Issue #2987208 by akalata, alexpott, sime, tim.plunkett, Charlie ChX Negyesi: ElementInfoManager does not provide an alter hook, cannot alter element plugin definitions

core/lib/Drupal/Core/Render/ElementInfoManager.php

@@ -65,6 +65,7 @@ class ElementInfoManager extends DefaultPluginManager implements ElementInfoMana
     $this->cacheTagInvalidator = $cache_tag_invalidator;
 
     parent::__construct('Element', $namespaces, $module_handler, 'Drupal\Core\Render\Element\ElementInterface', 'Drupal\Core\Render\Annotation\RenderElement');
+    $this->alterInfo('element_plugin');
   }
 
   /**

core/lib/Drupal/Core/Render/theme.api.php

@@ -810,6 +810,24 @@ function hook_element_info_alter(array &$info) {
   }
 }
 
+/**
+ * Alter Element plugin definitions.
+ *
+ * Whenever possible, hook_element_info_alter() should be used to alter the
+ * default properties of an element type. Use this hook only when the plugin
+ * definition itself needs to be altered.
+ *
+ * @param array $definitions
+ *   An array of Element plugin definitions.
+ *
+ * @see \Drupal\Core\Render\ElementInfoManager
+ * @see \Drupal\Core\Render\Element\ElementInterface
+ */
+function hook_element_plugin_alter(array &$definitions) {
+  // Use a custom class for the LayoutBuilder element.
+  $definitions['layout_builder']['class'] = '\Drupal\mymodule\Element\MyLayoutBuilderElement';
+}
+
 /**
  * Perform necessary alterations to the JavaScript before it is presented on
  * the page.

core/modules/system/tests/modules/element_info_test/element_info_test.module

@@ -14,6 +14,15 @@ function element_info_test_element_info_alter(array &$info) {
   $info['number']['#pre_render'][] = 'element_info_test_element_pre_render';
 }
 
+/**
+ * Implements hook_element_plugin_alter().
+ */
+function element_info_test_element_plugin_alter(array &$definitions) {
+  if (\Drupal::state()->get('hook_element_plugin_alter:remove_weight', FALSE)) {
+    unset($definitions['weight']);
+  }
+}
+
 /**
  * {@inheritdoc}
  *

core/tests/Drupal/KernelTests/Core/Render/Element/PluginAlterTest.php

@@ -0,0 +1,34 @@
+<?php
+
+namespace Drupal\KernelTests\Core\Render\Element;
+
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * @group Render
+ */
+class PluginAlterTest extends KernelTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['element_info_test'];
+
+  /**
+   * Tests hook_element_plugin_alter().
+   */
+  public function testPluginAlter() {
+    $info_manager = $this->container->get('plugin.manager.element_info');
+    $this->assertArrayHasKey('weight', $info_manager->getDefinitions());
+
+    // @see element_info_test_element_plugin_alter()
+    $this->container->get('state')->set('hook_element_plugin_alter:remove_weight', TRUE);
+    // The definition will be cached.
+    $this->assertArrayHasKey('weight', $info_manager->getDefinitions());
+
+    // Clearing the caches removes the definition.
+    $info_manager->clearCachedDefinitions();
+    $this->assertArrayNotHasKey('weight', $info_manager->getDefinitions());
+  }
+
+}