Document that cache warmers now must return an array of files to preload

Author: javiereguiluz

reference/dic_tags.rst

@@ -370,6 +370,17 @@ the :class:`Symfony\\Component\\HttpKernel\\CacheWarmer\\CacheWarmerInterface` i
         public function warmUp($cacheDirectory)
         {
             // ... do some sort of operations to "warm" your cache
+
+            // this method must return an array with the file paths of the
+            // PHP files to preload. If you don't have any, return an empty array
+            $filesToPreload = [];
+            foreach (scandir($someCacheDir) as $file) {
+                if (!is_dir($file = $someCacheDir.'/'.$file)) {
+                    $filesToPreload[] = $file;
+                }
+            }
+
+            return $filesToPreload;
         }
 
         public function isOptional()
@@ -378,6 +389,11 @@ the :class:`Symfony\\Component\\HttpKernel\\CacheWarmer\\CacheWarmerInterface` i
         }
     }
 
+.. deprecated:: 5.1
+
+    Not returning an array from the ``warmUp()`` method with the files to
+    preload is deprecated since Symfony 5.1.
+
 The ``isOptional()`` method should return true if it's possible to use the
 application without calling this cache warmer. In Symfony, optional warmers
 are always executed by default (you can change this by using the