IssueTracker

Jetpack Compose version: 1.3.0-alpha01

Jetpack Compose component(s) used: LazyColumn and NestedScrollConnection

Android Studio Build: Android Studio Chipmunk 2021.2.1 Patch 1

Kotlin version: 1.7.0

Hi,

NestedScrollConnection::onPreScroll is not called when manually scrolling to a specific position via scrollToItem or animateScrollToItem. It would be very convenient if NestedScrollConnection::onPreScroll was called

Code Sample to Reproduce:

val scope = rememberCoroutineScope()
val scrollState = rememberLazyListState()

Scaffold(
    floatingActionButton = {
        ExtendedFloatingActionButton(
            onClick = {
                scope.launch {
                    scrollState.animateScrollToItem(0)
                }
            }
        ) { Text("Scroll to top") }
    },
    content = { innerPadding ->
        val toolbarHeight = 48.dp
        val toolbarHeightPx =
            with(LocalDensity.current) { toolbarHeight.roundToPx().toFloat() }
        val toolbarOffsetHeightPx = remember { mutableStateOf(0f) }

        val nestedScrollConnection = remember {
            object : NestedScrollConnection {
                override fun onPreScroll(
                    available: Offset,
                    source: NestedScrollSource
                ): Offset {
                    val delta = available.y
                    val newOffset = toolbarOffsetHeightPx.value + delta
                    toolbarOffsetHeightPx.value =
                        newOffset.coerceIn(-toolbarHeightPx, 0f)
                    return Offset.Zero
                }
            }
        }
        Box(
            Modifier
                .fillMaxSize()
                .nestedScroll(nestedScrollConnection)
        ) {
            LazyColumn(
                state = scrollState,
                contentPadding = PaddingValues(top = toolbarHeight)
            ) {
                items(100) { index ->
                    Text(
                        "I'm item $index", modifier = Modifier
                            .fillMaxWidth()
                            .padding(16.dp)
                    )
                }
            }
            SmallTopAppBar(
                modifier = Modifier
                    .height(toolbarHeight)
                    .offset {
                        IntOffset(
                            x = 0,
                            y = toolbarOffsetHeightPx.value.roundToInt()
                        )
                    },
                title = { Text("Home") }
            )
        }
    }
)