What's the difference between `rememberUpdatedState` , `remember` and `rememberSaveable` in Android?...

# compose

[Bilagi]

What's the difference between rememberUpdatedState , remember and rememberSaveable in Android?
I've gone through the docs but don't understand properly when to use each.

I know rememberSaveable persist state across configuration changes
Can anyone please suggest some use cases? Also, does rememberUpdatedState persist state across configuration changes?

[Stylianos Gakis]

rememberUpdatedState has nothing to do with persisting things accross config changes. The docs that explain the use case the best are here https://developer.android.com/develop/ui/compose/side-effects#rememberupdatedstate.
But if this is too confusing to you, I think you can initially get away with not using it in the beginning before it starts making sense. I do remember it was a while before I properly understood what it does from when I started with compose.

[Stylianos Gakis]

Here is one example with it
https://kotlinlang.slack.com/archives/CJLTWPH7S/p1678750942883689?thread_ts=1678732457.665529&cid=CJLTWPH7S

And here is another where rememberUpdatedState fixes one bug
https://kotlinlang.slack.com/archives/CJLTWPH7S/p1640285517040200?thread_ts=1640270959.029600&cid=CJLTWPH7S

The tl;dr that made it click for me is that rememberUpdatedState turns T into State<T>. But I understand how this can be tricky by itself

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

• remember is a fundamental primitive of the compose runtime.
• rememberSaveable extends that primitive with support for a pluggable state persistence mechanism that requires support from the hosting app toolkit .
• rememberUpdatedState is just a convenience function for a certain pattern that’s useful to save a few lines of code in certain cases.

[Matthew Feinberg]

Just in case a different point of view helps, another way to look at it is what they are used for:
[disclaimer: I've been using Compose for a bit less than a year, so it's possible I may have some minor inaccuracies here]

💁 remember

Use remember when you want something to survive recomposition (instead of making a new instance with each recomposition). Note that if the remember call leaves and re-enters the composition, you'll still get a new instance.


💁 rememberSaveable

Use rememberSaveable when you want something to survive recomposition and certain kinds of platform-specific potential state loss.

This depends very much on the platform app toolkit. For example, Android provides a mechanism for saving state in a Bundle so it can survive a process getting killed and re-created when the system is low on RAM, or survive activity re-creation (such as for some kinds of configuration changes). For some platforms (I think!) this might just do the same thing as remember.

Use rememberSaveable to save ephemeral state like the scroll position of a list or the cursor position in a text field.

Don't use rememberSaveable for things that are affected by configuration changes, unless you have a separate way to handle those configuration changes yourself. For example, you probably don't want to use rememberSaveable for things like localized UI strings, because you want those to be re-created with the correct values when (for example) the user changes the system language.


💁 rememberUpdatedState

This is very different from the others. The most common use for this when you have a lambda that sticks around over recompositions, but you want the latest value (not the one captured by the lambda).

For example, this is very useful together with .pointerInput if you are handling a gesture that might affect the composition during the gesture: The pointer input lambda can't be re-created mid-gesture or you lose the gesture tracking, but it might need to access the latest values of state that change as a result of that gesture.

As Zach said, it's a convenience function. You can look at the implementation of it yourself to see how it works (from Android Studio or Idea, it's easy to jump into to the source code for any of the compose APIs...if you're not already doing it, it's a great way to learn how compose works):

@Composable
fun <T> rememberUpdatedState(newValue: T): State<T> = remember {
    mutableStateOf(newValue)
}.apply { value = newValue }

As you can see, it's just 4 lines of code! All it does is remember a State and then update that State with the latest value whenever there's a recomposition, so that if you capture it in a lambda, you're getting the latest value instead of the value when the lambda was created.