Hi everyone! I'm using the following code in Compose: ```val density = LocalDensity.current var text...

# compose

[Zhang Zihan]

Hi everyone! I'm using the following code in Compose:

val density = LocalDensity.current
var textHeight by remember { mutableIntStateOf(0) }
val iconHeight by remember { derivedStateOf { with(density) { textHeight.toDp() - 2.dp } } }

In theory, when the screen density changes (e.g. I move the window to another monitor with a different zoom), iconHeight should change and trigger a recompose.
But it doesn't, it keeps the old value.
I need to explicitly specify the key to cause remember to change:

val iconHeight by remember(density) { derivedStateOf { with(density) { textHeight.toDp() - 2.dp } } }

Is this a bug? (I've only tested Compose Desktop 1.9.0-beta01 so far)

[Adrien de Sentenac]

This is not a bug, this is because LocalDensity.current is not a Compose state object so derivedStateOf does not react when it changes.
Also, this is an incorrect usage of derivedStateOf. You only want to use derivedStateOf if the value it produces changes less often than the state it observes.
See https://developer.android.com/develop/ui/compose/side-effects#incorrect-usage

[Stylianos Gakis]

derivedState can only "magically" read updates on values it uses when they're backed by mutableState. Does not look to be the case for LocalDensity.current

[Zhang Zihan]

Thanks! I wrapped it in derivedStateOf because I need to call iconHeight in multiple Icons and I want to cache the value to reduce unnecessary calculations.
But as you said, this is not correct.
So should I write it like this?

val density = LocalDensity.current
var textHeight by remember { mutableIntStateOf(0) }
val iconHeight = with(density) { textHeight.toDp() - 2.dp }

This seems to be a bit confusing to me. I remember that some articles said that writing like this will cause additional calculations to be generated at each recompose, while other articles said that writing like this will cause some calculations to be skipped...

[Stylianos Gakis]

I don't know which ones you've read, but I'd personally recommend you just read this one https://blog.zachklipp.com/how-derivedstateof-works-a-deep-d-er-ive/

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

Additional calculations are fine if they’re cheap - dSO has its own costs, and computers happen to be really good at subtracting one number from another.

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

In this case even remembering the calculation (without dSO) isn’t worth it, it’s so simple

[Zhang Zihan]

What if I use remember instead? Is it worth it?

val density = LocalDensity.current
var textHeight by remember { mutableIntStateOf(0) }
val iconHeight = remember(density, textHeight) { with(density) { textHeight.toDp() - 2.dp } }

[Stylianos Gakis]

^^ https://kotlinlang.slack.com/archives/CJLTWPH7S/p1753281749189199?thread_ts=1753242520.510609&cid=CJLTWPH7S

[Nino]

remember still costs a bit to check between the (re)compositions the keys. You better let the computer do its best : crush numbers

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

You’re literally just doing like 2 math operations, not even allocating anything. I don’t know why you are so concerned with caching this.