LaunchedEffect not triggered when mutableStateOf h...

# compose

[Aaron Waller]

LaunchedEffect not triggered when mutableStateOf has same value?
I want to display a snackbar message when the user clicks on “Favorite Post”
It works fine the first time but after that it is not getting triggered anymore.

Here is my snackBarMessage State in ViewModel:

var snackBarMessage: MutableState<String?> = mutableStateOf(null)
    private set

and the set function:

fun setSnackbarMessage(msg: String?){
    snackBarMessage.value = null
    snackBarMessage.value = msg
}

Here is my Launcheffect which unfortunately is not getting triggered twice:

LaunchedEffect(key1 = viewModel.snackBarMessage.value){ 
    val message = viewModel.snackBarMessage.value
    message?.let {
        scaffoldState.snackbarHostState.showSnackbar(message, "Ok")
    }
}

and on Favorite button click I call:

viewModel.setSnackbarMessage("Added to Favorite")

Eventhough I set the snackBarMessage to null it is not getting retriggered, only if the string message is different from the one before.
I want to show the snackbar every time the user clicks on the button and not just the first time

[oianmol]

also you can collectAsState

[oianmol]

check here https://github.com/mutualmobile/praxis/blob/baseproject/ui-authentication/src/main/java/com/praxis/feat/authentication/ui/AuthenticationUI.kt

[ste]

Instead of relying on the message itself, you should have a isDisplayed variable.

[Aaron Waller]

with

LaunchedEffect(key1 = viewModel.snackBarMessage)

its not getting triggered at all.
I changed my mutableStateOf to a MutableStateFlow and collected it as a state but its still the same behaviour, it just gets called the very first time

[ste]

Also, I never used material's snackbar, but I guess your snackBarMessage is useless. You can directly call scaffoldState.snackbarHostState::showSnackbar inside your clickable or whatever

[Aaron Waller]

Yes my approach was actually unnecessary.
I now simplay call this from my onclick:

scope.launch {
scaffoldState.snackbarHostState.showSnackbar("Added to Favorite", actionLabel = "Ok")
}

and it gets called every time.
Thanks guys

[Casey Brooks]

I don’t think it’s not that the LaunchedEffect isn’t triggering properly, it’s that the mutableStateOf is optimizing itself when it sees that the new value is the same as the old value. mutableStateOf() by default uses structuralEqualityPolicy() to do that optimization, but you can use a different policy such as referentialEqualityPolicy() or neverEqualPolicy() to tweak it so that it triggers when you expect it should. In this case, to have it trigger twice on null, you’ll need the neverEqualPolicy()

var snackBarMessage: MutableState<String?> = mutableStateOf(null, policy = neverEqualPolicy())
    private set

[Zach Klippenstein (he/him) [MOD]]

Directly invoking the showSnackbar function from your event handler is the correct approach. Sure you can use neverEqualPolicy to maybe make this work, but there’s a reason that path has extra friction. State, by definition, is idempotent, so most types for working with state (MutableState, MutableStateFlow) automatically conflate equivalent values.