listener.ex

defmodule AlchemyWeb.UserChannelListener do
  use GenServer

  def start_link(name: name) do
    GenServer.start_link(__MODULE__, [], name: name)
  end

  def init(_) do
    {:ok, %{}}
  end

  def handle_info({:register, Registry.UserChannels, _key, pid, token}, state) do
    {:noreply, open_channel(pid, token, state)}
  end

  def handle_info({:unregister, Registry.UserChannels, _, pid}, state) do
    {:noreply, close_channel(pid, state)}
  end

  def handle_info({:DOWN, _, _, pid, _}, state) do
    {:noreply, close_channel(pid, state)}
  end

  defp open_channel(pid, token, state) do
    Process.monitor(pid)
    
    # Do stuff

    state |> Map.put(pid, token)
  end

  defp close_channel(pid, state) do
    case state |> Map.get(pid) do
      nil ->
        state

      token ->
        # do stuff
    end
  end
end