Phoenix LiveView and handle a broadcast from outside module

genserver
broadcast
liveview
#1

Hi guys, I got a question about something I was not able to solve at all. Am just starting in this world so, sorry if it’s so trivial. This is not a professional thing, am just trying things out to learn.

The point is, I have a Phoenix app running which shows some information when you visit the /feed url. At the moment what it is doing is so simple, using the :timer it refreshes my html every X ms. and it happens all in my LiveView module.

But now I have created a new module which uses GenServer and is started when the app starts (at the application.ex), this module contains all the :timer and command output extracted from the LiveView module and when handles the event I broadcast the information that I would to handle in my LiveView module when am subscribed and handling this event.

Unfortunately something is wrong, since the handle thing is not getting anything and my html never gets refreshed.

Summarizing:

GenServer module A which has a :timer, then handles this :tick event and broadcast some information. To broadcast am doing:
Endpoint.broadcast_from!(self(), @topic, "message", %{name: 'foo'})

Liveview module which renders some information that is expecting to get by handleing the broadcasted message above and assigns it. For that purpose at the mount() method I subscribe this module to the topic above and then created a handler for that event, looks something like this:

def handle_info("message", %{name: name}) do // .... {:noreply, []} end
But nothing comes up and my html keeps as it is. In console I can see the GenServer module is in the loop by the :timer added so that is broadcasting, but can not catch it.

What am I doing wrong?

#2

I think handle_info should be an handle_event(topic, msg, socket) ?

#3

:wave:

You might need to check if the socket is connected?(socket) before doing any statefull work like subscribing to a pubsub topic. Also note that the message you’d receive will be probably wrapped in the phoenix’s broadcast struct.

# ...
@impl true
def mount(_session, socket) do
  if connected?(socket) do
    Phoenix.PubSub.subscribe(YourApp.PubSub, "some_topic")
  end
  {:ok, socket}
end

@impl true
def handle_info(%Phoenix.Socket.Broadcast{event: "message", topic: topic, payload: %{name: name}}, socket) do
  # ...
  {:noreply, socket}
end

Check out a presense example in the live view demo repo: https://github.com/chrismccord/phoenix_live_view_example/blob/master/lib/demo_web/live/user_live/presence_index.ex.

1 Like
#4

This was exactly the problem, this is wrapped into broadcast struct, I tried this but in the wrong way, so after broadcasting this from my GenServer module:

LiveViewTodosWeb.Endpoint.broadcast_from!(self(), @topic, "message", %{name: 'grey'})

From my live view module I handle the event by this way and it works as expected:

def handle_info(%Broadcast{event: "message"}, socket) do
   IO.puts "GASTY !!!!"
   {:noreply, []}
end

Thanks a lot for the quick help guys!!!

1 Like