I’m trying to use a Phoenix Channel for the signaling part of a WebRTC application, as I thought Presence would be perfect for this.

The thing is: I’m trying to track a user join using Phoenix.js’ Presence object, like this:

  presence.onJoin(async (id, current, newPres) => {
    if (!current) {
      const peerConnection = await createPeerConnection(
        channel,
        sendMessage,
        config.onTrack,
        localStreamMedia
      );
      peerMap.set(id, peerConnection);
    } else {
    }
  });

The thing is: every time a user joins, this callback is fired multiple times, even with its own Presence object and the objects of users that were previously in the channel. I don’t know if this is the expected behavior and couldn’t find a reference about that.

My Channel looks like this right now:

defmodule ElmWebRtcWeb.VideoChannel do
  use Phoenix.Channel
  alias ElmWebrtcWeb.Presence

  def join("videoroom:" <> _channel, _message, socket) do
    send(self(), :after_join)
    {:ok, socket}
  end

  def handle_info(:after_join, socket) do
    {:ok, _} =
      Presence.track(socket, socket.assigns.user_id, %{
        online_at: inspect(System.system_time(:millisecond))
      })

    push(socket, "presence_state", Presence.list(socket))
    {:noreply, socket}
  end
end

and the JS Presence is created like this:

  const channel = socket.channel(`videoroom:${room}`, {});
  channel.join();
  const presence = new Presence(channel);

Does anyone have a clue about:

• Is this the expected behavior of Presence.js?
• If so, is there any way for me to achieve what I want with it?
• If not, what could be going wrong?

Thank you very much!