Hello, how one can define a multiple derivedStated...

# compose

[nuhkoca]

Hello, how one can define a multiple derivedStatedOf? I have a code like below but they are called constantly when I log and recomposition happening. As far as I understand correctly, recomposition should only happen when isPageTitleReached and isContentScrolled changed, right?

val isPageTitleReached by derivedStateOf {
    remember {
        lazyListState.firstVisibleItemScrollOffset > 500
    }
}
val isContentScrolled by derivedStateOf {
    remember {
        lazyListState.firstVisibleItemScrollOffset > 600
    }
}

[myanmarking]

you inverted the function calls. It should be remember{derivedStateOf ..}

[myanmarking]

remembers like that are useless. The block inside will only run once.

[nuhkoca]

ohh really? fixing this right away, thanks for the heads-up. Will update you!

[nuhkoca]

btw why it is like I did in the official doc?

val highPriorityTasks by remember(highPriorityKeywords) {
        derivedStateOf { todoTasks.filter { it.containsWord(highPriorityKeywords) } }
    }

[myanmarking]

the reason they are called constantly is because derivedState that in not remembered will create a new instance every recomposition

[myanmarking]

remember{derivedState}, you have derivedState{remember} ?

[nuhkoca]

ahh my bad sorry you are right

[nuhkoca]

Hey @myanmarking can I have something like this or is it a good way on Compose? Basically I’d like to hold all variables inside a data class but wondering if remember and derivedStateOf would work as expected in my example?

@Composable
private fun rememberScrollState(
    lazyListState: LazyListState,
    firstCondition: Int,
    secondCondition: Int
) = remember(firstCondition, secondCondition) {
    ScrollState(lazyListState, firstCondition, secondCondition)
}

@Immutable
private data class ScrollState(
    val lazyListState: LazyListState,
    val firstCondition: Int,
    val secondCondition: Int
) {
    val isPageTitleReached by derivedStateOf {
        if (lazyListState.firstVisibleItemIndex == 0) {
            lazyListState.firstVisibleItemScrollOffset > firstCondition
        } else {
            true
        }
    }
    val isHeroScrolled by derivedStateOf {
        if (lazyListState.firstVisibleItemIndex == 0) {
            lazyListState.firstVisibleItemScrollOffset > secondCondition
        } else {
            true
        }
    }
}

[myanmarking]

Ya it should work. You should probably add the lazyliststate to remeber param as well

[myanmarking]

And probably change immutable to stable

[nuhkoca]

Awesome, applying right now. Thanks for the quick turnaround, appreciate it 🙏

[Michael Paus]

Which leads me to the question whether a data class with only val parameters needs any of these qualifiers (@Immutable, @Stable) at all. I thought such a data class is considered stable by default.

[myanmarking]

Well, you can have a val that is unstable. It is not the only requirement. Im not sure about this case would have to check, but it doesnt hurt adding it

[Tash]

IIRC the Compose compiler can infer immutability and stability only on classes which are compiled in modules where the Compose compiler enabled. So even if your data class has `val`s where nothing is backed by mutable data, it won’t be inferred as immutable unless it is in a module w/ the Compose compiler.

Also if your data class contains a val list: List<Foo> the compiler can’t infer stability even if Foo is stable (there’s an open issue). For that you’d have to explicitly annotate that class with `@Immutable`/`@Stable`. Recently read about this here https://chris.banes.dev/composable-metrics/

[Michael Paus]

@Tash Very interesting reading, especially the link to https://issuetracker.google.com/issues/216791427 I proposed exactly that here some months ago but the idea was bluntly rejected.