I want to hide the bottom sheet from inside a ViewModel (after a backend call succeeded). To do this...

# compose

[Lukasz Kalnik]

I want to hide the bottom sheet from inside a ViewModel (after a backend call succeeded).
To do this, I need to emit a StateFlow from the ViewModel which contains a boolean property showBottomSheet.
In my screen's composable, I can then remember (and reinitialize) the bottom sheet state like this:

val modalBottomSheetState = rememberModalBottomSheetState(if (screenState.showBottomSheet) Expanded else Hidden)

That takes care of hiding the bottom sheet.
But how do I keep the showBottomSheet state in the ViewModel synchronized with the composable, when e.g. the user swipes down the bottom sheet? For this I included a callback in the composable: onBottomSheetDismissed.

I called this callback in the confirmStateChange lambda parameter of `rememberModalBottomSheetState()`:

// Composable

val modalBottomSheetState = rememberModalBottomSheetState(
    initialValue = if (screenState.showBottomSheet) Expanded else Hidden,
    confirmStateChange = {
        onBottomSheetDismissed()
        true
    }
)

// ViewModel

val screenState: StateFlow<ScreenState> = _screenState.asStateFlow()
private val _screenState: MutableStateFlow<ScreenState>

fun onBottomSheetDismissed() {
    _screenState.update { it.copy(showBottomSheet = false) }
}

But it appears that this callback is called very early in the dragging process, and my showBottomSheet variable in the ViewModel gets reset to false as soon as the user even drags the bottom sheet one pixel down and the bottom sheet immediately disappears (see video in thread)

Is there a better way to handle opening/closing the bottom sheet from a viewmodel and keeping the state synchronized between the two?

[Oleksandr Balan]

I was trying to sync those two in the ModalSheet lib, where we would like to have an easy API with only "visible" state (+ event lambda for update), but build on top of SheetState from the Compose.

In our inplementation we always start with a Hidden state, and use methods to show() / hide() the sheet when the boolean state changes.

You can check the code here, maybe it may help for your case:
https://github.com/oleksandrbalan/modalsheet/blob/main/modalsheet/src/main/java/eu/wewox/modalsheet/ModalSheet.kt#L118

[Lukasz Kalnik]

Thank you very much, I will take a look at it!

[Lukasz Kalnik]

I tried to use the LaunchedEffect block from your code, but generally I still have the same problem - onVisibleChange() callback is called too soon (as soon as the user starts dragging the finger down). So the callback to the viewmodel gets called immediately => the viewmodel emits the updated state (to hide the bottomsheet) => the bottomsheet disappears even though the user didn't finish dragging the finger down (or even moved his finger up again because he changed his mind).

[Lukasz Kalnik]

I see you actually wrap your ModalBottomSheetLayout into a FullScreenPopup, which gets the onDismiss callback. What is the reason for this?

[Oleksandr Balan]

the bottomsheet disappears

Hmm 🤔
Maybe you are removing the modal sheet from the composition when showBottomSheet flag is set to false? Could you share a code where ModalBottomSheetLayout is used?

FullScreenPopup … What is the reason for this?

The popup is a “trick” how to render the ModalSheet above all other UI. See Motivation & Solution sections.

[Lukasz Kalnik]

Ok, thanks for your explanation.
I change the bottomsheet state immediately when the showBottomSheet flag is false, yes:

val modalBottomSheetState = rememberModalBottomSheetState(
        initialValue = if (systemScreenState.showSensorSettings) Expanded else Hidden,
    )

[Oleksandr Balan]

And what if you try to set it always to Hidden and make it be driven with the LaunchedEffect and it’s .show() / .hide() ?

[Lukasz Kalnik]

Ah sorry, I actually started with the initialValue being always hidden, but it didn't work as well. The problem is simply the confirmStateChange callback is coming before the state change has been triggered, so the viewmodel state updates (hence emits) too early.

[Lukasz Kalnik]

I now fixed it using a DisposableEffect, which is kind of hacky, but works perfectly.
The fix is based on https://stackoverflow.com/a/69052933/4990683

[Lukasz Kalnik]

if (modalBottomSheetState.currentValue != ModalBottomSheetValue.Hidden) {
    DisposableEffect(Unit) {
        onDispose {
            onBottomSheetDismissed()
        }
    }
}

[Lukasz Kalnik]

This means that the DisposableEffect is added to the composition once the bottomsheet is shown. But once the bottomsheet is hidden, the DisposableEffect is removed from the composition, which triggers its onDispose method. In this way my viewmodel state is updated after the bottom sheet got dismissed.

[Lukasz Kalnik]

The alternative described in the Stackoverflow answer is to subscribe to snapshot changes inside a LaunchedEffect.

[Oleksandr Balan]

It is pretty weird, but only when I place the visible state and ModalBottomSheetState inside one Composable I get the behaviour you are describing 🤔
But when I extract the visible state to the outer Composable it suddenly works as I would expect 🤔

@Composable
private fun Demo() {
    var visible by remember { mutableStateOf(false) }

    DemoContent(
        visible = visible,
        onVisibleChange = { visible = it }
    )
}

@Composable
fun DemoContent(
    visible: Boolean,
    onVisibleChange: (Boolean) -> Unit,
) {
    val sheetState = rememberModalBottomSheetState(
        skipHalfExpanded = true,
        initialValue = ModalBottomSheetValue.Hidden,
        confirmStateChange = {
            onVisibleChange(it == ModalBottomSheetValue.Expanded)
            true
        }
    )

    LaunchedEffect(visible) {
        if (visible) {
            sheetState.show()
        } else {
            sheetState.hide()
        }
    }

    ModalBottomSheetLayout(
        sheetState = sheetState,
        sheetContent = {
            Box(
                modifier = Modifier
                    .height(500.dp)
                    .fillMaxWidth()
                    .background(Color.Cyan)
            )
        }
    ) {
        Box(
            contentAlignment = Alignment.Center,
            modifier = Modifier.fillMaxSize()
        ) {
            Button(onClick = { onVisibleChange(true) }) {
                Text("Open")
            }
        }
    }
}

[Lukasz Kalnik]

Interesting

[Lukasz Kalnik]

In the long run I want to migrate to Accompanist Navigation Material (or even Compose Destinations), and there I would have to do it completely differently anyway. The DisposableEffect now is just a temporary solution.

[Lukasz Kalnik]

This what you're talking about could be something in compose world called a deferred read.

[Oleksandr Balan]

Okay 👍

deferred read

Yea, I think it is due to changing the value right in the confirmStateChange lambda 🤔

[Lukasz Kalnik]

Which prevents recomposition as long as the value is not read inside of the composable