What are my options with interfaces to ensure some certain code to be executed when a class implements given interface? I’d like to avoid some abstract base class as this interface is intended to be implemented by viewmodels which already can inherit from two possible base classes. (sometimes I really miss multi-inheritance 🙂 )

When you said

code be executed when a class implements a given interface

- when exactly did you mean? Like at instantiation time? If that's the case, I don't know of any way (apart from using an abstract class instead). Could you please expand on your use case for needing this?

OK - let’s imagine an interface like this:

interface IBarcodeProcessor {
    val service: ScanService
}

Then there is this

ScanService

with this portion of code

class ScanService {
    val scanObservers = mutableListOf<(Scan) -> Unit>()

    var scan: Scan by Delegates.observable(
        Scan(
            "",
            "",
            ""
        )
    ) { _, _, new -> scanObservers.forEach { it(new) } }
}

I’d like to add the class which implements

IBarcodeProcessor

to add a method to the

ScanService.scanObservers

list

I don't quite see why you need to go through so much trouble. I mean adding the

IBarcodeProcessor

interface seems as much hassle as just registering a hook. Like in which way would your suggested setup be better than, say, simply this:

@HiltViewModel
class MyViewModel
@Inject
constructor(
    service: ScanService,
) : SomeBaseClass() {
    init {
        service.registerObserver(BarcodeType.CONTAINER, BarcodeType.SOMETHING_ELSE) {
            someHandlerFunNamedHoweverIWant(it)
        }
    }

    private fun someHandlerFunNamedHoweverIWant(scan: Scan) {
        TODO("Not yet implemented")
    }
}

People can forget things, but they can forget implementing

IBarcodeProcessor

just as much as forgetting to register the hook. Not sure why favor one over the other. IMO if you want your viewmodel to do something when a barcode event is emitted, it doesn't seem too farfetched to use the snippet above. Also, in this code the viewmodel authors control when they want to register