Added info on using config component's ConfigCache when dumping the service container

Author: richardmiller-zz

components/dependency_injection/compilation.rst

@@ -186,3 +186,44 @@ your application::
         }
     }
 
+This could be further improved by only recompiling the container in debug
+mode when changes have been made to its configuration rather than on every
+request. This can be done by caching the resource files used to configure
+the container in the way describe in ":doc:`/components/conf/caching`"
+in the config component documentation.
+
+You do not need to work out which files to cache as the container builder
+keeps track of all the resources used to configure it, not just the configuration
+files but the extension classes and compiler passes as well. This means that
+any changes to any of these files will invalidate the cache and trigger the
+container being rebuilt. You just need to ask the container for these resources
+and use them as metadata for the cache::
+
+    // ...
+
+    // set $isDebug based on something in your project
+
+    $file = __DIR__ .'/cache/container.php';
+    $containerConfigCache = new ConfigCache($file, $isDebug);
+
+    if ($cache->isFresh()) {
+        require_once $file;
+        $container = new MyCachedContainer();
+    } else {
+        $container = new ContainerBuilder();
+        //--
+        $container->compile();
+
+        $dumper = new PhpDumper($container);
+        $containerConfigCache->write(
+            $dumper->dump(array('class' => 'MyCachedContainer')),
+            $container->getResources()
+        );
+    }
+
+Now the cache is used regardless of whether debug mode is on or not. The difference
+is that the ``ConfigCache`` is set to debug mode with its second constructor
+argument. When the cache is not in debug mode the cached container will always
+be used if it exists. In debug mode, an additional metadata file is written with
+the timestamps of all the resource files. These are then checked to see if the files
+have changed, if they have the cache will be considered stale.