Compose - How to know when a sub-composable has been rendered

Question

Working on Compose, I'm trying to know when a sub-composable has been rendered to start my LaunchEffect

Here's a little snippet of what I'm trying to achieve :

fun MyCompo(
    screen: @Composable (Modifier) -> Unit,
) {
    val fr = remember { FocusRequester() }
    screen(Modifier.focusRequester(fr))

    LaunchEffect(Unit) {
        fr.requestFocus()
    }
}

My LaunchEffect is crashing because it's launched before screen() had time to render.

Any idea how to achieve this ? Thanks

Answer 1

How about adding a callback to your screen Composable and invoking it from LaunchedEffect from screen implementation? Does this work?

@Composable
fun MyComposable(
    screen: @Composable (Modifier, ()->Unit) -> Unit,
) {
    val fr = remember { FocusRequester() }
    screen(Modifier.focusRequester(fr)) {
        println("Child Composable")
    }

    LaunchedEffect(Unit) {
        println("Parent Composable")

        fr.requestFocus()
    }
}

And invoking it

MyComposable { modifier, onComposed->
    Box(modifier = modifier
        .size(200.dp)
        .border(3.dp, Color.Red)) {

        LaunchedEffect(key1 = Unit){
            onComposed()
        }
    }

}