Hi, question regarding the inner workings of `Andr...

# compose-android

[Fabio Berta]

Hi, question regarding the inner workings of AndroidView . I'm using it to wrap a maplibre (mapbox v9 fork) map in a composable Map. If I store the current camera position of the map as a mutableStateOf in Map , add a position listener that updates the state in the factory and set the position in the update block of the AndroidView, all is fine: the position updates when the state changes and the AndroidView itself does not recompose. But if that state is hoisted outside of Map and passed in as a param, AndroidView itself recomposes and the performance is bad. I checked that the factory is only called once though and update is called correctly. I'm trying to figure out 2 things, why does AndroidView itself recompose in this case and not just call update and why is the performance bad, it seems like factory is not called again but only updated which seems like what is supposed to happen.

[Jakub Syty]

Did you check the compiler metrics for the state you are passing? Maybe the function is not restartable?

[Jakub Syty]

that's the first thing i'd check - make sure your state is stable/immutable

[ephemient]

the https://developer.android.com/reference/kotlin/androidx/compose/ui/viewinterop/package-summary#AndroidView(kotlin.Function1,androidx.compose.ui.Modifier,kotlin.Function1,kotlin.Function1,kotlin.Function1) overload makes it possible to reuse Android views across recomposition, but you should check what Jakub says first

[Fabio Berta]

I did not check that yet, will do thanks! I saw about the overload with onReset but thought this was more appropriate for lazy layouts

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

It would be easier to figure this out with code, but it sounds like you might be pulling the state read up outside of the update function. If that’s not what’s going on, can you post some code?

[Fabio Berta]

sure, thanks for chipping in

[Fabio Berta]

@Composable
fun HoistedStateMap(
    position: MapPosition,
    onPositionChange: (MapPosition) -> Unit,
    modifier: Modifier = Modifier,
) {
    AndroidView(
        factory = { context ->
            MapView(context).also { mapView ->
                mapView.getMapAsync { map ->
                    map.apply {
                        addOnCameraMoveListener { onPositionChange(cameraPosition.toMapPosition()) }
                        setStyle(STYLE_URL)
                    }
                }
            }
        },
        update = { mapView ->
            mapView.getMapAsync { map ->
                position.toCameraPosition().let {
                    if (it != map.cameraPosition) {
                        map.cameraPosition = it
                    }
                }
            }
        },
        modifier = modifier,
    )
}

[Fabio Berta]

this is slow

[Fabio Berta]

@Composable
fun InternalStateMap(
    modifier: Modifier = Modifier,
) {
    var position by remember { mutableStateOf(initialPosition) }
    AndroidView(
        factory = { context ->
            MapView(context).also { mapView ->
                mapView.getMapAsync { map ->
                    map.apply {
                        addOnCameraMoveListener { position = cameraPosition.toMapPosition() }
                        setStyle(STYLE_URL)
                    }
                }
            }
        },
        update = { mapView ->
            mapView.getMapAsync { map ->
                position.toCameraPosition().let {
                    if (it != map.cameraPosition) {
                        map.cameraPosition = it
                    }
                }
            }
        },
        modifier = modifier,
    )
}

[Fabio Berta]

this is fast

[Fabio Berta]

I still need to look at the compiler metrics though

[Fabio Berta]

data class MapPosition(val coordinates: Coordinates, val zoom: Double)

[ephemient]

is MapPosition @Stable or @Immutable or neither?

[Fabio Berta]

data class Coordinates(val lat: Latitude, val lon: Longitude)

@JvmInline
value class Latitude(val value: Double)

@JvmInline
value class Longitude(val value: Double)

[Fabio Berta]

If it only contains primitives or other classes with only primitives, it wouldn't need it, no?

[Fabio Berta]

annotating MapPosition with Stable doesn't change anything

[Fabio Berta]

It's getting very late here. If you guys have any ideas/suggestions, please just dump it and I'll have a look tomorrow! I'll also have a look at the compiler metrics. Thanks!

[Fabio Berta]

So I finally looked at the compiler metric and MapPosition was indeed not stable because it came from a module without compose. I adjusted that and now it is stable and the composable is skippable. However, nothing changed in terms of performance. Obviously, HoistedStateMap can't be skipped because position actually changes. In that sense, it also makes sense that this change didn't have any effect.

[Fabio Berta]

Interestingly though, the following code is performant as well

[Fabio Berta]

@Composable
fun HoistedStateMap(
    position: MapPosition,
    onPositionChange: (MapPosition) -> Unit,
    modifier: Modifier = Modifier,
) {
    val mapPosition = rememberUpdatedState(position)
    AndroidView(
        factory = { context ->
            MapView(context).also { mapView ->
                mapView.getMapAsync { map ->
                    map.apply {
                        addOnCameraMoveListener { onPositionChange(cameraPosition.toMapPosition()) }
                        setStyle(STYLE_URL)
                    }
                }
            }
        },
        update = { mapView ->
            mapView.getMapAsync { map ->
                mapPosition.value.toCameraPosition().let {
                    if (it != map.cameraPosition) {
                        map.cameraPosition = it
                    }
                }
            }
        },
        modifier = modifier,
    )
}

[Fabio Berta]

I guess this is because mapPosition from rememberUpdatedState(position) is an object that itself doesn't change

[Fabio Berta]

but then I'm wondering how the update function of AndroidView is supposed to be used if this is required?

[Fabio Berta]

@Zach Klippenstein (he/him) [MOD] any ideas what I'm doing wrong here?

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

What sort of performance issues are you seeing? Dropped frames? Or lag between input and update?

[Fabio Berta]

lag between input and update

[Fabio Berta]

but I don't really understand why the is any kind of difference at all

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

The difference between the first two snippets you posted is that the one with the callback forces at least one frame of delay, assuming the caller is well-behaved. The third snippet also introduces that frame delay though, so if that isn’t affected by the lag then it’s probably not caused by that frame delay. Based on this code it does seem like the problem is caused by AndroidView not skipping, although I’m not sure why that would be. Have you attached a profiler?

[Fabio Berta]

I see, thanks! I have not yet attached a profiler, let me do that later tonight, I'll post my findings here, thanks!