无需重新组合 [Jetpack Compose]

Question

In my OpinionViewModel:

val labels = MutableStateFlow<MutableList<String>>(mutableListOf())

when actions come from UI (from BottomSheet item) I add item to my list:

 state.labels.value.add("Lorem Ipsu") 

My main @Composable:

fun OpinionScreen(viewModel: OpinionViewModel) {
        
            val scrollState = rememberScrollState()
            val bottomState = rememberModalBottomSheetState(ModalBottomSheetValue.Hidden)
        
            val items by viewModel.state.labels.collectAsState()
            
            AppTheme(isSystemInDarkTheme()) {
                ModalBottomSheetLayout(
                    sheetState = bottomState,
                    sheetContent = { AddPhotoBottomSheet()}) {
                    Scaffold(
                        topBar = { TopBar()},
                        content = {
                            Box(
                                modifier = Modifier.fillMaxSize()
                            ) {
                                Column(
                                    modifier = Modifier
                                        .fillMaxWidth()
                                        .wrapContentHeight()
                                        .verticalScroll(scrollState)
                                ) {
                                    FirstSection()
                                    HowLongUsageSection(photos)
                                    ThirdSection()
                                }
                            }
        
                        })
                }
            }
        }

@Composable Section:

fun HowLongUsageSection(labels: MutableList<String>) {
    Column(
        modifier = Modifier
            .fillMaxWidth()
            .padding(top = 10.dp, start = 10.dp, end = 10.dp),
        horizontalAlignment = Alignment.CenterHorizontally
    ) {
        labels.forEach {
            Text(text = it, color = Color.Blue)
        }
}

The problem is the no recomposition happening. I need to go back and open UI again to see added item. Am I missing something or the problem is related to bottom sheet?

Answer 1

The problem is that your MutableList does not change when you modify its contents (the reference still remains the same). To fix this, use a List instead of MutableList.

val labels = MutableStateFlow(listOf<String>())

To add an element to it:

label.value += "Lorem Ipsum"

Now the state flow will emit a new value and your UI will get updated.