I have a question about memory management for clas...

# compose

[Ryan Kay]

I have a question about memory management for classes which are instantiated within compose functions. I have a KMM project, and for the desktop version, I create a few pure Kotlin classes within my top level composable. I use a remember delegate which contains a enum class denoting each screen of the application (four screens total). When I want to navigate to a new screen in the desktop version, I just switch change that remembered enum and the recomposition occurs.

So the thing I'm wondering is how to make sure that the classes (basically a Presenter, ViewModel, and Container class for a given screen) are being GCed properly after recomposition occurs. It's worth noting that they are instantiated within a composable that gets recomposed. Is there a general rule about what happens to such classes? My guess is that they are held on to via the slotTable, but I don't know for sure.

I asked this in the #compose-desktop channel but it was suggested to ask here.

[Ryan Kay]

Code example

fun main() = application {
//...
    var windowState by remember {
        mutableStateOf(WindowState.VIEW_DAY)
    }
//..
    Window(onCloseRequest = ::exitApplication, title = "Todo", resizable = false) {

        SamsaraTheme {
            when (windowState) {
                WindowState.VIEW_DAY -> {
                    val vm = DayViewModel()
                    val container = buildDayFeature(
                        { newState: WindowState, arg: Int ->
                            windowState = newState
                            windowArgs = arg
                        },
                        vm,
                        storageService
                    )

                    DayViewScreen(
                        container.logic::onViewEvent,
                        vm
                    ) //...
                }}}}}


fun buildDayFeature(
    stateHandler: (WindowState, Int) -> Unit,
    vm: DayViewModel,
    storageService: StorageService
): DayViewContainer =
    DayViewContainer(stateHandler).start(storageService, vm)

[Adam Powell]

If you remember {} something it will be hard referenced by the composition. Any lambda captures you create that Compose might need to re-run to recompose will also be hard referenced by the composition. These references are dropped when the @Composable caller leaves the composition, or when the composition itself is disposed.

[Adam Powell]

Your val vm = DayViewModel() will create a new instance of DayViewModel each time the SamsaraTheme trailing lambda function recomposes and windowState == WindowState.VIEW_DAY. Generally when someone writes something called ViewModel this is not what they intend. 🙂 You might want to wrap a remember {} around that so that it keeps the same instance across recompositions.

[Adam Powell]

e.g. val vm = remember { DayViewModel() }

[Ryan Kay]

@Adam Powell Thanks Adam; appreciate the explanation!