Q: My MutableStateFlow Didn’t Emit I'm using this ...

# coroutines

[Clament John]

Q: My MutableStateFlow Didn’t Emit
I'm using this code snippet to update messages from two flows (using flatMapLatest)

private val searchString = MutableStateFlow<String>("")
    private val messages = searchString.flatMapLatest {
        if (it == "") {
            getAll()
        } else {
            search(it)
        }
    }.stateIn(viewModelScope, SharingStarted.Eagerly, null)

    suspend fun requestSearch(search: String) = searchString.emit(search)
    suspend fun requestAll() = searchString.emit("")

    fun getAll(): Flow<List<Message>> = TODO()
    fun search(search: String): Flow<List<Message>> = TODO()

But when I try emit to update searchString it doesn't work. Note: The initial condition of "" was successfully executed.

[Joffrey]

How do you consume messages?

[Joffrey]

Using stateIn for messages seems a bit strange, as messages feel more like a stream of things than a state. The consequence of using something meant for state is that if you have twice the same message, it won't trigger events in the consumers

[Clament John]

collectAsState in a composable.

I've set a breakpoint inside flatMapLatest to see if it is getting triggered. But that break point was never hit

[Clament John]

I need messages to be consumed at a composable. So I added a stateIn to convert it to a StateFlow, so that I can use collectAsState . How do you think I should do it so that the Flow from messages is consumed at the composable

[Joffrey]

So only the last message matters?

[Clament John]

Only the latest List<Message> matters. getAll and search are flows from the database.

[Joffrey]

Oh my bad, I didn't see that messages was a flow of lists, not just simple messages. How can you provide "" as initial value then? Does this code compile?

[Clament John]

Oh sorry typo. It is null . Edited the code snippet

[Joffrey]

I think requestSearch and requestAll are not supposed to use emit() , you should just set the value of the flow via its value property:

fun requestSearch(search: String) {
    searchString.value = search
}
fun requestAll() {
    searchString.value = ""
}

[Clament John]

I tried that too.

searchString.value = ""

But this doesn't work either

[Joffrey]

Just to be sure, you're using a non-empty value to test requestSearch, right? Because trying to send "" will not trigger events of the searchString flow if its last value was already "" (it only triggers when the new value is not equal to the current state)

[Clament John]

Yes. I did set a break point at requestSearch to check if search was a non empty value

[Clament John]

This is embarrassing. flatmapLatest is working (but my breakpoints did not work for some reason). My current problem is not related to this anymore.

It is related to channelFlow and me getting a ClosedChannelException . Will ask a different question for that.

[Joffrey]

Meh don't be embarassed, it happens all the time. Let's just be glad your problem is narrowed down 😉