Using rememberSaveable with mutableStateListOf

Question

I'm trying to add a mutable list of parcelable objects to my composable. I also want to be able to add objects to and remove objects from it.

Currently I'm using something like this:

val names = remember { mutableStateListOf<String>() }

names.add("Bill")
names.remove("Bill")

Now I want this list to survive configuration change, therefore it's perhaps a good idea to use rememberSaveable. Perhaps something like this:

val names = rememberSaveable { mutableStateListOf<String>() }

names.add("Bill")
names.remove("Bill")

But this does not work, it throws the following exception:

androidx.compose.runtime.snapshots.SnapshotStateList cannot be saved using the current SaveableStateRegistry. The default implementation only supports types which can be stored inside the Bundle. Please consider implementing a custom Saver for this class and pass it to rememberSaveable().

This means, that SnapshotStateList (the result of mutableStateListOf) is not saveable.

So far I can think of few ways how to work around this:

1. Actually implementing a saver for SnapshotStateList.
2. Using something like val namesState = rememberSaveable { mutableStateOf(listOf<String>()) }. This does indeed work flawlessly, however updating the list requires setting the value, which is both slow and inconvenient (e.g. namesState.value = namesState.value + "Joe" for just adding a single element).

Both these ways seem too complicated for a seemingly small task. I wonder what is the best way to do what I want. Thanks.

Answer 1

There should be no data inside remember or rememberSaveable that you are afraid of losing: it's gonna be destroyed as soon as your Composable disappears (goes out of composition, to be precise). Consider using view models in this case.

---

If you are still interested in storing mutableStateListOf inside rememberSaveable, I suggest that you follow your error recommendation and create a Saver for SnapshotStateList:

@Composable
fun <T: Any> rememberMutableStateListOf(vararg elements: T): SnapshotStateList<T> {
  return rememberSaveable(saver = snapshotStateListSaver()) {
    elements.toList().toMutableStateList()
  }
}

private fun <T : Any> snapshotStateListSaver() = listSaver<SnapshotStateList<T>, T>(
  save = { stateList -> stateList.toList() },
  restore = { it.toMutableStateList() },
)

Then, you can use it like this:

val names = rememberMutableStateListOf<String>()
LaunchedEffect(Unit) {
    names.add("Bill")
}
Text(names.joinToString { it })

The expected behaviour for this sample: each time you rotate your device - one more element gets added.

Don't use any state modifications inside the composable like you did when you added and removed an item. You should only do that inside side-effects, like I did here with LaunchedEffect, or callbacks, like onClick.

Note that saveableMutableStateListOf is still limited to Bundle-saveable types, like String, Int, etc. If you need to store a custom type inside, you will need to modify Saver to save/recreate it too.