feat: include last_seen in tag responses

lib/localiser/localisation/tag/filter.ex

@@ -11,7 +11,8 @@ defmodule Localiser.Localisation.Tag.Filter do
     :filter_module,
     :filter_state,
     :rooms,
-    current_room_id: nil
+    current_room_id: nil,
+    last_seen: nil
   ]
 
   def start_link(tag) do
@@ -28,6 +29,13 @@ defmodule Localiser.Localisation.Tag.Filter do
     GenServer.cast(via(tag_id), {:ingest, measurements})
   end
 
+  def get_live_state(tag_id) do
+    case Registry.lookup(Localiser.Registry, {:filter, tag_id}) do
+      [{pid, _}] -> GenServer.call(pid, :get_live_state)
+      []         -> nil
+    end
+  end
+
   # Hot-swap the filter module. Reinitialises filter state with new module.
   def swap_filter(tag_id, new_module, opts \\ []) do
     GenServer.call(via(tag_id), {:swap_filter, new_module, opts})
@@ -67,10 +75,14 @@ defmodule Localiser.Localisation.Tag.Filter do
       if new_room_id, do: RoomServer.tag_entered(new_room_id, state.tag_id)
     end
 
-    {:noreply, %{state | filter_state: new_filter_state, current_room_id: new_room_id}}
+    {:noreply, %{state | filter_state: new_filter_state, current_room_id: new_room_id, last_seen: DateTime.utc_now()}}
   end
 
   @impl true
+  def handle_call(:get_live_state, _from, state) do
+    {:reply, %{room_id: state.current_room_id, last_seen: state.last_seen}, state}
+  end
+
   def handle_call({:swap_filter, new_module, opts}, _from, state) do
     case new_module.init([], opts) do
       {:ok, new_filter_state} ->

lib/localiser/web/controllers/tag_controller.ex

@@ -95,10 +95,14 @@ defmodule Localiser.Web.Controllers.TagController do
   end
 
   defp render_tag(tag) do
+    live = TagFilter.get_live_state(tag.tag_id)
+
     %{
-      id:     tag.id,
-      tag_id: tag.tag_id,
-      name:   tag.name
+      id:        tag.id,
+      tag_id:    tag.tag_id,
+      name:      tag.name,
+      room_id:   live && live.room_id,
+      last_seen: live && live.last_seen
     }
   end