I appear to be confused about snapshot state The following c

Question

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 lt Thing gt val sortedList by derivedStateOf list sorted ```

tad · Answer

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

tad · Answer

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 · Answer

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 · Answer

List sorted doesnt realy make sense for derived state amp 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 · Answer

That s backward MutableList sort is the in place version

efemoney · Answer

Oh woops my bad then something is wrong somewhere

tad · Answer

Yeah it s because I need to call `list toList ` to get the underlying immutable list for the snapshot

efemoney · Answer

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] · Answer

This seems like a bug to me if sorted doesn t mutate the underlying list Can you file

tad · Answer

Sure

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

and please post here

tad · Answer

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 · Answer

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

tad · Answer