From d359f58e8805d3e89a2df9bcd991a55aa5f7af29 Mon Sep 17 00:00:00 2001
From: Joshua Young <djry1999@gmail.com>
Date: Thu, 3 Oct 2024 17:01:28 +1000
Subject: [PATCH] Feature | `lazy_load_previews_and_pages` config option

---
 config/app.yml                    |  1 +
 docs/src/_data/config_options.yml | 11 +++++++-
 lib/lookbook/engine.rb            | 42 +++++++++++++++++++++++++------
 lib/lookbook/reloaders.rb         |  2 ++
 4 files changed, 48 insertions(+), 8 deletions(-)

diff --git a/config/app.yml b/config/app.yml
index 5d074eaf7..6579ccf0e 100644
--- a/config/app.yml
+++ b/config/app.yml
@@ -50,6 +50,7 @@ shared:
   component_paths: [app/views, app/components]
 
   reload_on_change: ~
+  lazy_load_previews_and_pages: false
   live_updates: false
   listen_paths: []
   listen_extensions: [rb, html.*]
diff --git a/docs/src/_data/config_options.yml b/docs/src/_data/config_options.yml
index 21bd6f3b3..119ffebee 100644
--- a/docs/src/_data/config_options.yml
+++ b/docs/src/_data/config_options.yml
@@ -261,7 +261,16 @@ system:
     example: config.lookbook.reload_on_change = true
     description: |
       By default Lookbook uses the value of the `cache_classes` and `reload_classes_only_on_change` Rails config options to decide if
-      it should attempt to update the preview data after changes. If set the value of this config option will take precedence and be used instead.
+      it should attempt to update the preview data after changes. If set, the value of this config option will take precedence and be used instead.
+
+  - name: lazy_load_previews_and_pages
+    types: Boolean
+    default: "false"
+    example: config.lookbook.lazy_load_previews_and_pages = true
+    description: |
+      By default Lookbook loads all preview and page data on application initialization, and then on changes depending
+      on the value of `config.lookbook.reload_on_change`. If set to `true`, Lookbook will defer the initial loading of
+      preview and page data until it is first requested.
 
   - name: live_updates
     types: Boolean
diff --git a/lib/lookbook/engine.rb b/lib/lookbook/engine.rb
index 6585f9abc..9ca3dd43b 100644
--- a/lib/lookbook/engine.rb
+++ b/lib/lookbook/engine.rb
@@ -68,13 +68,15 @@ class Engine < Rails::Engine
     end
 
     config.after_initialize do
-      if Engine.reloading?
-        reloaders.add(:previews, Engine.preview_watch_paths, opts.listen_extensions, &Engine.method(:load_previews))
-        reloaders.add(:pages, Engine.page_watch_paths, opts.page_extensions, &Engine.method(:load_pages))
-        reloaders.execute
-      else
-        Engine.load_previews
-        Engine.load_pages
+      unless opts.lazy_load_previews_and_pages
+        if Engine.reloading?
+          reloaders.add(:previews, Engine.preview_watch_paths, opts.listen_extensions, &Engine.method(:load_previews))
+          reloaders.add(:pages, Engine.page_watch_paths, opts.page_extensions, &Engine.method(:load_pages))
+          reloaders.execute
+        else
+          Engine.load_previews
+          Engine.load_pages
+        end
       end
 
       Engine.run_hooks(:after_initialize)
@@ -196,10 +198,36 @@ def preview_watch_paths
 
       def pages
         @_pages ||= PageCollection.new
+
+        if opts.lazy_load_previews_and_pages && !@_loaded_pages
+          @_loaded_pages = true
+
+          if reloading?
+            reloader = reloaders.add(:pages, Engine.page_watch_paths, opts.page_extensions, &Engine.method(:load_pages))
+            reloader.execute
+          else
+            load_pages
+          end
+        end
+
+        @_pages
       end
 
       def previews
         @_previews ||= PreviewCollection.new
+
+        if opts.lazy_load_previews_and_pages && !@_loaded_previews
+          @_loaded_previews = true
+
+          if reloading?
+            reloader = reloaders.add(:previews, Engine.preview_watch_paths, opts.listen_extensions, &Engine.method(:load_previews))
+            reloader.execute
+          else
+            load_previews
+          end
+        end
+
+        @_previews
       end
 
       def preview_controller
diff --git a/lib/lookbook/reloaders.rb b/lib/lookbook/reloaders.rb
index 8ef8a5a79..a3bf4f7f8 100644
--- a/lib/lookbook/reloaders.rb
+++ b/lib/lookbook/reloaders.rb
@@ -16,6 +16,8 @@ def add(name, directories, extensions, &callback)
         Rails.application.reloaders << reloader
         Rails.application.reloader.to_run { reloader.execute_if_updated }
       end
+
+      reloader
     end
 
     def execute