How to use Presence to track users

elie · May 8, 2017, 9:54pm

I have code like this in my app:

{:ok, _} = Presence.track(socket, socket.assigns.user_id, %{
      online_at: inspect(System.system_time(:milli_seconds))
    })

Presence.list(socket) |> IO.inspect

I’d like yo use the info from Presence.list in another module in the app. How do I go about doing this?

And will online_at be the time the user was last online?

Thanks

OvermindDL1 · May 8, 2017, 10:37pm

You can use it from anywhere else just fine. Presence.list/1 accepts a topic. It also has an override that grabs a topic out of a socket then just calls itself again as seen at:

github.com

phoenixframework/phoenix/blob/main/lib/phoenix/presence.ex#L163


      
                  # event to all subscribers
                  for {user_id, presence} <- leaves do
                    metas =
                      case Map.fetch(presences, user_id) do
                        {:ok, presence_metas} -> presence_metas
                        :error -> []
                      end
          
                    user_data = %{user: presence.user, metas: metas}
                    msg = {MyApp.PresenceClient, {:leave, user_data}}
                    Phoenix.PubSub.local_broadcast(MyApp.PubSub, topic, msg)
                  end
          
                  {:ok, state}
                end
              end
          
          The `handle_metas/4` callback receives the topic, presence diff, current presences
          for the topic with their metadata, and any user-land state accumulated from init and
          subsequent `handle_metas/4` calls. In our example implementation, we walk the `:joins` and
          `:leaves` in the diff, and populate a complete presence from our known presence information.

But you can pass it the topic directly from anywhere.

You’ll have a set of those online_at’s for each one being tracked and they will each have a time based on when that code was executed (I.E. the tracking started).

elie · May 9, 2017, 3:43pm

So to get the latest time a user was online I’d have to do track on every action the user makes?

But if I just want to know if the user was online I can just if the user is in the list?

OvermindDL1 · May 9, 2017, 4:04pm

Well if they are still in the list at all then they ‘are’ still active.

If they are not in the presence list then they are gone

If you want to store when they were ‘last’ active instead of when they became active then store that on disconnect, like in a database.