Jetpack Compose - Make the first element in a LazyRow be aligned to the center of the screen

Question

I want to obtain a LazyRow that looks like this:

|--aaa-b|bb-cccc|-dd... ...|w--x---|

|-------| is one screen width

The size of the elements varies but they have a fixed spacing between them. I thought I could add some start content padding to the LazyRow so that the "aaa" Composable is aligned to the center of the screen, but I don't know its width.

If you think it's not clear what I'm asking, please drop a comment.

UPDATE

Added a gif for better understanding

Answer 1

You can use the BoxWithConstraints to get screen width. Then you can use Layout to properly positioning the item in the list.

@Composable
fun BigCarousel() {
    val items = (0..10).map { "Item $it" }
    BoxWithConstraints {
        LazyRow {
            itemsIndexed(items) { index, item ->
                Layout(
                    content = {
                        // Here's the content of each list item.
                        Box(
                            Modifier
                                .size(200.dp)
                                .padding(8.dp)
                                .background(Color.Gray)
                        ) {
                            Text(text = item, Modifier.align(Alignment.Center))
                        }
                    },
                    measurePolicy = { measurables, constraints ->
                        // I'm assuming you'll declaring just one root 
                        // composable in the content function above
                        // so it's measuring just the Box
                        val placeable = measurables.first().measure(constraints)
                        // maxWidth is from the BoxWithConstraints
                        val maxWidthInPx = maxWidth.roundToPx()
                        // Box width
                        val itemWidth = placeable.width
                        // Calculating the space for the first and last item
                        val startSpace =
                            if (index == 0) (maxWidthInPx - itemWidth) / 2 else 0
                        val endSpace =
                            if (index == items.lastIndex) (maxWidthInPx - itemWidth) / 2 else 0
                        // The width of the box + extra space
                        val width = startSpace + placeable.width + endSpace
                        layout(width, placeable.height) {
                            // Placing the Box in the right X position
                            val x = if (index == 0) startSpace else 0
                            placeable.place(x, 0)
                        }
                    }
                )
            }
        }
    }
}

Here's the result: