If I'm using `Flow` as the bridge between a VM and a fragment, how can I ensure that the callback I ...

# android

[kevin.cianfarini]

If I'm using Flow as the bridge between a VM and a fragment, how can I ensure that the callback I use for the flow is initialized before I need It? The way I'm approaching it now seems like an antipattern.

private var loadedPosts: List<Post> = emptyList()
    private lateinit var loadedPostsCallback: LoadedPostsCallback
    val posts: Flow<List<Post>> = callbackFlow {
        val callback = object : LoadedPostsCallback {
            override fun setLoadedPosts(posts: List<Post>) {
                loadedPosts = loadedPosts + posts
                offer(loadedPosts)
            }
        }
        loadedPostsCallback = callback

        awaitClose()
    }

[kevin.cianfarini]

when setting everything up in the fragment, I start a new coroutine to observe the posts. However, the loadedPostsCallback isn't initialized when I try to do the initial load from loadPosts().

[kevin.cianfarini]

The idea I have right now is to suspendCoroutine until I can ensure everything is initialized correctly, but I feel like there's a better way

[kevin.cianfarini]

nevermind. Avoiding callbackFlow and using a backing channel of my own made this easier

[Dominaezzz]

Why can't you init loadedPostsCallback inside callbackFlow?

[Dominaezzz]

Yeah, was about to hint at that.

[kevin.cianfarini]

@Dominaezzz I did, but the flow wasn't actually being initialized because that block didn't get run until collect was being called

[kevin.cianfarini]

using my own backing channel makes it much easier to read too 😅

[Dominaezzz]

You could call produceIn and turn the flow into a channel.

[kevin.cianfarini]

I ended up with this

[kevin.cianfarini]

private val postsChannel: Channel<List<Post>> = Channel(CONFLATED)
    val posts: Flow<List<Post>> = flow {
        postsChannel.consumeEach {
            loadedPosts = it
            emit(it)
        }
    }

[kevin.cianfarini]

and just use Channel.offer as the callback

[Dominaezzz]

Hmm, I feel like this could definitely be improved somehow.

[kevin.cianfarini]

yeah it definitely doesn't feel as easy as LiveData. There's got to be something I'm missing

[Dominaezzz]

You should be able to do this with, callbackFlow {}.onEach {}.launchIn(coroutineScope).

[kevin.cianfarini]

not quite sure how to apply that to something like the snippit above

[Dominaezzz]

Can loadedPostsCallback be initialised and registered inside a flow? (Do you need a context or smth)

[kevin.cianfarini]

no, but I did need to make the callback from within the flow or else I wouldn't be able to offer in the flow scope

[kevin.cianfarini]

right?

[Dominaezzz]

Ah, I misread. Channel seems to be what you need.

[kevin.cianfarini]

cool 🙂