Maybe a stupid question, but is it possible to have a

map

function on

SnapshotStateList

instance that would be itself

state list

(but not mutable), so that it only calls map’s lambda when original list changes? Kinda like

val files = mutableStateListOf("/path/to/file1", "/path/to/file2"); val loadedFiles = files.map { loadFile(it) }

and so that it calls

loadFile

only on inserted/modified elements of

files

.

Not sure if I get it right. Out of my head something like the below with a combination of

derivedStateOf

and

toMutableStateList

could work. It's still a mutable state list, but should change when the files state list changes.

val files = remember { mutableStateListOf("/path/to/file1", "/path/to/file2") }

val loadedFile = remember {
    derivedStateOf {
        files.map { path ->
            File(path)
        }
    }.value.toMutableStateList()
}

Nope, it will rebuild the whole loadedFile list once any change occurs in the source mutable state list, because

map

there is just an ordinary collection function.

You could make a List implementation that delegates to your state list and does the mapping on access.

@Zach Klippenstein (he/him) [MOD] but I want it to be a state, i.e. memoized

Sure:

val loadedFile = remember(files) { MappingList(files) { doTransform(it) } }

Hm, let me try to explain what I want differently, probably I was not clear enough 🙂 Given a

SnapshotStateList<T>

as a source, I would like to synchronize it to another

SnapshotStateList<R>

with a transformation function

T->R

. So that if an item is added to the source, the transformation function is called just once on the new value and it is placed in the target. If the item is removed, it is removed from both lists (by index, since they are synced). If the item is updated, the transformation function is called and value is replaced at target by index again.

You can keep an internal list of derived states for individual transform results, but keeping it trimmed might be kind of hacky (do it on each read?)

JavaFX has this built in, you could look at their code for inspiration: https://openjfx.io/javadoc/18/javafx.base/javafx/collections/transformation/package-summary.html