Why the

Modifier.onSizeChange()

doesn't work in Preview?

var cardSize by remember { mutableStateOf<IntSize?>(null) }
    Box(
        modifier = Modifier
            .background(Color.White)
            .aspectRatio(1f)
            .fillMaxSize()
            .onSizeChanged { cardSize = it }
    ) {
        Text(text = "Size is = $cardSize")
    }

the preview always show "Size is null" while if I run it I get the size as I expect? Is there a way to fix the preview?

Did you try it on a device or an emulator?

I guess you need the interactive mode for the text to get updated.

yes in both emulator and device work as expected.

You can enable it in settings.

oh thank you, I had missed that

The reason is because the preview only runs a single composition. The size changed event isn’t sent until after the first composition-layout completes, which would normally trigger a recomposition but not in previews. This means if you’re using that state in the composition (as you are) then it will probably flicker in even on a real device because that first frame will be drawn without the size value.

BoxWithConstraints

might be a better approach if it works for your use case since it gives you the constraints immediately, on the first composition. No missed frame, no flicker

What about that preview they were showing with an animated 3D content being previewed? is that gonna come later?

that relies on specific integrations with the animation primitives