What is the difference between the lifecycle of a viewModel and the remember function?

I think I can use either a ViewModel or the remember function to persist states, so I can share some states and parameters among functions using the following code.

Also, I think I use the remember function instead of ViewModel sometimes.

@Composable
fun ScreenHome(  
    recordSoundViewModel: RecordSoundViewModel = hiltViewModel(),
    lineParState: LineParameterState = rememberLineParameterState()    
) {

}

@Composable
fun ItemContent(
    index:Int,
    lineParState: LineParameterState = rememberLineParameterState(),
    recordSoundViewModel: RecordSoundViewModel = hiltViewModel()
) {

}



@HiltViewModel
class RecordSoundViewModel @Inject constructor(
    private val appContext: Application,
): ViewModel()
{

}

class LineParameterState private constructor() {
    var isShowFirstRunPrompt by mutableStateOf(false)

    val fontSizeSecondary = 12.sp

    companion object {    
        val instance: LineParameterState by lazy { LineParameterState() }
    }

}


@Composable
fun rememberLineParameterState(): LineParameterState {
     return rememberSaveable {
        LineParameterState.instance
    }
}

What are the differences in lifecycle between ViewModel and the remember function?

remember is intended for UI-related things, like keeping track of the position of a UI element being dragged or what index of a drop-down should be shown as selected. You can theoretically use it to store your data/model, but that would be poor separation of concerns and make it very difficult to unit test either your UI or your model. All of your examples above fit this usage.

Conversely, it would also be poor separation of concerns to be storing your UI status in the ViewModel.

ViewModel is not only the more sensible place to store the data/model, but since it is a class you can easily connect to databases and web APIs and do complex logic to populate your State there.

Both ViewModel and remember can be scoped to either a Fragment or an Activity.

<h2>Lifecycle</h2>

<h3>ViewModel</h3>

ViewModel's lifecycle is scoped to ViewModelStore when it's destroyed onClear method of every ViewModel it stores are called.

    getLifecycle().addObserver(new LifecycleEventObserver() {
        @Override
        public void onStateChanged(@NonNull LifecycleOwner source,
                @NonNull Lifecycle.Event event) {
            if (event == Lifecycle.Event.ON_DESTROY) {
                // Clear out the available context
                mContextAwareHelper.clearAvailableContext();
                // And clear the ViewModelStore
                if (!isChangingConfigurations()) {
                    getViewModelStore().clear();
                }
                mReportFullyDrawnExecutor.activityDestroyed();
            }
        }
    });

But ViewModel can survive configuration changes such as rotation and using SavedStateHandle you can get data back from SavedStateHandle or navigation. And using a Activity scoped ViewModel you can shared data between data fragments using same ViewModel.

@Override
@Nullable
@SuppressWarnings("deprecation")
public final Object onRetainNonConfigurationInstance() {
    // Maintain backward compatibility.
    Object custom = onRetainCustomNonConfigurationInstance();

    ViewModelStore viewModelStore = mViewModelStore;
    if (viewModelStore == null) {
        // No one called getViewModelStore(), so see if there was an existing
        // ViewModelStore from our last NonConfigurationInstance
        NonConfigurationInstances nc =
                (NonConfigurationInstances) getLastNonConfigurationInstance();
        if (nc != null) {
            viewModelStore = nc.viewModelStore;
        }
    }

    if (viewModelStore == null && custom == null) {
        return null;
    }

    NonConfigurationInstances nci = new NonConfigurationInstances();
    nci.custom = custom;
    nci.viewModelStore = viewModelStore;
    return nci;
}

<h3>remember</h3>

remember's lifecycle is scoped to Composable its in, when the Composable it's in exits composition remember is forgotten. You can observe when data is remembered, forgotten or abandoned with RememberObserver.

private class SampleUiState : RememberObserver {

    var counter by mutableStateOf(0)


    override fun onAbandoned() {
        println("🔥 onAbandoned")
    }

    override fun onForgotten() {
        println("🏈 onForgotten")
    }

    override fun onRemembered() {
        println("🔥 onRemembered")
    }

}

If you create a function that remembers SampleUiState as

@Preview
@Composable
private fun Sample() {

    var showSample by remember {
        mutableStateOf(false)
    }
    Column(
        modifier = Modifier
            .fillMaxSize()
            .padding(16.dp)
            .verticalScroll(rememberScrollState())
    ) {
        Button(onClick = { showSample = showSample.not() }) {
            Text(text = "Show Composable")
        }
        if (showSample) {
            RememberObserverSample()
        }
    }
}

@Composable
private fun RememberObserverSample() {
    val sampleUiState = remember {
        SampleUiState()
    }

    Button(
        modifier = Modifier.fillMaxWidth(),
        onClick = { sampleUiState.counter++ }
    ) {
        Text(text = "Increase Counter")
    }

    Text(text = "Counter: ${sampleUiState.counter}")
}

In the sample above RememberObserverSample lifecycle so the remember it contains is limited to if block returning true as you check out logs returned in each RememberObserver callback.

<h2>Selecting between ViewModel and remember</h2>

ViewModel is suitable as presentation layer to hold data from data layer while remember is preferred with generally with Ui related class but it's not always the case since LaunchedEffect and DisposableEffect uses remember under the hood and these functions can be used to call analytics, doing for measurements, calculations, sorting, calling an api request and more non-ui related utilities as well.

DisposableEffect uses onForgotten to call dispose function.

When building an ui library or a Composable or Modifier remember is preferred over ViewModel as you can see with default rememberXState functions such as

@Composable
fun rememberScrollState(initial: Int = 0): ScrollState {
    return rememberSaveable(saver = ScrollState.Saver) {
        ScrollState(initial = initial)
    }
}

Using a separate state lets you do calculations or animations inside a class instead of a very big Composable and also making this as a Modifier gives more flexibility over building a Composable too.

<h2>Summary</h2>
If you wish to manipulate business data you can select ViewModel or any presentation layer if you are using a MVVM/MV/MVP architecture.

If you are holding state for a screen states such as loading, success, error i still think ViewModel is better option.

When you build a custom Composable or a library custom remember passed to Modifier gives a lot of flexibility and with a library it's redundant to create ViewModel.

And last but not least, remember function is not only for ui data, it's for creating memory for Composable functions when you don't have or don't want to use an external persistent state storage like ViewModel or other presentation layer classes.

You can create a timer, one time effect with scope as LaunchedEffect does, observe forgotten with DisposableEffect or create state with

@Composable
fun <T> produceState(
    initialValue: T,
    @BuilderInference producer: suspend ProduceStateScope<T>.() -> Unit
): State<T> {
    val result = remember { mutableStateOf(initialValue) }
    LaunchedEffect(Unit) {
        ProduceStateScopeImpl(result, coroutineContext).producer()
    }
    return result
}

which in its core based on remember because of LaunchedEffect, an explicit remember.

@Composable
@NonRestartableComposable
@OptIn(InternalComposeApi::class)
fun LaunchedEffect(
    key1: Any?,
    block: suspend CoroutineScope.() -> Unit
) {
    val applyContext = currentComposer.applyCoroutineContext
    remember(key1) { LaunchedEffectImpl(applyContext, block) }
}

this is a example

if (anything) {
    var field1 by remember { mutableStateOf(1) }
    Text(field1.toString(), modifier = Modifier.clickable { field1++ })
} else {
    var field2 by remember { mutableStateOf(1) }
    Text(field2.toString(), modifier = Modifier.clickable { field2++ })
}

if anything change, field1 or field2 will reset to 1

because the remember function will be forget when leave composable

but ViewModel is not, its lifecycle as long as Activity, or even longer