I need to perform some actions when the user leaves a channel (in most cases where they close the tab voluntarily, but there may also be a connection loss/timeout etc.)

According to posts like Phoenix.Presence run some code when user leaves the channel and https://stackoverflow.com/questions/33934029/how-to-detect-if-a-user-left-a-phoenix-channel-due-to-a-network-disconnect, intercepting the "presence_diff" event from Presence seems to be a foolproof way to go, as it should also cover the cases where the connection terminates abnormally.

Strangely, the presence_diff event seems to only be triggered when I track the user via Presence.track (i.e. when the user joins), but not when the user leaves (e.g. when I close my browser tab under dev environment)

Meanwhile, adding a terminate(reason, socket) callback in my channel correctly catches the leave event.

I wonder what could be wrong in my configuration. Or did I not understand the use of Presence correctly?

Example code:

  def join("participant:" <> participant_id, _payload, socket) do
    if socket.assigns.participant_id == participant_id do
      send(self(), :after_participant_join)
      {:ok, socket}
    else
      {:error, %{reason: "unauthorized"}}
    end
  end
  
  def handle_info(:after_participant_join, socket) do
    experiment_id = socket.assigns.experiment_id

    Presence.track(socket, experiment_id, %{
      # keys to track
    })

    # Broadcast something
    # broadcast(socket, ...)
    
    {:noreply, socket}
  end

  intercept(["presence_diff"])

  def handle_out("presence_diff", payload, socket) do
    # Only gets triggered at Presence.track, but not when the connection is closed.
    IO.puts("presence_diff triggered, payload is #{inspect(payload)}")

    leaves = payload.leaves

    for {experiment_id, meta} <- leaves do
      IO.puts("Leave information: #{meta}")
      
      # Do stuffs
    end

    {:noreply, socket}
  end
  
  # This works, however.
  def terminate(reason, socket) do
    IO.puts("terminated. #{inspect(reason)}")

    # Do stuffs.
  end