You guys always have very good explanations for my...

# getting-started

[Norbi]

You guys always have very good explanations for my generics-related questions and misunderstandings, so the next question is... 😉 😄

I have a simple event bus implementation (backed by MutableSharedFlow):

interface InProcessEventBus<E: Any> {

    suspend fun events(): Flow<E>

    suspend fun publish(event: E)
}

What I would like to achieve is to create a "view" of this event bus to handle only specific types of events, like:

val globalEventBus: InProcessEventBus<Any> = ...
val constrainedEventBus: InProcessEventBus<SomeCustomEventClass> = globalEventBus // Do something with globalEventBus to make this compile

So the constrainedEventBus would be a view of the less specific globalEventBus, allowing only to publish and listen to more specific event types.
Although I think this "subtyping" would be correct, the compiler does not think so 🙂
Currently my best solution is an unsafe cast but I hope you have a better idea:

@Suppress("UNCHECKED_CAST")
fun <E: B, B: LocalEvent> InProcessEventBus<B>.constrain(): InProcessEventBus<E> = this as InProcessEventBus<E>

val constrainedEventBus = globalEventBus.constrain<SomeCustomEventClass, _>()

Thanks.

[ephemient]

Nope, that's not safe.

constrainedEventBus.events().collect { event: SomeCustomEventClass ->

would crash when

globalEventBus.publish(NotSomeCustomEventClass())

is called

[ephemient]

if you want to receive a subset of the events, you'll have to do so yourself.

inline fun <reified E: Any> InProcessEventBus<in E>.constrain(): InProcessEventBus<E> =
    object : InProcessEventBus<E> {
        override val events: Flow<E>
            get() = [e-mail]().filterIsInstance<E>()

        override fun publish(event: E) {
            [e-mail](event)
        }
    }