IssueTracker

At the moment it is not easy to tell the direction the LazyList was scrolled.

It is currently only possible by:

1. Launching an effect
2. Creating a snapshot flow of listState.firstVisibleItemScrollOffset
3. transforming that flow with a scan operator to compare previous and current values, and then deriving the scroll direction

The above is not bullet proof either as listState.firstVisibleItemScrollOffset when going between sections with sticky headers first increases monotonically, and then resets itself to 0.

Currently, to be able to fully infer scroll direction, the following is needed:

private data class ScrollState(
    val offset: Int = 0,
    val minViewPortIndex: Int = 0,
    val maxViewPortIndex: Int = 0,
    val isDownward: Boolean = true,
)

private val ScrollState.key get() = "$minViewPortIndex-$maxViewPortIndex-$isDownward

// Check if there is a reset of offset
private fun ScrollState.updateDirection(new: ScrollState) = new.copy(
    isDownward = when {
        // Use the last value of isDownward in case there was a reset of offset to 0
        abs(new.offset - offset) > 10 -> isDownward
        // Scroll position is downward if the new offset is greater than the old
        else -> new.offset > offset
    }
)


LaunchedEffect(listState) {
    snapshotFlow {
        ScrollState(
            offset = listState.firstVisibleItemScrollOffset,
            minViewPortIndex = listState.firstVisibleItemIndex,
            maxViewPortIndex = listState.layoutInfo.visibleItemsInfo.lastOrNull()?.index
                ?: 0
        )
    }
        .scan(ScrollState(), ScrollState::updateDirection)
        .distinctUntilChangedBy(ScrollState::key)
        .collect { scrollInfo ->
            onScrollStateChanged(scrollInfo, viewModel.accept)
        }
}