Jetpack Compose FAB hide on scroll behavior

Question

In the XML-based UI, FAB had a field to override the custom scroll behavior. Since there is no such parameter in the function in Compose FAB implementation, it became a little unclear how to implement the hide on scroll behavior.

I use Scaffold with the nestedScroll modifier in order for my СollapsingAppBar to work. I assumed at first that it was necessary to create an object inherited from the NestedScrollConnection interface. And also connect it with the nestedScroll modifier to Scaffold. But unfortunately, as I understand it, it is impossible to connect several scrollBehavior objects to one Scaffold at once.

val scrollBehavior = TopAppBarScrollBehavior()
val scrollBehavior1 = FloatingActionButtonScrollBehavior()

Scaffold(
    modifier = Modifier
        .nestedScroll(scrollBehavior.nestedScrollConnection)
        //this one won't work
        .nestedScroll(scrollBehavior1.nestedScrollConnection)
        ...

Answer 1

If someone is interested in how to implement this behavior, I found this solution:

@Composable
fun AnimatedFloatingActionButton(
    ...
    listState: LazyListState
) {
    AnimatedVisibility(
        visible = listState.isScrollingUp().value,
        enter = //TODO: your enter animation,
        exit = //TODO: your exit animation
    ) {
        FloatingActionButton(
            ...
        )
    }
}

You only need to create a LazyListState inside your Composable parent, and pass it inside the AnimatedFloatingActionButton. And also define an extension function for LazyListState.

@Composable
fun LazyListState.isScrollingUp(): State<Boolean> {
    return produceState(initialValue = true) {
        var lastIndex = 0
        var lastScroll = Int.MAX_VALUE
        snapshotFlow {
            firstVisibleItemIndex to firstVisibleItemScrollOffset
        }.collect { (currentIndex, currentScroll) ->
            if (currentIndex != lastIndex || currentScroll != lastScroll) {
                value = currentIndex < lastIndex ||
                        (currentIndex == lastIndex && currentScroll < lastScroll)
                lastIndex = currentIndex
                lastScroll = currentScroll
            }
        }
    }
}