In Compose how to access drawable once Coil loads image from URL

Question

I am using accompanist-coil:0.12.0. I want to load image from a url and then pass the drawable to a method. I am using this:

val painter = rememberCoilPainter(
        request = ImageRequest.Builder(LocalContext.current)
            .data(imageUrl)
            .target {
                viewModel.calcDominantColor(it) { color ->
                    dominantColor = color
                }
            }
            .build(),
        fadeIn = true
    )

and then passing the painter to Image like this:

Image(
   painter = painter,
   contentDescription = "Some Image",
)

The image loads without any problem but the method calcDominantColor is never called.
Am I doing it the wrong way?

UPDATE:
I was able to call the method using Transformation in requestBuilder but I am not sure, if this is how it is supposed to be done because I am not actually transforming the Bitmap itself:

val painter = rememberCoilPainter(
        request = entry.imageUrl,
        requestBuilder = {
            transformations(
                object: Transformation{
                    override fun key(): String {
                        return entry.imageUrl
                    }
                    override suspend fun transform(
                        pool: BitmapPool,
                        input: Bitmap,
                        size: Size
                    ): Bitmap {
                        viewModel.calcDominantColor(input) { color ->
                            dominantColor = color
                        }
                        return input
                    }
                }
            )
        }
    )

This works fine for first time but when the composable recomposes, transformation is returned from cache and my method doesn't run.

Answer 1

I think you want to use LaunchedEffect along with an ImageLoader to access the bitmap from the loader result.

    val context = LocalContext.current

    val imageLoader = ImageLoader(context)

    val request = ImageRequest.Builder(context)
        .transformations(RoundedCornersTransformation(12.dp.value))
        .data(imageUrl)
        .build()

    val imagePainter = rememberCoilPainter(
        request = request,
        imageLoader = imageLoader
    )

    LaunchedEffect(key1 = imagePainter) {
        launch {
            val result = (imageLoader.execute(request) as SuccessResult).drawable
            val bitmap = (result as BitmapDrawable).bitmap
            val vibrant = Palette.from(bitmap)
                .generate()
                .getVibrantColor(defaultColor)
            // do something with vibrant color
        }
    }