How to observe Ktor download progress by a Flow

I want to observe the download progress by a Flow,
so I wrote a function like this:

suspend fun downloadFile(file: File, url: String): Flow<Int>{
        val client = HttpClient(Android)
        return flow{
            val httpResponse: HttpResponse = client.get(url) {
                onDownload { bytesSentTotal, contentLength ->
                    val progress = (bytesSentTotal * 100f / contentLength).roundToInt()
                    emit(progress)
                }
            }
            val responseBody: ByteArray = httpResponse.receive()
            file.writeBytes(responseBody)
        }
}

but the onDownload will be called only once, and the file will not be downloaded. If I remove the emit(progress) it will work.

io.ktor:ktor-client-android:1.6.7

>Solution :

Use callbackFlow instead of flow. A regular flow can’t launch background code, and can only emit values from code inside the flow itself. Meanwhile, a callback flow can launch other work in the background, and then receive callbacks from it.