Flush data added to websocket - dart

Question

I'm writing a speed test, but i'm having trouble on the client side for uploading.
I have a the following setup, which basically continues to write data into the socket while a condition is true, and then closes the socket:
var ws = await createWebSocket(sb.serverAddress, sb.authToken);
while (condition) {
var bytes = generateRandomBytes(_BUFFER_SIZE_BYTES);
ws.add(bytes);
print('added');
var megabits = (bytes.length * 8) / 1000000;
channel.sink.add(megabits);
}
await ws.close();
My problem is that I can't work out how to wait for the bytes to be accepted by the underlying buffer. Even if I set _BUFFER_SIZE_BYTES to an huge size it still loops at break neck speed printing out added, where I really want to wait until all the bytes are accepted by the send buffer (having been accepted by the server) before adding a new list of bytes.
With an http post request you can do: await postReq.flush();, but I don't see any such method for web sockets.

Answer 1

Ok so I think I have a reasonable solution to this problem.
Client side has to wait for a response from the server before sending more bytes:
var bytes = generateRandomBytes(_CHUNK_SIZE_BYTES);
ws.listen((data) async {
ws.add(bytes);
var megabits = (bytes.length * 8) / 1000000;
channel.sink.add(megabits);
}
});
Server (Go) sends a message to the client signalling that it can send a chunk, and then reads the entire response from the client, before signalling to the client that it is ready to accept another one:
for start := time.Now(); time.Since(start) < time.Second*maxDuration; {
err := conn.WriteMessage(websocket.TextMessage, []byte("next"))
if err != nil {
break
}
// will get an error if try writing to closed socket
_, bytes, err := conn.ReadMessage()
if err != nil {
fmt.Println(err)
break
}
fmt.Println(len(bytes))
}
I think this solution is ok. I've set the chunk size to 10Mb which seems to work ok. Let me know if anyone has a better idea.