A function to encode text as a QR code using BitmapPainter and zxing-core for use in Jetpack Compose with Image composable. Note: This function has been optimized to set all pixels at once using setPixels, which significantly improves performance, especially when used in a LazyColumn. Thanks @dev-niiaddy!

QRPainter.kt

@Composable
fun rememberQrBitmapPainter(
    content: String,
    size: Dp = 150.dp,
    padding: Dp = 0.dp
): BitmapPainter {

    val density = LocalDensity.current
    val sizePx = with(density) { size.roundToPx() }
    val paddingPx = with(density) { padding.roundToPx() }


    var bitmap by remember(content) {
        mutableStateOf<Bitmap?>(null)
    }

    LaunchedEffect(bitmap) {
        if (bitmap != null) return@LaunchedEffect

        launch(Dispatchers.IO) {
            val qrCodeWriter = QRCodeWriter()

            val encodeHints = mutableMapOf<EncodeHintType, Any?>()
                .apply {
                    this[EncodeHintType.MARGIN] = paddingPx
                }

            val bitmapMatrix = try {
                qrCodeWriter.encode(
                    content, BarcodeFormat.QR_CODE,
                    sizePx, sizePx, encodeHints
                )
            } catch (ex: WriterException) {
                null
            }

            val matrixWidth = bitmapMatrix?.width ?: sizePx
            val matrixHeight = bitmapMatrix?.height ?: sizePx

            val newBitmap = Bitmap.createBitmap(
                bitmapMatrix?.width ?: sizePx,
                bitmapMatrix?.height ?: sizePx,
                Bitmap.Config.ARGB_8888,
            )
            
            val pixels = IntArray(matrixWidth * matrixHeight)

            for (x in 0 until matrixWidth) {
                for (y in 0 until matrixHeight) {
                    val shouldColorPixel = bitmapMatrix?.get(x, y) ?: false
                    val pixelColor = if (shouldColorPixel) Color.BLACK else Color.WHITE

                    pixels[y * matrixWidth + x] = pixelColor
                }
            }

            newBitmap.setPixels(pixels, 0, matrixWidth, 0, 0, matrixWidth, matrixHeight)

            bitmap = newBitmap
        }
    }

    return remember(bitmap) {
        val currentBitmap = bitmap ?: Bitmap.createBitmap(
            sizePx, sizePx,
            Bitmap.Config.ARGB_8888,
        ).apply { eraseColor(Color.TRANSPARENT) }

        BitmapPainter(currentBitmap.asImageBitmap())
    }
}