Given phoenix already comes with a sophisticated solution for tracking presences (Phoenix.Presence) I would just use that one.

You can do it by defining your own Phoenix.Socket and implementing init with a track call. I didn’t want to do all of that, and was okay with a warning appearing, so I just defined the function before the use Phoenix.Socket call. pushex/lib/push_ex_web/channels/push_socket.ex at master · pushex-project/pushex · GitHub

That isn’t best practice, but demonstrates how you can track a Socket instead of a Channel. You enter a world of “may not be supported” once you do this, but I think it’s pretty small here.

This alternative does not issue a warning and it is not coupled to the implementation of the original init/1, only to its return value:

defmodule MyAppWeb.DriverSocket do
  use Phoenix.Socket
  defoverridable init: 1

  def init(state) do
    res = {:ok, {_, socket}} = super(state)
    MyAppWeb.SocketMonitor.monitor(socket.transport_pid, socket.assigns.driver_id)
    res
  end
end

That is exactly what I meant