From 0ec176e4120ce33971faea07d3c41094a8c20c13 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Lui=CC=81s=20Arteiro?= <lucho14garfield@gmail.com>
Date: Mon, 13 Feb 2023 12:52:43 +0000
Subject: [PATCH] feat: Adding basic Presence for list of logged in users. #14

---
 assets/js/app.js                      |  9 +++++++
 lib/chat/application.ex               |  1 +
 lib/chat_web/channels/room_channel.ex | 38 +++++++++++++++++++++++----
 lib/chat_web/presence.ex              |  5 ++++
 4 files changed, 48 insertions(+), 5 deletions(-)
 create mode 100644 lib/chat_web/presence.ex

diff --git a/assets/js/app.js b/assets/js/app.js
index 8e96f7a..ebb41cb 100644
--- a/assets/js/app.js
+++ b/assets/js/app.js
@@ -54,6 +54,15 @@ channel.on('shout', function (payload) {
   render_message(payload)
 });
 
+// Listening to presence events
+channel.on('presence_diff', function (payload) {
+  console.log(payload)
+});
+
+channel.on('presence_state', function (payload) {
+  console.log(payload)
+});
+
 
 // Send the message to the server on "shout" channel
 function sendMessage() {
diff --git a/lib/chat/application.ex b/lib/chat/application.ex
index 4a96a44..1923c94 100644
--- a/lib/chat/application.ex
+++ b/lib/chat/application.ex
@@ -14,6 +14,7 @@ defmodule Chat.Application do
       Chat.Repo,
       # Start the PubSub system
       {Phoenix.PubSub, name: Chat.PubSub},
+      ChatWeb.Presence,
       # Start the Endpoint (http/https)
       ChatWeb.Endpoint
       # Start a worker by calling: Chat.Worker.start_link(arg)
diff --git a/lib/chat_web/channels/room_channel.ex b/lib/chat_web/channels/room_channel.ex
index daf6a81..6c10c85 100644
--- a/lib/chat_web/channels/room_channel.ex
+++ b/lib/chat_web/channels/room_channel.ex
@@ -1,5 +1,6 @@
 defmodule ChatWeb.RoomChannel do
   use ChatWeb, :channel
+  alias ChatWeb.Presence
 
   @impl true
   def join("room:lobby", payload, socket) do
@@ -22,16 +23,23 @@ defmodule ChatWeb.RoomChannel do
   # broadcast to everyone in the current topic (room:lobby).
   @impl true
   def handle_in("shout", payload, socket) do
-    Chat.Message.changeset(%Chat.Message{}, payload) |> Chat.Repo.insert()
-    broadcast(socket, "shout", payload)
+    # Insert message in database
+    {:ok, msg} = Chat.Message.changeset(%Chat.Message{}, payload) |> Chat.Repo.insert()
+
+    # Assigning username to socket assigns and tracking presence
+    socket
+      |> assign(:user_name, msg.name)
+      |> track_presence()
+      |> broadcast("shout", Map.put_new(payload, :id, msg.id))
+
     {:noreply, socket}
   end
 
   @impl true
   def handle_info(:after_join, socket) do
+    # Get messages and list them
     Chat.Message.get_messages()
-    # revers to display the latest message at the bottom of the page
-    |> Enum.reverse()
+    |> Enum.reverse()     # reverts the enum to display the latest message at the bottom of the page
     |> Enum.each(fn msg ->
       push(socket, "shout", %{
         name: msg.name,
@@ -40,7 +48,9 @@ defmodule ChatWeb.RoomChannel do
       })
     end)
 
-    # :noreply
+    # Send currently online users in lobby
+    push(socket, "presence_state", Presence.list("room:lobby"))
+
     {:noreply, socket}
   end
 
@@ -48,4 +58,22 @@ defmodule ChatWeb.RoomChannel do
   defp authorized?(_payload) do
     true
   end
+
+
+  # Add a track in Presence when user joins the channel
+  # Ideally this should be on joining "room:lobby" but the socket has no info as of now
+  # to associate with a user.
+
+  defp track_presence(socket) do
+    case do_track(socket) do
+      {:ok, _ref} -> socket
+      {:error, {:already_tracked, _pid, _channel, _user}} -> socket
+    end
+  end
+
+  defp do_track(%{assigns: %{user_name: user_name}} = socket) when not is_nil(user_name) do
+    Presence.track(socket, user_name, %{
+      online_at: inspect(System.system_time(:second))
+    })
+  end
 end
diff --git a/lib/chat_web/presence.ex b/lib/chat_web/presence.ex
new file mode 100644
index 0000000..262fa91
--- /dev/null
+++ b/lib/chat_web/presence.ex
@@ -0,0 +1,5 @@
+defmodule ChatWeb.Presence do
+  use Phoenix.Presence,
+    otp_app: :chat,
+    pubsub_server: Chat.PubSub
+end