Hi, im currently learning to test StateFlow and and got into a problem where only the initialValue i...

# coroutines

[ABRAR WIRYAWAN]

Hi, im currently learning to test StateFlow and and got into a problem where only the initialValue is emitted. The StateFlow is produced from combining multiple snapshotFlow and use stateIn. I have manually collected the Flow and set the dispatcher to Unconfined but still no luck. Anyone has any idea on what am i missing?

[ABRAR WIRYAWAN]

The code:

@Test
fun testStateFlow2() = runTest {
    var emailState by mutableStateOf("")
    var passwordState by mutableStateOf("")

    val allInputValid: StateFlow<Boolean> = combine(
        snapshotFlow { emailState },
        snapshotFlow { passwordState }
    ) { emailValue, passwordValue ->
        emailValue.isNotEmpty() && passwordValue.isNotEmpty()
    }.stateIn(
        CoroutineScope(UnconfinedTestDispatcher(testScheduler)),
        SharingStarted.WhileSubscribed(5000),
        false
    )

    val test = mutableListOf<String>()
    val job = launch {
        allInputValid.collect {
            test.add("Value emitted: $it")
            println(it)
        }
    }

    advanceUntilIdle()
    emailState = "test_email"
    passwordState = "test_password"

    advanceUntilIdle()
    println(test)

    job.cancel()
}

[Rob]

StateFlow conflates values so maybe it's because the value hasn't changed?

[Rob]

Put a println inside combine lambda to see if it is getting invoked with new values and what it is emitting.

[ABRAR WIRYAWAN]

yes it only invoked once with empty value

email: - pass: // inside combine
false // inside collect
[Value emitted: false] // last println before cancel

[ABRAR WIRYAWAN]

so the emailState and passwordState update value doesnt do anything, why that happens?

[Rob]

never mind. I didn't read that correctly.

[Rob]

I think it's because the test method is not a composable.

[ABRAR WIRYAWAN]

so snapshotFlow can only be triggered from composable only?

[Rob]

From the example here they're observing the State<T> from an observable. I don't think snapshotFlow is going to work unless State<T> is in a composable. But I'm am not an expert.
https://developer.android.com/develop/ui/compose/side-effects#snapshotFlow

[ABRAR WIRYAWAN]

It makes sense, thanks. I'll try to search/ask in the compose channel

[ABRAR WIRYAWAN]

Ah you are right, it needs a compose observer, now im confused why it is work on standard flow. Will try to ask in compose channel

https://kotlinlang.slack.com/archives/CJLTWPH7S/p1614530850199700?thread_ts=1614530680.199100&cid=CJLTWPH7S

[Zach Klippenstein (he/him) [MOD]]

Yea you need to bootstrap snapshot observation.

Here’s how molecule does it (look for registerGlobalWriteObserver in that file, GitHub app apparently can’t link to lines), you can do something similar (probably don’t even need to launch just for your test)

[ABRAR WIRYAWAN]

Thanks for the reference, TIL we can register our own observer for snapshot