Scaffold body behind AppBar in Android Jetpack Compose?

Question

I want to extend my Scaffold body behind the AppBar because of the curve in the bottom right corner:

I tried Modifier.offset(y = (-25).dp). It works fine, but the BG image not fills the area behind the bottom navbar anymore:

 Scaffold( topBar = { ... }, ) 
        { padding ->
            Box(
                Modifier.padding(padding),
                contentAlignment = Alignment.TopCenter
            ) {
                Box(
                    modifier = Modifier
                        .fillMaxSize()
                        .offset(y = (-25).dp),  // BG image behind AppBar
                ) {
                    Image(
                        modifier = Modifier.fillMaxSize(),
                        painter = painterResource(R.drawable.bg),
                        contentDescription = null,
                        contentScale = ContentScale.FillBounds
                    )
                }

How can I do both?

Answer 1

You can use Modifier.graphicsLayer {scaleY = // scale ratio greater than 1f}

But this option requires you to be able to find exact ratio(height of Scaffold / height of Box with Image) you need to use a way for finding height of both Scaffold and Box then you can scale exactly.

@Composable
private fun BoxSample() {
    BoxWithConstraints(modifier = Modifier.fillMaxSize()) {

        val parentHeight = maxHeight
        Scaffold(
            topBar = {
                Box(
                    modifier = Modifier
                        .fillMaxWidth()
                        .height(50.dp)
                        .background(Color.Black),
                    contentAlignment = Alignment.Center
                ) {
                    Text("TITLE", fontSize = 20.sp, color = Color.White)
                }
            }
        )
        { padding ->
            BoxWithConstraints(
                Modifier.padding(padding),
                contentAlignment = Alignment.TopCenter
            ) {
                val childHeight = this.maxHeight

                val scaleY = parentHeight.value / childHeight.value
                Box(
                    modifier = Modifier
                        .fillMaxSize()
                        .graphicsLayer {
                            this.scaleY = scaleY
                        }
                ) {
                    Image(
                        modifier = Modifier.fillMaxSize(),
                        painter = painterResource(R.drawable.bg),
                        contentDescription = null,
                        contentScale = ContentScale.FillBounds
                    )
                }
            }
        }
    }
}

Second option is placing your appbar not inside topBar property of Scaffold but inside a Box in content.

@Composable
private fun Sample() {
    Box(
        Modifier.padding(padding),
        contentAlignment = Alignment.TopCenter
    ) {
        Box(
            modifier = Modifier
                .fillMaxSize(),
        contentAlignment = Alignment.TopStart

        ) {
            Image(
                modifier = Modifier.fillMaxSize(),
                painter = painterResource(R.drawable.bg),
                contentDescription = null,
                contentScale = ContentScale.FillBounds
            )
        }
        TopAppbar()
    }
}

Third option is putting Box with Image in topBar of Scaffold