Is there a generic way to know when the scroll sta...

# compose

[julioromano]

Is there a generic way to know when the scroll state of a scrollable composable is at its topmost position?

Thing is Lazy lists hold scroll state in the LazyListState type whilst composables that scroll using modifiers hold scroll state in the ScrollState  type.
Even though both types implement  ScrollableState  the way they expose the current scroll position differs:
LazyListState exposes firstVisibleItemIndex and firstVisibleItemScrollOffset for this purpose, while ScrollState exposes value instead.

This is making it more difficult to implement a generic solution that works with both e.g. a LazyColumn and a Column which uses the verticalScroll modifier.

In case you’re wondering what I’d like to accomplish you’ll find more info in the thread.

[julioromano]

I'd like to change the `topBar`'s elevation inside a Scaffold depending on whether its main content has been scrolled all the way to the top or not.

I managed to do that by hoisting the `LazyColumn`'s LazyListState up into the Scaffold but I'd like a generic solution that works for any scrollable composable that could be hosted in the Scaffold's content. Do you think it's possible?

[julioromano]

I’ve also tried with the nested scroll subsystem but it seems it only informs about scroll events, and not scroll position. So it’s not possible to extract a scroll position from it.

[Albert Chang]

You can increase the elevation in onPostScroll when there's offset available in a downwards scroll, and decrease it in onPreScroll when it's a upwards scroll.

[julioromano]

This actually works! But I still don’t understand why 🙂. I think I’m missing something about the nested scrolling subsystem.

[Timo Drick]

I think there is this nestedScroll modifier for such use cases. But i think it is not working reliable for now to identify the exact scrolling position:
https://developer.android.com/reference/kotlin/androidx/compose/ui/input/nestedscroll/package-summary#(androidx.compose.ui.[…]estedScrollDispatcher)

[julioromano]

Yes it works @Timo Drick, that’s what @Albert Chang suggested and that’s what I’ve done in the end.
I still don’t fully understand how the offsets in those callbacks work, but I was able to accomplish what I originally asked for.