The following expected function, which writes data to a file read from the ByteReadChannel, can be added to the common code:

expect suspend fun ByteReadChannel.writeToFile(filepath: String)

Here’s an example of the above function usage:

val client = HttpClient()

client.prepareGet("https://httpbin.org/image/jpeg").execute {
    it.bodyAsChannel().writeToFile("/path/to/file")
}

For JVM and Android, the File.writeChannel can be used to stream the ByteReadChannel to the file:

actual suspend fun ByteReadChannel.writeToFile(filepath: String) {
    this.copyTo(File(filepath).writeChannel())
}

For iOS, the dispatch_write function can be used to asynchronously write to the file while reading chunks from the ByteReadChannel:

private const val BUFFER_SIZE = 4096

@OptIn(ExperimentalForeignApi::class)
actual suspend fun ByteReadChannel.writeToFile(filepath: String) {
    val channel = this
    val queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT.convert(), 0u)
    memScoped {
        val dst = allocArray<ByteVar>(BUFFER_SIZE)
        val fd = open(filepath, O_RDWR)

        try {
            while (!channel.isClosedForRead) {
                val rs = channel.readAvailable(dst, 0, BUFFER_SIZE)
                if (rs < 0) break

                val data = dispatch_data_create(dst, rs.convert(), queue) {}

                dispatch_write(fd, data, queue) { _, error ->
                    if (error != 0) {
                        channel.cancel(IllegalStateException("Unable to write data to the file $filepath"))
                    }
                }
            }
        } finally {
            close(fd)
        }
    }
}