In this example I have a mapping method that takes in a Response<> from retrofit, and maps it to an `AlbumsView` state if successful, and otherwise throws an error.

mapper.kt

fun mapFacebookAlbumsResponse(response: Response<FacebookAllAlbumsResponse>): SocialPhotoUploadState {
    return if (response.isSuccessful) {
        mapFacebookAlbumsSuccess(response)
    } else {
        mapFacebookAlbumsError(response)
    }
}

private fun mapFacebookAlbumsError(response: Response<FacebookAllAlbumsResponse>): SocialPhotoUploadState {
    if (response.code() == HttpURLConnection.HTTP_BAD_REQUEST) {
        val error = SocialAuthenticationError("Need FB permission.")
        throw error
    } else {
        throw Throwable("Error fetching albums")
    }
}

private fun mapFacebookAlbumsSuccess(response: Response<FacebookAllAlbumsResponse>): SocialPhotoUploadState {
    // We should only concern ourselves with albums that have photos.
    val albums = response.body()?.data?.filter { it.count > 0 } ?: ArrayList()

    return if (albums.isEmpty()) {
        throw Throwable("Empty albums error.")
    } else {
        SocialPhotoUploadState.AlbumsView(albums, Constants.SOCIAL_PLATFORM_FACEBOOK, "Your FacebookAlbums")
    }
}

Example unit test:

@Test
    fun mapFacebookAlbumsAuthErrorResponse() {
        val authErrorResponse: Response<FacebookAllAlbumsResponse> = Response.error(HttpURLConnection.HTTP_BAD_REQUEST, errorResponseBody)
        assertFailsWith(SocialAuthenticationError::class, "If the server returns a 400, this means we have an authentication error.") {
            responseMapper.mapFacebookAlbumsResponse(authErrorResponse)
        }
    }