Track presence on UserSocket

I am trying to track the presence of a user on the UserSocker using Phoenix.Presence without the need for the client to be present on a specific channel.

Later on I want to subscribe to the presence of a user in different channels to be informed about the users presence.

I got this working except receiving a presence_diff when a user disconnects. What I am doing is tracking the presence from the UserSocket on a distinct topic for each user:

defmodule MyAppWeb.UserSocket do

  # ...

  def connect(%{"user_id" => user_id, "password" => password}, socket) do
    case Accounts.authenticate(user_id, password) do
      {:ok, user} ->
        track_user_presence(socket.transport_pid, user)
        {:ok, assign(socket, :user, user)}
      _error -> :error
    end
  end

  defp user_presence_topic(user_id) do
    "user_presence:#{user_id}"
  end

  defp track_user_presence(pid, user) do
    MyApp.Presence.track(pid, user_presence_topic(user.id), user.id, %{
      online_at: inspect(System.system_time(:seconds))
    })
  end
end

From my channel I am subscribing to the users distinct presence topics:

defmodule MyAppWeb.RoomChannel do
  # ...

  def join("room:" <> room_id, payload, socket) do
    send(self(), :after_join)
    {:ok, assign(socket, :room_id, room_id)}
  end

  def handle_info(:after_join, socket) do
    user_ids = ~w(1 2)
    presence_state = get_and_subscribe_to_user_presence(socket, user_ids)
    push(socket, "presence_state", presence_state)
    {:noreply, socket}
  end

  def get_and_subscribe_to_user_presence(socket, user_ids) do
    user_ids
    |> Enum.map(&user_presence_topic/1)
    |> Enum.map(fn topic ->
      Phoenix.PubSub.subscribe(
        socket.pubsub_server,
        topic,
        fastlane: {socket.transport_pid, socket.serializer, []}
      )
      Presence.list(topic)
    end)
    |> Enum.reduce(%{}, fn map, acc -> Map.merge(acc, map) end)
  end

  defp user_presence_topic(user_id) do
    "user_presence:#{user_id}"
  end
end

I came to the conclusion that I somehow need to monitor the sockets transport_pid and send the presence diff myself when the socket terminates.

Another idea was to join the client to a separate presence channel from the UserSocket.connect/2 function but I did not found out how to archive that so far.

I hacked together a simple phoenix app with tests outlining the issue: https://github.com/kbredemeier/socket_presence

Any advice on this would be appreciated.

Figured it out. Instead of using PubSub.subscribe/3 I ended up using MyApp.Endpoint.subscribe/1 with the topic I wanted to subscribe to and implementing handle_info/2 on my subscribing channel. handle_info/2 will then receive the brodcasts from the subscribed channel which in my case were the presence_diffs.

Chris McCord pointed out to me on slack that the fastlane option for Phoenix.PubSub.subscribe/3 allows to message a different process than the subscriber when broadcasting. In my case this resulted in my client receiving the presence_diff not on the channel I was expecting.