docs: add docs

Author: kenjis

user_guide_src/source/concepts/autoloader.rst

@@ -96,3 +96,50 @@ the value defined in **app/Config/Constants.php**.
 
 .. note:: If the same namespace is defined in both CodeIgniter and Composer, CodeIgniter's autoloader will be
     the first one to get a chance to locate the file.
+
+.. _file-locator-caching:
+
+FileLocator Caching
+*******************
+
+.. versionadded:: 4.5.0
+
+**FileLocator** is responsible for finding files or getting a classname from a file,
+which cannot be achieved with PHP autoloading.
+
+To improve its performance, FileLocator Caching has been implemented.
+
+How It Works
+============
+
+- Save the all found data by FileLocator into a cache file when destructing,
+  if the cache data is updated.
+- Restore cached data when instantiating if cached data is available.
+
+The cached data are used permanently.
+
+How to Delete Cached Data
+=========================
+
+Once stored, the cached data never expire.
+
+So if you add or remove files or change existing file paths, or namespaces, old
+cached data will be returned and your app may not work properly.
+
+In that case, you must manually delete the cache file. If you add a CodeIgniter
+package via Composer, you also need to delete the cache file.
+
+You can use the ``spark cache:clear`` command:
+
+.. code-block:: console
+
+    php spark cache:clear
+
+Or simply delete the **writable/cache/FileLocatorCache** file.
+
+How to Enable FileLocator Caching
+=================================
+
+Add the following code in **app/Config/Services.php**:
+
+.. literalinclude:: autoloader/004.php

user_guide_src/source/concepts/autoloader/004.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Config;
+
+use CodeIgniter\Autoloader\FileLocator;
+use CodeIgniter\Autoloader\FileLocatorCached;
+use CodeIgniter\Config\BaseService;
+
+class Services extends BaseService
+{
+    // ...
+
+    public static function locator(bool $getShared = true)
+    {
+        if ($getShared) {
+            if (! isset(static::$instances['locator'])) {
+                static::$instances['locator'] = new FileLocatorCached(new FileLocator(static::autoloader()));
+            }
+
+            return static::$mocks['locator'] ?? static::$instances['locator'];
+        }
+
+        return new FileLocator(static::autoloader());
+    }
+}