I have the strangest bugs again. I have a `MutableState` and two nested derived states reading it. I...

# compose

[vide]

I have the strangest bugs again. I have a MutableState and two nested derived states reading it. I read the deepest-nested derived state with snapshotFlow but it doesn't get updates. What is wrong? I'm running out of theories... 🧵

[vide]

CoroutineScope(Dispatchers.Main).launch {
    val truth = mutableStateOf(true)
    val derived1 = derivedStateOf { truth.value }
    val derived2 = derivedStateOf { derived1.value }

    launch {
        snapshotFlow { derived2.value }.collect {
            Log.d("wat", "job1: $it")
        }
    }

    launch {
        snapshotFlow { derived2.value }.collect {
            Log.d("wat", "job2: $it")
        }
    }

    delay(100)
    truth.value = false
}

[vide]

I would expect to see

job1: true

job2: true

job1: false

job2: false

[vide]

In reality I see
> job1: true
> job2: true
> job1: false

[shikasd]

Interesting, that sounds like a caching bug to me

[vide]

If it passes your sanity check on something that should definitely work, I guess it really could be a bug. I can file on the tracker

[vide]

https://issuetracker.google.com/issues/332772495

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

what happens if you wrap your truth.value = false in a withMutableSnapshot?

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

i’m guessing it won’t fix it but there was another discussion this week about a race condition here so wondering if it’s related

[vide]

yeah, same result

[vide]

@shikasd thanks for taking a look, we really appreciate it! I don't really understand the fix though, should probably dive deeper to be able to debug these. Are there any public docs on how snapshots work apart from in-code comments?

[shikasd]

well you don't have to understand it, tbh, it was caused by somewhat weird interaction of nested derived states and a snapshot flow
I think Zach's articles are the best source of info we have: https://dev.to/zachklipp/introduction-to-the-compose-snapshot-system-19cn

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

Unfortunately I don’t have one on derived state… yet. I’ll be giving a talk at droidcon sf about it, planning to publish a blog post version after as well