The Custom Modifier for Box/Drop Shadows in Jetpack Compose

CustomShadowModifier.kt

import android.graphics.BlurMaskFilter
import androidx.compose.foundation.background
import androidx.compose.foundation.layout.Box
import androidx.compose.foundation.layout.size
import androidx.compose.material3.Text
import androidx.compose.runtime.Composable
import androidx.compose.ui.Alignment
import androidx.compose.ui.Modifier
import androidx.compose.ui.draw.drawBehind
import androidx.compose.ui.graphics.Color
import androidx.compose.ui.graphics.Paint
import androidx.compose.ui.graphics.asComposePaint
import androidx.compose.ui.graphics.drawscope.drawIntoCanvas
import androidx.compose.ui.graphics.toArgb
import androidx.compose.ui.text.font.FontWeight
import androidx.compose.ui.tooling.preview.Preview
import androidx.compose.ui.unit.Dp
import androidx.compose.ui.unit.dp
import androidx.compose.ui.unit.sp

@Preview
@Composable
private fun BoxShadowTest() {
    Box(modifier = Modifier
        .size(400.dp)
        .background(color = Color.Magenta), contentAlignment = Alignment.Center) {
        Box(
            modifier = Modifier
                .size(200.dp)
                .shadow(
                    blurAmount = 15.dp,
                    offsetX = 5.dp,
                    offsetY = 5.dp,
                    spread = 3.dp
                )
                .background(color = Color.Blue),
            contentAlignment = Alignment.Center
        ) {
            Text(
                text = "A",
                fontSize = 150.sp,
                fontWeight = FontWeight.Bold,
                color = Color.White
            )
        }
    }
}

fun Modifier.shadow(
    offsetX: Dp = 0.dp,
    offsetY: Dp = 0.dp,
    blurAmount: Dp = 0.dp,
    spread: Dp = 0.dp,
    shadowColor: Color = Color.Black
): Modifier {
    return this.drawBehind {
        drawIntoCanvas {
            val paint = Paint()
            val frameworkPaint = paint.asFrameworkPaint()

            if(blurAmount != 0.dp) {
                frameworkPaint.maskFilter = BlurMaskFilter(blurAmount.toPx(), BlurMaskFilter.Blur.NORMAL)
            }
            frameworkPaint.color = shadowColor.toArgb()

            val spreadPixel = spread.toPx()
            val left = (0f - spreadPixel) + offsetX.toPx()
            val top = (0f - spreadPixel) + offsetY.toPx()
            val right = size.width + spreadPixel + offsetX.toPx()
            val bottom = size.height + spreadPixel + offsetY.toPx()

            it.drawRect(
                left = left,
                top = top,
                right = right,
                bottom = bottom,
                paint = frameworkPaint.asComposePaint()
            )
        }
    }
}