How to use a mutableStateList / mutableStateMap as...

# compose

[Andy Himberger]

How to use a mutableStateList / mutableStateMap as a key for remember or side effect? or you just shouldn't try?

We have a number of places with a mutableStateMap in a viewmodel, that gets exposed as just a Map<> to the consumer. If someone tries to do something like remember(map) { // create something base on map }, it doesn't get invalidated when changes to the map are made.

[Andy Himberger]

Here's more of an example:

class VM ... {
    val componentMap: Map<String, Component>
        get() = _componentMap
    private val _componentMap = mutableStateMapOf<String, Component>()
}

@Composable
fun UsingMap() {
    val vm = viewModels<VM>()
    val componentMap = vm.componentMap
    val filteredMap = remember(componentMap) {
        componentMap.values.filterIsInstance<FooComponent>()
    }
    filteredMap.forEach{
        //
    }
}

Doing this instead works:

// Works, only way?
    val filteredMap by remember {
        derivedStateOf {            componentMap.values.filterIsInstance<FooComponent>()
        }
    }

Which solves the remember case. Not sure about an Effect

[Alex Vanyo]

There’s nothing wrong with it directly, but it depends on the behavior you want.

remember, key and the effects use .equals to determine if the argument(s) have changed, and .equals is different than being @Stable and being observable for snapshot state

For arguments that are mutable, .equals must return true for the same instance, so remember will only run once for the same instance.

LazyListState is a good example where that is useful:

LaunchedEffect(lazyListState) {
    snapshotFlow { lazyListState.layoutInfo.totalItemsCount }
        .collect {
            // use the new items count
        }
}

The LaunchedEffect here will restart whenever the instance of LazyListState changes.
But then the internal properties of LazyListState can themselves change and we can observe them with snapshotFlow.

So something else you could try is remember(componentMap.toMap()) which creates an immutable copy to compare against for .equals

[Andy Himberger]

yeah I was thinking .toMap() maybe another alternative once we move to Compose 1.4 and toMap won't always make a new copy.

snapshotFlow is a good reminder too will try that out