Optimize presence.

Avoid fetching each user by passing in full pre-fetched
presences from Presence.fetch/2 callback.
Use temporary assigns in ProfileLive to avoid duping
presences in memeory.
Handle removes by a small hook event

assets/js/app.js

@@ -195,6 +195,7 @@ window.addEventListener("phx:page-loading-stop", info => topbar.hide())
 window.addEventListener("phx:page-loading-stop", routeUpdated)
 
 window.addEventListener("js:exec", e => e.target[e.detail.call](...e.detail.args))
+window.addEventListener("phx:remove-el", e => document.getElementById(e.detail.id).remove())
 
 // connect if there are any LiveViews on the page
 liveSocket.getSocket().onOpen(() => execJS("#connection-status", "js-hide"))

lib/live_beats/presence/phoenix_presence_client.ex

@@ -2,8 +2,10 @@ defmodule Phoenix.Presence.Client do
   use GenServer
 
   @callback init(state :: term) :: {:ok, new_state :: term}
-  @callback handle_join(topic :: String.t(), key :: String.t(), meta :: [map()], state :: term) :: {:ok, term}
-  @callback handle_leave(topic :: String.t(), key :: String.t(), meta :: [map()], state :: term) :: {:ok, term}
+  @callback handle_join(topic :: String.t(), key :: String.t(), meta :: [map()], state :: term) ::
+              {:ok, term}
+  @callback handle_leave(topic :: String.t(), key :: String.t(), meta :: [map()], state :: term) ::
+              {:ok, term}
 
   @doc """
   TODO
@@ -19,7 +21,8 @@ defmodule Phoenix.Presence.Client do
       {:ok, name} ->
         GenServer.start_link(__MODULE__, opts, name: name)
 
-      :error -> GenServer.start_link(__MODULE__, opts)
+      :error ->
+        GenServer.start_link(__MODULE__, opts)
     end
   end
 
@@ -113,7 +116,9 @@ defmodule Phoenix.Presence.Client do
     updated_state =
       update_topics_state(:add_new_presence_or_metas, state, topic, joined_key, joined_meta)
 
-    {:ok, updated_client_state} = state.client.handle_join(topic, joined_key, joined_meta, state.client_state)
+    {:ok, updated_client_state} =
+      state.client.handle_join(topic, joined_key, meta, state.client_state)
+
     updated_state = Map.put(updated_state, :client_state, updated_client_state)
 
     {updated_state, topic}
@@ -122,7 +127,9 @@ defmodule Phoenix.Presence.Client do
   defp handle_leave({left_key, meta}, {state, topic}) do
     updated_state = update_topics_state(:remove_presence_or_metas, state, topic, left_key, meta)
 
-    {:ok, updated_client_state} = state.client.handle_leave(topic, left_key, meta, state.client_state)
+    {:ok, updated_client_state} =
+      state.client.handle_leave(topic, left_key, meta, state.client_state)
+
     updated_state = Map.put(updated_state, :client_state, updated_client_state)
 
     {updated_state, topic}

lib/live_beats/presence/presence_client.ex

@@ -15,25 +15,34 @@ defmodule LiveBeats.PresenceClient do
   end
 
   @impl Phoenix.Presence.Client
-  def handle_join(topic, key, _meta, state) do
+  def handle_join(topic, _key, presence, state) do
     active_users_topic =
       topic
       |> profile_identifier()
       |> active_users_topic()
 
-    Phoenix.PubSub.local_broadcast(@pubsub, active_users_topic, {__MODULE__, %{user_joined: key}})
+    Phoenix.PubSub.local_broadcast(
+      @pubsub,
+      active_users_topic,
+      {__MODULE__, %{user_joined: presence}}
+    )
 
     {:ok, state}
   end
 
   @impl Phoenix.Presence.Client
-  def handle_leave(topic, key, _meta, state) do
+  def handle_leave(topic, _key, presence, state) do
     active_users_topic =
       topic
       |> profile_identifier()
       |> active_users_topic()
 
-    Phoenix.PubSub.local_broadcast(@pubsub, active_users_topic, {__MODULE__, %{user_left: key}})
+    Phoenix.PubSub.local_broadcast(
+      @pubsub,
+      active_users_topic,
+      {__MODULE__, %{user_left: presence}}
+    )
+
     {:ok, state}
   end
 

lib/live_beats_web/channels/presence.ex

@@ -5,8 +5,9 @@ defmodule LiveBeatsWeb.Presence do
   See the [`Phoenix.Presence`](http://hexdocs.pm/phoenix/Phoenix.Presence.html)
   docs for more details.
   """
-  use Phoenix.Presence, otp_app: :live_beats,
-                        pubsub_server: LiveBeats.PubSub
+  use Phoenix.Presence,
+    otp_app: :live_beats,
+    pubsub_server: LiveBeats.PubSub
 
   import Phoenix.LiveView.Helpers
   import LiveBeatsWeb.LiveHelpers
@@ -18,10 +19,16 @@ defmodule LiveBeatsWeb.Presence do
     ~H"""
     <!-- users -->
     <div class="px-4 mt-6 sm:px-6 lg:px-8">
-      <h2 class="text-gray-500 text-xs font-medium uppercase tracking-wide">Who's Listening</h2>
-      <ul role="list" class="grid grid-cols-1 gap-4 sm:gap-4 sm:grid-cols-2 xl:grid-cols-5 mt-3" x-max="1">
+      <h2 class="text-gray-500 text-xs font-medium uppercase tracking-wide">Here now</h2>
+      <ul
+        id="listening-now"
+        phx-update="prepend"
+        role="list"
+        x-max="1"
+        class="grid grid-cols-1 gap-4 sm:gap-4 sm:grid-cols-2 xl:grid-cols-5 mt-3"
+      >
         <%= for presence <- @presences do %>
-          <li class="relative col-span-1 flex shadow-sm rounded-md overflow-hidden">
+          <li id={"presence-#{presence.id}"} class="relative col-span-1 flex shadow-sm rounded-md overflow-hidden">
             <.link navigate={profile_path(presence)} class="flex-1 flex items-center justify-between border-t border-r border-b border-gray-200 bg-white rounded-r-md truncate">
               <img class="w-10 h-10 flex-shrink-0 flex items-center justify-center rounded-l-md bg-purple-600" src={presence.avatar_url} alt="">
               <div class="flex-1 flex items-center justify-between text-gray-900 text-sm font-medium hover:text-gray-600 pl-3">

lib/live_beats_web/live/profile_live.ex

@@ -88,7 +88,9 @@ defmodule LiveBeatsWeb.ProfileLive do
       MediaLibrary.subscribe_to_profile(profile)
       Accounts.subscribe(current_user.id)
       LiveBeatsWeb.Presence.subscribe(profile)
-      Phoenix.Presence.Client.track(topic(profile.user_id),
+
+      Phoenix.Presence.Client.track(
+        topic(profile.user_id),
         current_user.id,
         %{}
       )
@@ -111,7 +113,7 @@ defmodule LiveBeatsWeb.ProfileLive do
       |> list_songs()
       |> assign_presences()
 
-    {:ok, socket, temporary_assigns: [songs: []]}
+    {:ok, socket, temporary_assigns: [songs: [], presences: []]}
   end
 
   def handle_params(params, _url, socket) do
@@ -147,21 +149,14 @@ defmodule LiveBeatsWeb.ProfileLive do
     {:noreply, socket}
   end
 
-  def handle_info({LiveBeats.PresenceClient, %{user_joined: user_id}}, socket) do
-    new_user = Accounts.get_user!(user_id)
-    updated_presences =
-    if new_user in socket.assigns.presences do
-      socket.assigns.presences
-    else
-      [new_user | socket.assigns.presences]
-    end
-    {:noreply, assign(socket, :presences, updated_presences)}
+  def handle_info({LiveBeats.PresenceClient, %{user_joined: presence}}, socket) do
+    %{user: user} = presence
+    {:noreply, update(socket, :presences, &[user | &1])}
   end
 
-  def handle_info({LiveBeats.PresenceClient, %{user_left: user_id}}, socket) do
-    updated_presences = socket.assigns.presences
-    |> Enum.reject(fn user -> user.id == String.to_integer(user_id) end)
-    {:noreply, assign(socket, :presences, updated_presences)}
+  def handle_info({LiveBeats.PresenceClient, %{user_left: presence}}, socket) do
+    %{user: user} = presence
+    {:noreply, push_event(socket, "remove-el", %{id: "presence-#{user.id}"})}
   end
 
   def handle_info({Accounts, %Accounts.Events.ActiveProfileChanged{} = event}, socket) do
@@ -264,10 +259,11 @@ defmodule LiveBeatsWeb.ProfileLive do
   end
 
   defp assign_presences(socket) do
-    presences = socket.assigns.profile.user_id
-    |> topic()
-    |> LiveBeats.PresenceClient.list()
-    |> Enum.map(fn {_key, meta} -> meta.user end)
+    presences =
+      socket.assigns.profile.user_id
+      |> topic()
+      |> LiveBeats.PresenceClient.list()
+      |> Enum.map(fn {_key, meta} -> meta.user end)
 
     assign(socket, presences: presences)
   end

test/live_beats_web/live/profile_live_test.exs

@@ -51,18 +51,13 @@ defmodule LiveBeatsWeb.ProfileLiveTest do
                }
              }) =~ "can't be blank"
 
-      assert {:ok, new_lv, html} =
-               lv |> form("#song-form") |> render_submit() |> follow_redirect(conn)
-
-      assert_redirected(lv, "/#{current_user.username}")
-      assert html =~ "1 song(s) uploaded"
-
-      assert html =~ "silence1s"
+      assert lv |> form("#song-form") |> render_submit() =~ "silence1s"
+      assert_patch(lv, "/#{current_user.username}")
 
       # deleting songs
 
       song = MediaLibrary.get_first_song(profile)
-      assert new_lv |> element("#delete-modal-#{song.id}-confirm") |> render_click()
+      assert lv |> element("#delete-modal-#{song.id}-confirm") |> render_click()
 
       {:ok, refreshed_lv, _} = live(conn, LiveHelpers.profile_path(current_user))
       refute render(refreshed_lv) =~ "silence1s"