Whats the recommended way to compute values in com...

# compose

[Zoltan Demant]

Whats the recommended way to compute values in compose? If I have a Lazy<Foo> in my MainActivity that takes 1000 ms to resolve; how can I .get() it inside a @Composable function such that it happens in the background (e.g. with Dispatchers.Default)?

[Tiago Nunes]

You need to find a way to convert your Lazy<Foo> into something like State<Foo?>, that starts with null, and gets updated from a coroutine. For this, you can use Side Effects (https://developer.android.com/jetpack/compose/side-effects). The easiest to implement in your case should be produceState . Or you could create a StateFlow in the viewModel, launch a coroutine and update the value, and collect it as state in the @Composable function.

[Zoltan Demant]

Something like this? Im doing this before viewmodels (etc) get into the picture, it just takes a while to create the dependency graph.

@Composable
private fun rootController(
    context: Context
): RootController? {
    val controller by produceState<RootController?>(
        initialValue = null,
        producer = {
            value = withContext(Default) {
                PhoneComponent
                    .get(context)
                    .rootController()
            }
        }
    )

    return controller
}

[Tiago Nunes]

Yes, exactly. But the second approach with StateFlow being collected as State is best in most situations

[Tiago Nunes]

So you’d do something like:

private fun getRootControllerFlow(context: Context) = 
  flow { emit(PhoneComponent.get(context)) }
  .flowOn(Dispatchers.Default)
  .stateIn(viewModelScope, SharingStarted.WhileSubscribed(), null)

@Composable
fun Screen() {
  val rootController by getRootControllerFlow(LocalContext.current).collectAsState()
}

[Zoltan Demant]

I think the flow variant breaks due to a new flow being created everytime the function is recomposed?

[Tiago Nunes]

You’re right, it should be

val context = LocalContext.current
val rootController by remember(context) { getRootControllerFlow(context) }.collectAsState()

[Zoltan Demant]

That works 👍🏽 If you dont mind elaborating, how is this better than produceState?