I appear to be confused about snapshot state. The following code crashes with a ConcurrentModificati...

# compose

[tad]

I appear to be confused about snapshot state. The following code crashes with a ConcurrentModificationException, presumably when the source SnapshotStateList is modified while sorting in derivedStateOf.

private val list = mutableStateListOf<Thing>()

val sortedList by derivedStateOf {
    list.sorted()
}

[tad]

Do I need to be protecting against modifications to the state I read in derivedStateOf blocks?

[tad]

I see now that toList() is intended to be used to get the PersistentList for the current snapshot record, so you can iterate without dealing with concurrent modifications. It's a bit concerning that using standard Kotlin library functions such as map, fold, filter, etc. run the risk of crashing unless this is used. Would it make sense to have iterator() return toList().iterator() instead of the SnapshotStateList's ListIterator implementation?

[tad]

Ah, that wouldn't work because of the MutableList interface's contract that iterator() returns a MutableIterator. Feels like a footgun inherited by Java.

[efemoney]

List.sorted doesnt realy make sense for derived state & snapshot state list because you are modifying the snapshot state list while its trying to run the derived block.

You need List.sort() which creates a new list that is sorted from the first one.

[tad]

That's backward, MutableList.sort is the in-place version

[efemoney]

Oh woops, my bad, then something is wrong somewhere

[tad]

Yeah, it's because I need to call list.toList() to get the underlying immutable list for the snapshot

[efemoney]

Yeah I use the toList() version everywhere (i also have a derived sorted list too) so I haven’t run into this. But I agree it should be somewhat clarified

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

This seems like a bug to me… if sorted doesn’t mutate the underlying list. Can you file?

[tad]

Sure

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

and please post here

[tad]

I'm reporting now, but as I'm writing this it strikes me that it's not really a problem with Compose, but more of a problem with the List/MutableList interface.

[tad]

Like, this would happen if it were a MutableList instead of a SnapshotStateList.

[tad]

https://issuetracker.google.com/issues/272334463