Issue #2760659 by lauriii, yogeshmpawar, vmachado, anya_m, alexpott, Fabianx,...

Issue #2760659 by lauriii, yogeshmpawar, vmachado, anya_m, alexpott, Fabianx, joelpittet, xjm: Allow the use of callbacks instead of global functions in preprocess function callbacks

core/lib/Drupal/Core/Theme/ThemeManager.php

@@ -281,8 +281,8 @@ public function render($hook, array $variables) {
     }
     if (isset($info['preprocess functions'])) {
       foreach ($info['preprocess functions'] as $preprocessor_function) {
-        if (function_exists($preprocessor_function)) {
-          $preprocessor_function($variables, $hook, $info);
+        if (is_callable($preprocessor_function)) {
+          call_user_func_array($preprocessor_function, [&$variables, $hook, $info]);
         }
       }
       // Allow theme preprocess functions to set $variables['#attached'] and

core/modules/system/tests/modules/theme_test/src/ThemeTestController.php

@@ -132,6 +132,15 @@ public function preprocessSuggestions() {
     ];
   }
 
+  /**
+   * Controller for testing callable preprocess functions.
+   */
+  public function preprocessCallback() {
+    return [
+      '#theme' => 'theme_test_preprocess_callback',
+    ];
+  }
+
   /**
    * Controller for testing a namespaced class in a theme.
    */

core/modules/system/tests/modules/theme_test/src/ThemeTestPreprocess.php

+<?php
+
+namespace Drupal\theme_test;
+
+/**
+ * Class to test preprocess callbacks.
+ */
+class ThemeTestPreprocess {
+
+  /**
+   * Preprocess callback for testing preprocess callbacks.
+   *
+   * @param $variables
+   *   An associative array containing:
+   *   - foo: Text for testing preprocess callback.
+   */
+  public static function preprocess(&$variables) {
+    $variables['foo'] = 'Make Drupal full of kittens again!';
+  }
+
+}

core/modules/system/tests/modules/theme_test/templates/theme-test-preprocess-callback.html.twig

+{{ foo }}

core/modules/system/tests/modules/theme_test/theme_test.module

@@ -48,6 +48,11 @@ function theme_test_theme($existing, $type, $theme, $path) {
       'bar' => '',
     ],
   ];
+  $items['theme_test_preprocess_callback'] = [
+    'variables' => [
+      'foo' => '',
+    ],
+  ];
   $items['theme_test_registered_by_module'] = [
     'render element' => 'content',
     'base hook' => 'container',
@@ -60,6 +65,13 @@ function theme_test_theme($existing, $type, $theme, $path) {
   return $items;
 }
 
+/**
+ * Implements hook_theme_registry_alter().
+ */
+function theme_test_theme_registry_alter(&$registry) {
+  $registry['theme_test_preprocess_callback']['preprocess functions'][] = ['\Drupal\theme_test\ThemeTestPreprocess', 'preprocess'];
+}
+
 /**
  * Implements hook_preprocess_HOOK() for HTML document templates.
  */

core/modules/system/tests/modules/theme_test/theme_test.routing.yml

@@ -88,6 +88,13 @@ theme_test.preprocess_suggestions:
   requirements:
     _access: 'TRUE'
 
+theme_test.preprocess_callback:
+  path: '/theme-test/preprocess-callback'
+  defaults:
+    _controller: '\Drupal\theme_test\ThemeTestController::preprocessCallback'
+  requirements:
+    _access: 'TRUE'
+
 theme_test.test_theme_class:
   path: '/theme-test/test-theme-class'
   defaults:

core/modules/system/tests/src/Functional/Theme/ThemeTest.php

@@ -196,4 +196,12 @@ public function testSuggestionPreprocessForDefaults() {
     }
   }
 
+  /**
+   * Ensures that preprocess callbacks can be defined.
+   */
+  public function testPreprocessCallback() {
+    $this->drupalGet('theme-test/preprocess-callback');
+    $this->assertSession()->pageTextContains('Make Drupal full of kittens again!');
+  }
+
 }