How to retrieve the scrolling direction for LazyRow

user_15880865 0
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For a LazyRow, or Column, how to I know whether the user has scrolled left or right ( or up or... you know). We do not need callbacks in compose for stuff like that, since mutableStateOf objects always anyway trigger recompositions so I just wish to know a way to store it in a variable. Okay so there's lazyRowState.firstVisibleItemScrollOffset, which can be used to mesaure it in a way, but I can't find a way to store its value first, and then subtract the current value to retrieve the direction (based on positive or negative change). Any ideas on how to do that, thanks

android-jetpack-compose-list android-jetpack-compose android
2 Answers

Currently (1.0.x) there is no built-in function to get this info from LazyListState.

You can use something like:

@Composable
private fun LazyListState.isScrollingUp(): Boolean {
    var previousIndex by remember(this) { mutableStateOf(firstVisibleItemIndex) }
    var previousScrollOffset by remember(this) { mutableStateOf(firstVisibleItemScrollOffset) }
    return remember(this) {
        derivedStateOf {
            if (previousIndex != firstVisibleItemIndex) {
                previousIndex > firstVisibleItemIndex
            } else {
                previousScrollOffset >= firstVisibleItemScrollOffset
            }.also {
                previousIndex = firstVisibleItemIndex
                previousScrollOffset = firstVisibleItemScrollOffset
            }
        }
    }.value
}

Then just use listState.isScrollingUp() to get the info about the scroll.

This snippet is used in a google codelab.

user_20165620

Got it

{ //Composable Scope
val lazyRowState = rememberLazyListState()
    val pOffset = remember { lazyRowState.firstVisibleItemScrollOffset }
    val direc = lazyRowState.firstVisibleItemScrollOffset - pOffset
    val scrollingRight /*or Down*/ = direc > 0 // Tad'aa
}
user_158808650
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