Hi all! I have a KMP with a react website and an A...

# multiplatform

[Joost Klitsie]

Hi all! I have a KMP with a react website and an Android application. To support the functional components, I make my props (RProps) external interfaces in the common module. However, when I try to build the Android application, it complains about these external interfaces:

Modifier 'external' is not applicable to 'class'

It looks (for example) like this:

external interface EventDetailsContractProps: BaseProps {
    var eventId: UUID
    var handler: EventDetailsContract.Handler
}

interface EventDetailsContract {
    interface Handler: EventSummaryContract.Handler {
        fun onEventDetailsFinished()
    }
}

data class EventDetailsProps constructor(
    override var eventId: UUID,
    override var handler: EventDetailsContract.Handler
): EventDetailsContractProps

This code is shared between my javascript and android targets. Is there any way to resolve this? I could try:

expect interface EventDetailsContractProps {
    var eventId: UUID
    var handler: EventDetailsContract.Handler
}

And then make a platform specific actual Interface, but that would mean a lot of boilerplate (As I would have to rewrite the interface 3 times).

[Marc Knaup]

You can’t use external in outside of Kotlin/JS. Also not with expect / actual.

You’ll have to wrap your props.

external interface MyProps<T> {
   var value: T
}

Then your component has MyProps<EventDetailsContractProps> as props where EventDetailsContractProps is a regular class or interface.

kotlin-react is annoying in that regard. For that reason and others I’ve created my own 🙂
https://github.com/fluidsonic/fluid-react
(no class components though and requires IR compiler)

[Joost Klitsie]

I think I can still write: actual external interface EventDetailsContractProps: BaseProps

[Marc Knaup]

If that would be possible then common code is broken because things like value is EventDetailsContractProps or value.equals(…) etc. won’t work and just crash.

[Joost Klitsie]

well to support the external interfaces I already took away all reified things and comparisons

[Joost Klitsie]

so I think in that case I should always remember to keep it like that

[Marc Knaup]

Yeah I didn’t want to make all the compromises like that or losing type safety b/c of external. Hence I’ve made my own library 😉