PlacementWithinAnchor

PlacementWithinAnchor.kt

@Composable
fun PlacementWithinAnchor(
    modifier: Modifier,
    content: @Composable PlacementWithinAnchorScope.() -> Unit
) {
    CompositionLocalProvider(
        LocalLayoutCoordinatesHolder provides ContainerLayoutCoordinatesHolder()
    ) {
        Box(
            modifier = modifier
        ) {
            PlacementWithinAnchorScope.content()
        }
    }
}

private val LocalLayoutCoordinatesHolder = compositionLocalOf<ContainerLayoutCoordinatesHolder> {
    throw IllegalStateException("No LayoutCoordinates provided")
}

object PlacementWithinAnchorScope {
    @Stable
    fun Modifier.placementAnchor() = composed {
        val currentLayoutCoordinatesHolder = LocalLayoutCoordinatesHolder.current
        onPlaced {
            currentLayoutCoordinatesHolder.containerLayoutCoordinates = it
        }
    }

    @Stable
    fun Modifier.onPlacedWithinAnchor(
        onPlaced: (WithinAnchor) -> Unit
    ) = composed {
        val currentLocalLayoutCoordinatesHolder = LocalLayoutCoordinatesHolder.current
        onGloballyPositioned {
            val anchorLayoutCoordinates = currentLocalLayoutCoordinatesHolder.containerLayoutCoordinates

            val boundsWithinAnchor = anchorLayoutCoordinates
                .localBoundingBoxOf(it, false)

            val positionWithinAnchor = Offset(
                boundsWithinAnchor.left,
                boundsWithinAnchor.top
            )

            val corners = with(boundsWithinAnchor) { listOf(topLeft, topRight, bottomLeft, bottomRight) }
            val anchorBounds = Rect(Offset.Zero, anchorLayoutCoordinates.size.toSize())
            val isVisible = corners.any { anchorBounds.contains(it) }
            onPlaced(WithinAnchor(positionWithinAnchor, boundsWithinAnchor, isVisible))
        }
    }
}

@Immutable
data class WithinAnchor(
    val position: Offset = Offset.Zero,
    val bounds: Rect = Rect.Zero,
    val isVisible: Boolean = false
)

class ContainerLayoutCoordinatesHolder {
    lateinit var containerLayoutCoordinates: LayoutCoordinates
}