onPlacedDemo

Demo.kt

@Preview
@Composable
private fun LazyColumnDemo() {
    var spacerPositionOffset by remember {
        mutableStateOf(Offset.Unspecified)
    }
    val configuration = LocalConfiguration.current
    val windowSize = remember(configuration) {
        DpSize(configuration.screenWidthDp.dp, configuration.screenHeightDp.dp)
    }

    Box(Modifier.fillMaxSize()) {
        LazyColumn {
            items(10) {
                Box(Modifier.padding(4.dp)) {
                    Card {
                        Box(
                            Modifier
                                .background(Color.LightGray)
                                .fillMaxWidth()
                                .height(100.dp)
                        )
                    }
                }
            }

            item {
                Spacer(modifier = Modifier
                    .fillMaxWidth()
                    .height(40.dp)
                    .onPlaced {
                        spacerPositionOffset = it.positionInWindow()
                    }
                )

                val density = LocalDensity.current
                DisposableEffect(density) {
                    // TODO: this is practically (un)placed
                    onDispose {
                        with(density) {
                            if (spacerPositionOffset != Offset.Unspecified
                                    && spacerPositionOffset.y < windowSize.height.roundToPx() / 2) {
                                spacerPositionOffset = Offset(0f, -40.dp.toPx())
                            } else {
                                spacerPositionOffset = Offset.Unspecified
                            }
                        }
                    }
                }
            }

            items(10) {
                Box(Modifier.padding(4.dp)) {
                    Card {
                        Box(
                            Modifier
                                .background(Color.LightGray)
                                .fillMaxWidth()
                                .height(100.dp)
                        )
                    }
                }
            }
        }

        Button(
            onClick = {}, modifier = Modifier
                .align(Alignment.BottomCenter)
                .height(40.dp)
                .fillMaxWidth()
                .layout { measurable, constraints ->
                    val placeable = measurable.measure(constraints)
                    layout(placeable.width, placeable.height) {
                        val position = coordinates?.positionInWindow() ?: Offset.Zero
                        val spacerPosition =
                            if (spacerPositionOffset == Offset.Unspecified) position else spacerPositionOffset
                        // windowSize is 25 pixels smaller on my emulator (no idea why), so offsetting it by 25
                        val offset = if (spacerPosition.y > windowSize.height.roundToPx() + 25) {
                            IntOffset.Zero
                        } else {
                            IntOffset(
                                (spacerPosition.x - position.x).roundToInt(),
                                (spacerPosition.y - position.y).roundToInt()
                            )
                        }
                        placeable.place(offset)
                    }
                }
        ) {
            Text("Test")
        }
    }
}