feat(chat): use Phoenix.Presence to show online users

lib/slax_web/online_users.ex

@@ -0,0 +1,48 @@
+defmodule SlaxWeb.OnlineUsers do
+  alias Phoenix.Presence
+  alias SlaxWeb.Presence
+
+  @topic "online_users"
+
+  def list do
+    @topic
+    |> Presence.list()
+    |> Enum.into(
+      %{},
+      fn {id, %{metas: metas}} ->
+        {String.to_integer(id), length(metas)}
+      end
+    )
+  end
+
+  def track(pid, user) do
+    with {:ok, _} <- Presence.track(pid, @topic, user.id, %{}) do
+      :ok
+    end
+  end
+
+  def online?(online_users, user_id) do
+    Map.get(online_users, user_id, 0) > 0
+  end
+
+  def subscribe do
+    Phoenix.PubSub.subscribe(Slax.PubSub, @topic)
+  end
+
+  def update(online_users, %{joins: joins, leaves: leaves}) do
+    online_users
+    |> process_updates(joins, &Kernel.+/2)
+    |> process_updates(leaves, &Kernel.-/2)
+  end
+
+  defp process_updates(online_users, updates, operation) do
+    Enum.reduce(updates, online_users, fn {id, %{metas: metas}}, acc ->
+      Map.update(
+        acc,
+        String.to_integer(id),
+        length(metas),
+        &operation.(&1, length(metas))
+      )
+    end)
+  end
+end