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kotlinlang

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... :thread:

```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
}```


I would _expect_ to see
&gt; job1: true
&gt; job2: true
&gt; job1: false
&gt; job2: false

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

Interesting, that sounds like a caching bug to me

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

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

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

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

<@UJNU95VNC> 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?

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>

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