How to set a drawable object instance as a background of a LazyColumn?

I mean a Drawable generated dinamically, not a file from res/drawable.

I tryed setting it as a background of the modifier, but it doesn't accept a Drawable instance object. In java, it was done like this: layout.setBackground(bg.getDrawable())

In this case, I have a LazyColumn, and it haves some items. Ok, I need the drawable to be the background of the LazyColumn. The background must fit the height and the width of the LazyColumn

I thought a possibility is to do it using a Box, placing a Image with the Drawable inside the Box, and over it, the LazyColumn, but the drawable must be the exact size of the LazyColumn, and I don't know how to do that.

Any ideas about how to achieve this will be appreciated.

You could use drawBehind modifier which provides access to the the underlying Canvas with DrawScope.drawIntoCanvas. It's important to also add clipToBounds to avoid drawing outside of the element.

.clipToBounds()
.drawBehind {
    drawIntoCanvas { canvas ->
       drawable.bounds = canvas.nativeCanvas.clipBounds
       drawable.draw(canvas.nativeCanvas)
    }
}

Full example:

@Composable
fun DrawableBackground() {
    val drawable = object : Drawable() {
        val paint: Paint = Paint().apply { setARGB(255, 0, 255, 0) }

        override fun draw(canvas: Canvas) {
            val top = bounds.top.toFloat()
            val bottom = bounds.bottom.toFloat()
            val left = bounds.left.toFloat()
            val right = bounds.right.toFloat()
            canvas.drawLine(right, top, left, bottom, paint)
            canvas.drawLine(left, top, right, bottom, paint)
        }

        override fun setAlpha(alpha: Int) {}
        override fun setColorFilter(colorFilter: ColorFilter?) {}
        override fun getOpacity(): Int = PixelFormat.OPAQUE
    }

    Column {
        var items by remember { mutableIntStateOf(5) }
        Row {
            Button(onClick = { ++items }) { Text("+") }
            Button(onClick = { --items }) { Text("-") }
        }
        LazyColumn(modifier = Modifier
            .fillMaxWidth()
            .clipToBounds()
            .drawBehind {
                drawIntoCanvas { canvas ->
                    drawable.bounds = canvas.nativeCanvas.clipBounds
                    drawable.draw(canvas.nativeCanvas)
                }
            }
        ) {
            items(items) { Text("Item $it") }
        }
    }
}