Following the discussion at <https://kotlinlang.sl...

# compose-android

[ascii]

Following the discussion at https://kotlinlang.slack.com/archives/CJLTWPH7S/p1693324536742409 and many others before, what's the best way to deal with a situation where you need to adjust things based on State in both the parent & child composables, but the State's value changes only in the child?

I'll take a simple example first, to explain what I mean (code in thread). I'm displaying a banner ad and I need to conditionally apply a Modifier to AdView as well as some changes here and there in the parent composable, based on whether or not the ad has loaded.

[ascii]

@Composable
fun ColumnScope.BannerAd(
  adUnitId: String,
  adLoaded: MutableState<Boolean>? = null,
  viewUpdated: (AdView) -> Unit,
) = if (LocalInspectionMode.current) {
  Text("AdView")
} else AndroidView(factory = {
  AdView(it).apply {
    …init…

    if (adLoaded != null) adListener = object : AdListener() {
      override fun onAdFailedToLoad(error: LoadAdError) { adLoaded.value = false }
      override fun onAdLoaded() { adLoaded.value = true }
    }
  }
}, if (adLoaded?.value == true) Modifier.something() else Modifier, viewUpdated)

I could pass adListener and Modifier from the parent composable itself, which bypasses the need to pass a MutableState to this child, but that introduces the bigger question in more complicated situations: what do I do if it's not always possible to move things out? Do I forego this separate child composable entirely, and put everything in the parent composable? Or do I deconstruct MutableState and pass value/setter separately?

From what I've seen with benchmarks and Layout Inspector, there aren't any performance issues or unnecessary recompositions when I pass a MutableState.

[Stylianos Gakis]

The rules are mostly simple.

Hoist the state to the lowest possible ancestor (in your case the parent)
Pass down to the child composable a way to read the state (This can be T or () -> T)
Also pass down a way for the child to change the state by passing a (T) -> Unit if the child also needs to change the value.

Never pass State<T> and especially not MutableState<T> down. For the first just because () -> T is better, and for the second don’t do it because you introduce a very hard to reason about flow of where you state changes.

[ascii]

I see, it would also make testing a bit annoying. Thank you!