I want to remember scroll state across configurati...

# compose

[Rick Regan]

I want to remember scroll state across configuration changes within a Text with modifier .verticalScroll(scrollState).

• val scrollState = rememberScrollState() does not survive a configuration change.
• val scrollState = rememberSaveable { ScrollState(0) } gets an exception "ScrollState@fb46372 cannot be saved using the current SaveableStateRegistry".
• val scrollState = ScrollState(0) globally outside of any Composable works, but this is putting the scroll state outside the scope of the Composable. Is it OK to do that? Is there another way?

[Ian Lake]

rememberScrollState uses rememberSaveable internally, so it certainly does save and restore its state after a config change: https://cs.android.com/androidx/platform/frameworks/support/+/androidx-main:compose/foundation/foundation/src/commonMain/kotlin/androidx/compose/foundation/Scroll.kt;l=81

[Rick Regan]

Hmmm, thanks. It did not work for me...I'll try it again with a minimal repro.

[Ian Lake]

Is what you're trying to scroll also immediately available after a config change? If there's a composition with no data, then that restored scroll position is going to be reset to the max available (i.e., to the top if there's no content at all)

[Rick Regan]

I have a fixed height Text that I've scrolled down into. When I rotate, the text is still there and in its proper scroll position when I use the third option.

[Rick Regan]

rememberScrollState does work in a small example, but not in my app. I'll keep looking into it...

[Rick Regan]

I know what it is. I use the same element in both configurations, with logic like

if (maxWidth < 500.dp) {
  MyComposable()
} else {
  MyComposable()
}

So each one gets its own fresh ScrollState. (The text they display is in my app state.) To fix it, I have to hoist rememberScrollState. If I put it in that top-level composable then I have to pass it down through many layers of composables (not shown). That makes option 3 above (val scrollState = ScrollState(0) in my app state) more appealing. But is that bad practice?

[Ian Lake]

Well, option 3 won't actually help with process death and recreation (i.e., what you test when you enable the 'Don't keep activties' developer option); you really do need to save it and restore it at some level

[Ian Lake]

State hoisting is a natural part of Compose: https://developer.android.com/jetpack/compose/state#state-hoisting

[Rick Regan]

Thanks, I was overlooking process death (which is down there on my todo list 🙂 ). Obviously then I'll also have to save the accompanying text with rememberSaveable. I have the text as text by mutableStateOf() in my "view model" (not actually ViewModel); so looking ahead: is rememberSaveable(text) { text } (at an appropriate-level composable) the way to do it so that it keeps updating the "save"?

[Ian Lake]

If that text hasn't been actually persisted to disk, yep, you'll want to use rememberSaveable

[Rick Regan]

Thanks so much for the help.