If i have a SharedFlow of sealed class, can i subscribe to just specific event? If so how?

# coroutines

[Gordon]

If i have a SharedFlow of sealed class, can i subscribe to just specific event? If so how?

[hfhbd]

Use filterIsInstance, mapNotNull + when or filter + when, but I would not use filterIsInstance with sealed classes to not forget to update the check when adding new cases.

[Gordon]

I tried adding generic function to my class, but due to type erasue I can't forward the type to sharedflow

[Gordon]

fun <Event: AppEventData> subscribeForSpecificEvent(scope: CoroutineScope,  block: suspend (AppEventData) -> Unit) =
        _eventFlow.mapNotNull { if (it is Event) it else null}

[Gordon]

Cannot check for instance due to type erasure

[gildor]

Could you show some self contained example
It doesn’t look as something Flow specific, after all Flow callback is not different from any another lambda used as callback

[Gordon]

sealed class AppEvent {
    data class Event1(val name: String) : AppEvent()
    data class OtherEvent(val id: Int, val data: Long) : AppEvent()
}

class AppEventBus {
    private val _eventFlow = MutableSharedFlow<AppEvent>()

    fun subscribe(scope: CoroutineScope, block: suspend (AppEvent) -> Unit) = _eventFlow.onEach(block).launchIn(scope)

    suspend fun emit(appEvent: AppEvent) = _eventFlow.emit(appEvent)
}

Let's say i have something like this

[Gordon]

Now everyone can subscribe to ALL events. And respond only to the event it's actually interested on.
But I do not want to notify listeners about irrelevant events.
Let's say someone is interested in only Evet1 events. How would one subscribe to specific event types.

[gildor]

in general, filter is way to go

[gildor]

Why would you create AppEventBus instead of using Flow, it looks as a huge step backward

[Gordon]

I Am using Flow

[gildor]

I also don’t see any erasure in your case

[Gordon]

Because I deleted it.

[gildor]

I Am using Flow

Well, no, you hide it behind far inferior callback based API

[gildor]

You deleted erasure?
In general, I don’t see anything Flow specific in this case, yes, erasure can be an issue if you have some generic events, but it would be an issue without flow too

[Gordon]

fun <Event:AppEvent> subscribeForEvent(scope: CoroutineScope, block: suspend (AppEventData) -> Unit) = _eventFlow.filter { /*TYPE ERASURE :(*/ )}

[gildor]

Yes, pass class as argument

[gildor]

+ use inline to avoid erasure

[Gordon]

Can you show me example? I don't get it

[gildor]

Yes, give me a minute

[hfhbd]

https://github.com/Kotlin/kotlinx.coroutines/blob/d3a9c9d118bb7d16ddef56b7f42723ab598facba/kotlinx-coroutines-core/common/src/flow/operators/Transform.kt#L35

[hfhbd]

It was only the showcase for the signature

[gildor]

Ahh, right

[Gordon]

inline fun <reified Event:AppEvent> subscribeFor(scope: CoroutineScope, block: suspend (AppEventData) -> Unit) : Flow<Event> = _eventFlow.filterIsInstance<Event>().onEach(block).launchIn(scope)

Suspend inline lambda parameters of non-suspend functions are not supported

[gildor]

inline fun <reified Event : AppEvent> subscribeForEvent(
            scope: CoroutineScope,
            noinline block: suspend (AppEvent) -> Unit
    ) = subscribe(scope) { if (it is Event) block(it) }

[hfhbd]

then make subscribeFor suspend too or noinline

[gildor]

You should mark it as noinline

[gildor]

Making it suspend doesn’t really work for this API, it should be non suspend subscription

[hfhbd]

Oh, yeah in this context it is useless

[Gordon]

only type parameters of inline functions can be reified

[Gordon]

can't mark it noinline

[gildor]

Well, this function is inline, see above

[gildor]

just insert it into your class, it works for your current implementation

[Gordon]

This compiles: inline fun <reified Event:AppEvent> subscribeFor(scope: CoroutineScope, noinline block: suspend (Event) -> Unit) = events.filterIsInstance<Event>().onEach(block).launchIn(scope)

[gildor]

ahh, right, I see now

[Gordon]

The only last problem: I Hate it 🙂
It looks like over engineering.
Do you have a good example of how I can pass events from various classes to other classes

[gildor]

No issue

[gildor]

Back to my original suggestion, by splitting it up

[gildor]

fun <Event : AppEvent> subscribeForEvent(
        scope: CoroutineScope,
        eventType: Class<Event>,
        block: suspend (Event) -> Unit
) = subscribe(scope) {
    if (eventType.isInstance(it)) {
        @Suppress("UNCHECKED_CAST")
        block(it as Event)
    }
}

[gildor]

So now you filter it by class, and on top it add inline version:

inline fun <reified Event : AppEvent> subscribeForEvent(
            scope: CoroutineScope,
            noinline block: suspend (Event) -> Unit
    ) = subscribeForEvent(scope, Event::class.java, block)

[gildor]

Wait, no, I was confused, having single method also works, no issue (just fixed type of lambda from my previous example):

inline fun <reified Event : AppEvent> subscribeForEvent(
        scope: CoroutineScope,
        noinline block: suspend (Event) -> Unit
) = subscribe(scope) { if (it is Event) block(it) }

[gildor]

The only last problem: I Hate it 🙂

It looks like over engineering.

Do you have a good example of how I can pass events from various classes to other classes

What is over engineering?

[gildor]

You pass class explicitly as param, or you use inline class to do this for you, I really don’t see issue with it

[Gordon]

What is over engineering? A Solution more complex than a problem 😉

[gildor]

Nice to mock me, even if I try to help

[gildor]

I don’t see it over engineering at all, it’s just usage of Kotlin reified types

[Gordon]

What? I was not talking about you

[Gordon]

About me

[Gordon]

I tend to take simple problems and come up with solutions more complex

[gildor]

reified types used in a lot of Kotlin stdlib functions and in our code too, it’s just a way to avoid passing explicit class

[gildor]

> Do you have a good example of how I can pass events from various classes to other classes

I feel that this question not about rified types, right? But about EventBus

[gildor]

Then yes, as I said above, I feel that downgrade stream from flow to event bus with callbacks is bad solution

[Gordon]

Right.
I Mean Ii have a niche class, somewhere deep deep in the classes tree.
It needs sensor data, rarely. it wants to ask the data from provider, and be notified about events.
But i do not wish to pass this data all the way down. So my question is how the data flow is usually done in android apps.

[gildor]

pass events from various classes to other classes

Yes, create a class with Flow (which can be implemented in any kind of flow), inject it where you need events

[gildor]

so yeah, as you said, you have to pass it all way down in this case (manually or automatically with DI library)

[Gordon]

Ok, thanks a lot.

[gildor]

EventBus also should be injected somehow, so I don’t see big difference in this case

[gildor]

In extreme case, well, you can use global property after all to access this repository of events (not that I would suggest it, but at least it will be properly typed and will have only one use case, unlike event bus which delivers all events for everyone)