridge? Is it only a subset of objects, or does anything work?

Objects that make sense in both worlds usually could be transferred, but this question better be more specifics, if you need more specific answer.

You just need to return it in the function, and generate an iOS framework from it. And inspect the obj-c definition. Sometimes it has a great bridge, sometimes you need to implement an interface. It depends.

If I have an immutable data class from the kotlin framework, will I have a problem transferring that object to swift?

In most cases you won’t have any problems transferring Kotlin object to Swift. @olonho says about very special objects, e.g. object which inherits

Set

and

List

interfaces at the same time (because Objective-C object can’t be

NSArray

and

NSSet

at the same time).

I see. makes sense.