create user tracker

lib/live_beats/user_tracker.ex

@@ -0,0 +1,77 @@
+defmodule LiveBeats.UserTracker do
+
+  require Logger
+  use GenServer
+  @pubsub LiveBeats.PubSub
+  @poll_interval :timer.seconds(5)
+
+  @doc """
+  TODO
+  """
+
+  def subscribe() do
+    Phoenix.PubSub.subscribe(@pubsub, topic())
+  end
+
+  def start_link(_opts) do
+    GenServer.start_link(__MODULE__, name: __MODULE__)
+  end
+
+  @impl true
+  def init(_opts) do
+    {:ok, schedule_updates(%{})}
+  end
+
+  @impl true
+  def handle_call({:user_joined, user}, _from, state) do
+    {:reply, :ok, handle_join(state, user) |> IO.inspect}
+  end
+
+  @impl true
+  def handle_call({:user_left, user}, _from, state) do
+    {:reply, :ok, handle_leave(state, user) |> IO.inspect}
+  end
+
+  @impl true
+  def handle_info(:send_updates, state) do
+    broadcast_updates(state)
+    {:noreply, schedule_updates(state)}
+  end
+
+  defp schedule_updates(state) do
+    Process.send_after(self(), :send_updates, @poll_interval)
+    state
+  end
+
+  defp handle_join(state, user) do
+    if Map.has_key?(state, user.id) do
+      state
+    else
+      Map.put_new(state, user.id, user)
+    end
+  end
+
+  defp handle_leave(state, user) do
+    if Map.has_key?(state, user.id) do
+      Map.delete(state, user.id)
+    else
+      state
+    end
+  end
+
+  defp topic() do
+    "active_users"
+  end
+
+  defp broadcast_updates(state) do
+    active_users =
+    state
+    |> Enum.map(fn {_key, value} -> value end)
+
+    Logger.info(
+          "broadcasting updates"
+        )
+    Phoenix.PubSub.local_broadcast(@pubsub, topic(), active_users)
+  end
+
+end