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
