ThingCounter.kt

@Composable
fun Parent() {
  Column {
    var length by remember { mutableStateOf(1) }
    val stringy by remember { derivedStateOf { { "a".repeat(length) } } }
    ThingCounter(stringy)
    Button(onClick = { length++ }) {
      Text(text = "Change stringy")
    }
  }
}

@SuppressLint("UnrememberedMutableState")
@Composable
fun ThingCounter(stringy: () -> String) {
  var count by mutableStateOf(0)
  Button(onClick = { count++ }) {
    Text("You've had $count ${stringy()}.")
  }
}