Trying to stream data from ESP device over a web socket

jim · January 31, 2020, 6:14am

I’m trying to allow one or more client devices (like tablets and phones) to connect to an ESP32 via a websocket, via wifi or BT, and then have the ESP32 stream data to the client in JSON.

From searching it seems that I do this using the RPC libraries, including one called rpc-ws which I can not find in the API documentation. Regardless, it seems that I have to create a callback and then use mg_rpc_send_responsef() from within the callback.

But this only seems to work if the client takes some action to poll the server. How can I cause the server to stream data to the clients? Would I just use a timer with a callback that calls mg_rpc_send_responsef() even if there is nothing from the client?

Is there an example? After searching I have not found much.

Thank you…

cpq · January 31, 2020, 9:31am

Are you trying to make something like https://vcon.io ?

nliviu · January 31, 2020, 9:45am

You don’t need rpc-ws.

You need your client to open a websocket connection and your application to send data over it.

jim · February 1, 2020, 3:23am

No, I’m trying to have an uC stream data to an iPad for display, so the UI is on the iPad (or other device).

jim · February 1, 2020, 3:25am

Right, that’s what I’m trying to figure out how to do. More searching found some more hints so I’m still at it.

cpq · February 1, 2020, 8:29am

The mgos_register_http_endpoint() should do the job:

github.com

mongoose-os-libs/http-server/blob/master/include/mgos_http_server.h#L34-L57


/*
 * Register HTTP endpoint handler `handler` on URI `uri_path`
 *
 * Example:
 * ```c
 * static void foo_handler(struct mg_connection *c, int ev, void *p,
 *                         void *user_data) {
 *   (void) p;
 *   if (ev != MG_EV_HTTP_REQUEST) return;
 *   LOG(LL_INFO, ("Foo requested"));
 *   mg_send_response_line(c, 200,
 *                         "Content-Type: text/html\r\n");
 *   mg_printf(c, "%s\r\n", "Fooooo");
 *   c->flags |= (MG_F_SEND_AND_CLOSE | MGOS_F_RELOAD_CONFIG);
 *   (void) user_data;
 * }
 *
 * // Somewhere else:
 * mgos_register_http_endpoint("/foo/", foo_handler, NULL);
 * ```

This file has been truncated. show original

As for the event handler function, see example:

github.com

cesanta/mongoose/blob/master/examples/websocket_chat/websocket_chat.c#L36


                      MG_SOCK_STRINGIFY_IP | MG_SOCK_STRINGIFY_PORT);


  snprintf(buf, sizeof(buf), "%s %.*s", addr, (int) msg.len, msg.p);
  printf("%s\n", buf); /* Local echo. */
  for (c = mg_next(nc->mgr, NULL); c != NULL; c = mg_next(nc->mgr, c)) {
    if (c == nc) continue; /* Don't send to the sender. */
    mg_send_websocket_frame(c, WEBSOCKET_OP_TEXT, buf, strlen(buf));
  }
}


static void ev_handler(struct mg_connection *nc, int ev, void *ev_data) {
  switch (ev) {
    case MG_EV_WEBSOCKET_HANDSHAKE_DONE: {
      /* New websocket connection. Tell everybody. */
      broadcast(nc, mg_mk_str("++ joined"));
      break;
    }
    case MG_EV_WEBSOCKET_FRAME: {
      struct websocket_message *wm = (struct websocket_message *) ev_data;
      /* New websocket message. Tell everybody. */
      struct mg_str d = {(char *) wm->data, wm->size};