diff --git a/core/includes/ajax.inc b/core/includes/ajax.inc
deleted file mode 100644
index 186d1f3..0000000
--- a/core/includes/ajax.inc
+++ /dev/null
@@ -1,342 +0,0 @@
-<?php
-
-/**
- * @file
- * Functions for use with Drupal's Ajax framework.
- */
-
-use Drupal\Core\Form\FormStateInterface;
-use Drupal\Core\Render\Element;
-
-/**
- * @defgroup ajax Ajax API
- * @{
- * Overview for Drupal's Ajax API.
- *
- * @section sec_overview Overview of Ajax
- * Ajax is the process of dynamically updating parts of a page's HTML based on
- * data from the server. When a specified event takes place, a PHP callback is
- * triggered, which performs server-side logic and may return updated markup or
- * JavaScript commands to run. After the return, the browser runs the JavaScript
- * or updates the markup on the fly, with no full page refresh necessary.
- *
- * Many different events can trigger Ajax responses, including:
- * - Clicking a button
- * - Pressing a key
- * - Moving the mouse
- *
- * @section sec_framework Ajax responses in forms
- * Forms that use the Drupal Form API (see the
- * @link form_api Form API topic @endlink for more information about forms) can
- * trigger AJAX responses. Here is an outline of the steps:
- * - Add property '#ajax' to a form element in your form array, to trigger an
- *   Ajax response.
- * - Write an Ajax callback to process the input and respond.
- * See sections below for details on these two steps.
- *
- * @subsection sub_form Adding Ajax triggers to a form
- * As an example of adding Ajax triggers to a form, consider editing a date
- * format, where the user is provided with a sample of the generated date output
- * as they type. To accomplish this, typing in the text field should trigger an
- * Ajax response. This is done in the text field form array element
- * in \Drupal\config_translation\FormElement\DateFormat::getFormElement():
- * @code
- * '#ajax' => array(
- *   'callback' => 'Drupal\config_translation\FormElement\DateFormat::ajaxSample',
- *   'event' => 'keyup',
- *   'progress' => array(
- *     'type' => 'throbber',
- *     'message' => NULL,
- *   ),
- * ),
- * @endcode
- *
- * As you can see from this example, the #ajax property for a form element is
- * an array. Here are the details of its elements, all of which are optional:
- * - callback: The callback to invoke to handle the server side of the
- *   Ajax event. More information on callbacks is below in @ref sub_callback.
- * - path: The URL path to use for the request. If omitted, defaults to
- *   'system/ajax', which invokes the default Drupal Ajax processing (this will
- *   call the callback supplied in the 'callback' element). If you supply a
- *   path, you must set up a routing entry to handle the request yourself and
- *   return output described in @ref sub_callback below. See the
- *   @link menu Routing topic @endlink for more information on routing.
- * - wrapper: The HTML 'id' attribute of the area where the content returned by
- *   the callback should be placed. Note that callbacks have a choice of
- *   returning content or JavaScript commands; 'wrapper' is used for content
- *   returns.
- * - method: The jQuery method for placing the new content (used with
- *   'wrapper'). Valid options are 'replaceWith' (default), 'append', 'prepend',
- *   'before', 'after', or 'html'. See
- *   http://api.jquery.com/category/manipulation/ for more information on these
- *   methods.
- * - effect: The jQuery effect to use when placing the new HTML (used with
- *   'wrapper'). Valid options are 'none' (default), 'slide', or 'fade'.
- * - speed: The effect speed to use (used with 'effect' and 'wrapper'). Valid
- *   options are 'slow' (default), 'fast', or the number of milliseconds the
- *   effect should run.
- * - event: The JavaScript event to respond to. This is selected automatically
- *   for the type of form element; provide a value to override the default.
- * - prevent: A JavaScript event to prevent when the event is triggered. For
- *   example, if you use event 'mousedown' on a button, you might want to
- *   prevent 'click' events from also being triggered.
- * - progress: An array indicating how to show Ajax processing progress. Can
- *   contain one or more of these elements:
- *   - type: Type of indicator: 'throbber' (default) or 'bar'.
- *   - message: Translated message to display.
- *   - url: For a bar progress indicator, URL path for determining progress.
- *   - interval: For a bar progress indicator, how often to update it.
- *
- * @subsection sub_callback Setting up a callback to process Ajax
- * Once you have set up your form to trigger an Ajax response (see @ref sub_form
- * above), you need to write some PHP code to process the response. If you use
- * 'path' in your Ajax set-up, your route controller will be triggered with only
- * the information you provide in the URL. If you use 'callback', your callback
- * method is a function, which will receive the $form and $form_state from the
- * triggering form. You can use $form_state to get information about the
- * data the user has entered into the form. For instance, in the above example
- * for the date format preview,
- * \Drupal\config_translation\FormElement\DateFormat\ajaxSample() does this to
- * get the format string entered by the user:
- * @code
- * $format_value = \Drupal\Component\Utility\NestedArray::getValue(
- *   $form_state->getValues(),
- *   $form_state->getTriggeringElement()['#array_parents']);
- * @endcode
- *
- * Once you have processed the input, you have your choice of returning HTML
- * markup or a set of Ajax commands. If you choose to return HTML markup, you
- * can return it as a string or a renderable array, and it will be placed in
- * the defined 'wrapper' element (see documentation above in @ref sub_form).
- * In addition, any messages returned by drupal_get_messages(), themed as in
- * status-messages.html.twig, will be prepended.
- *
- * To return commands, you need to set up an object of class
- * \Drupal\Core\Ajax\AjaxResponse, and then use its addCommand() method to add
- * individual commands to it. In the date format preview example, the format
- * output is calculated, and then it is returned as replacement markup for a div
- * like this:
- * @code
- * $response = new AjaxResponse();
- * $response->addCommand(new ReplaceCommand(
- *   '#edit-date-format-suffix',
- *   '<small id="edit-date-format-suffix">' . $format . '</small>'));
- * return $response;
- * @endcode
- *
- * The individual commands that you can return implement interface
- * \Drupal\Core\Ajax\CommandInterface. Available commands provide the ability
- * to pop up alerts, manipulate text and markup in various ways, redirect
- * to a new URL, and the generic \Drupal\Core\Ajax\InvokeCommand, which
- * invokes an arbitrary jQuery command.
- *
- * As noted above, status messages are prepended automatically if you use the
- * 'wrapper' method and return HTML markup. This is not the case if you return
- * commands, but if you would like to show status messages, you can add
- * @code
- * array('#theme' => 'status_messages')
- * @endcode
- * to a render array, use drupal_render() to render it, and add a command to
- * place the messages in an appropriate location.
- *
- * @section sec_other Other methods for triggering Ajax
- * Here are some additional methods you can use to trigger Ajax responses in
- * Drupal:
- * - Add class 'use-ajax' to a link. The link will be loaded using an Ajax
- *   call. When using this method, the href of the link can contain '/nojs/' as
- *   part of the path. When the Ajax JavaScript processes the page, it will
- *   convert this to '/ajax/'. The server is then able to easily tell if this
- *   request was made through an actual Ajax request or in a degraded state, and
- *   respond appropriately.
- * - Add class 'use-ajax-submit' to a submit button in a form. The form will
- *   then be submitted via Ajax to the path specified in the #action.  Like the
- *   ajax-submit class on links, this path will have '/nojs/' replaced with
- *   '/ajax/' so that the submit handler can tell if the form was submitted in a
- *   degraded state or not.
- * - Add property '#autocomplete_route_name' to a text field in a form. The
- *   route controller for this route must return an array of options for
- *   autocomplete, as a \Symfony\Component\HttpFoundation\JsonResponse object.
- *   See the @link menu Routing topic @endlink for more information about
- *   routing.
- */
-
-/**
- * Form element processing handler for the #ajax form property.
- *
- * @deprecated Use \Drupal\Core\Render\Element\RenderElement::processAjaxForm().
- */
-function ajax_process_form(&$element, FormStateInterface $form_state, &$complete_form) {
-  return Element\RenderElement::processAjaxForm($element, $form_state, $complete_form);
-}
-
-/**
- * Adds Ajax information about an element to communicate with JavaScript.
- *
- * If #ajax['path'] is set on an element, this additional JavaScript is added
- * to the page header to attach the Ajax behaviors. See ajax.js for more
- * information.
- *
- * @param $element
- *   An associative array containing the properties of the element.
- *   Properties used:
- *   - #ajax['event']
- *   - #ajax['prevent']
- *   - #ajax['path']
- *   - #ajax['options']
- *   - #ajax['wrapper']
- *   - #ajax['parameters']
- *   - #ajax['effect']
- *   - #ajax['accepts']
- *
- * @return
- *   The processed element with the necessary JavaScript attached to it.
- */
-function ajax_pre_render_element($element) {
-  // Skip already processed elements.
-  if (isset($element['#ajax_processed'])) {
-    return $element;
-  }
-  // Initialize #ajax_processed, so we do not process this element again.
-  $element['#ajax_processed'] = FALSE;
-
-  // Nothing to do if there are no Ajax settings.
-  if (empty($element['#ajax'])) {
-    return $element;
-  }
-
-  // Add a reasonable default event handler if none was specified.
-  if (isset($element['#ajax']) && !isset($element['#ajax']['event'])) {
-    switch ($element['#type']) {
-      case 'submit':
-      case 'button':
-      case 'image_button':
-        // Pressing the ENTER key within a textfield triggers the click event of
-        // the form's first submit button. Triggering Ajax in this situation
-        // leads to problems, like breaking autocomplete textfields, so we bind
-        // to mousedown instead of click.
-        // @see http://drupal.org/node/216059
-        $element['#ajax']['event'] = 'mousedown';
-        // Retain keyboard accessibility by setting 'keypress'. This causes
-        // ajax.js to trigger 'event' when SPACE or ENTER are pressed while the
-        // button has focus.
-        $element['#ajax']['keypress'] = TRUE;
-        // Binding to mousedown rather than click means that it is possible to
-        // trigger a click by pressing the mouse, holding the mouse button down
-        // until the Ajax request is complete and the button is re-enabled, and
-        // then releasing the mouse button. Set 'prevent' so that ajax.js binds
-        // an additional handler to prevent such a click from triggering a
-        // non-Ajax form submission. This also prevents a textfield's ENTER
-        // press triggering this button's non-Ajax form submission behavior.
-        if (!isset($element['#ajax']['prevent'])) {
-          $element['#ajax']['prevent'] = 'click';
-        }
-        break;
-
-      case 'password':
-      case 'textfield':
-      case 'number':
-      case 'tel':
-      case 'textarea':
-        $element['#ajax']['event'] = 'blur';
-        break;
-
-      case 'radio':
-      case 'checkbox':
-      case 'select':
-        $element['#ajax']['event'] = 'change';
-        break;
-
-      case 'link':
-        $element['#ajax']['event'] = 'click';
-        break;
-
-      default:
-        return $element;
-    }
-  }
-
-  // Attach JavaScript settings to the element.
-  if (isset($element['#ajax']['event'])) {
-    $element['#attached']['library'][] = 'core/jquery.form';
-    $element['#attached']['library'][] = 'core/drupal.ajax';
-
-    $settings = $element['#ajax'];
-
-    // Assign default settings. When 'path' is set to NULL, ajax.js submits the
-    // Ajax request to the same URL as the form or link destination is for
-    // someone with JavaScript disabled. This is generally preferred as a way to
-    // ensure consistent server processing for js and no-js users, and Drupal's
-    // content negotiation takes care of formatting the response appropriately.
-    // However, 'path' and 'options' may be set when wanting server processing
-    // to be substantially different for a JavaScript triggered submission.
-    // One such substantial difference is form elements that use
-    // #ajax['callback'] for determining which part of the form needs
-    // re-rendering. For that, we have a special 'system/ajax' route.
-    $settings += array(
-      'path' => isset($settings['callback']) ? 'system/ajax' : NULL,
-      'options' => array(),
-      'accepts' => 'application/vnd.drupal-ajax'
-    );
-
-    // @todo Legacy support. Remove in Drupal 8.
-    if (isset($settings['method']) && $settings['method'] == 'replace') {
-      $settings['method'] = 'replaceWith';
-    }
-
-    // Change path to URL.
-    $settings['url'] = isset($settings['path']) ? url($settings['path'], $settings['options']) : NULL;
-    unset($settings['path'], $settings['options']);
-
-    // Add special data to $settings['submit'] so that when this element
-    // triggers an Ajax submission, Drupal's form processing can determine which
-    // element triggered it.
-    // @see _form_element_triggered_scripted_submission()
-    if (isset($settings['trigger_as'])) {
-      // An element can add a 'trigger_as' key within #ajax to make the element
-      // submit as though another one (for example, a non-button can use this
-      // to submit the form as though a button were clicked). When using this,
-      // the 'name' key is always required to identify the element to trigger
-      // as. The 'value' key is optional, and only needed when multiple elements
-      // share the same name, which is commonly the case for buttons.
-      $settings['submit']['_triggering_element_name'] = $settings['trigger_as']['name'];
-      if (isset($settings['trigger_as']['value'])) {
-        $settings['submit']['_triggering_element_value'] = $settings['trigger_as']['value'];
-      }
-      unset($settings['trigger_as']);
-    }
-    elseif (isset($element['#name'])) {
-      // Most of the time, elements can submit as themselves, in which case the
-      // 'trigger_as' key isn't needed, and the element's name is used.
-      $settings['submit']['_triggering_element_name'] = $element['#name'];
-      // If the element is a (non-image) button, its name may not identify it
-      // uniquely, in which case a match on value is also needed.
-      // @see _form_button_was_clicked()
-      if (!empty($element['#is_button']) && empty($element['#has_garbage_value'])) {
-        $settings['submit']['_triggering_element_value'] = $element['#value'];
-      }
-    }
-
-    // Convert a simple #ajax['progress'] string into an array.
-    if (isset($settings['progress']) && is_string($settings['progress'])) {
-      $settings['progress'] = array('type' => $settings['progress']);
-    }
-    // Change progress path to a full URL.
-    if (isset($settings['progress']['path'])) {
-      $settings['progress']['url'] = url($settings['progress']['path']);
-      unset($settings['progress']['path']);
-    }
-
-    $element['#attached']['js'][] = array(
-      'type' => 'setting',
-      'data' => array('ajax' => array($element['#id'] => $settings)),
-    );
-
-    // Indicate that Ajax processing was successful.
-    $element['#ajax_processed'] = TRUE;
-  }
-  return $element;
-}
-
-/**
- * @} End of "defgroup ajax".
- */
diff --git a/core/includes/install.core.inc b/core/includes/install.core.inc
index b538cb4..8fbf561 100644
--- a/core/includes/install.core.inc
+++ b/core/includes/install.core.inc
@@ -301,7 +301,6 @@ function install_begin_request(&$install_state) {
   require_once __DIR__ . '/database.inc';
   require_once __DIR__ . '/form.inc';
   require_once __DIR__ . '/batch.inc';
-  require_once __DIR__ . '/ajax.inc';
 
   // Load module basics (needed for hook invokes).
   include_once __DIR__ . '/module.inc';
diff --git a/core/lib/Drupal/Core/DrupalKernel.php b/core/lib/Drupal/Core/DrupalKernel.php
index 19a0fa9..b49aa70 100644
--- a/core/lib/Drupal/Core/DrupalKernel.php
+++ b/core/lib/Drupal/Core/DrupalKernel.php
@@ -372,7 +372,6 @@ public function boot() {
     require_once DRUPAL_ROOT . '/core/includes/unicode.inc';
     require_once DRUPAL_ROOT . '/core/includes/form.inc';
     require_once DRUPAL_ROOT . '/core/includes/mail.inc';
-    require_once DRUPAL_ROOT . '/core/includes/ajax.inc';
     require_once DRUPAL_ROOT . '/core/includes/errors.inc';
     require_once DRUPAL_ROOT . '/core/includes/schema.inc';
     require_once DRUPAL_ROOT . '/core/includes/entity.inc';
diff --git a/core/lib/Drupal/Core/Render/Element/Link.php b/core/lib/Drupal/Core/Render/Element/Link.php
index 2a06eb6..446b542 100644
--- a/core/lib/Drupal/Core/Render/Element/Link.php
+++ b/core/lib/Drupal/Core/Render/Element/Link.php
@@ -67,7 +67,7 @@ public static function preRenderLink($element) {
       $element['#options']['attributes']['id'] = $element['#id'];
     }
 
-    // Conditionally invoke ajax_pre_render_element(), if #ajax is set.
+    // Conditionally invoke self::preRenderAjaxForm(), if #ajax is set.
     if (isset($element['#ajax']) && !isset($element['#ajax_processed'])) {
       // If no HTML ID was found above, automatically create one.
       if (!isset($element['#id'])) {
@@ -78,7 +78,7 @@ public static function preRenderLink($element) {
         $element['#ajax']['path'] = $element['#href'];
         $element['#ajax']['options'] = $element['#options'];
       }
-      $element = ajax_pre_render_element($element);
+      $element = static::preRenderAjaxForm($element);
     }
 
     if (isset($element['#route_name'])) {
diff --git a/core/lib/Drupal/Core/Render/Element/RenderElement.php b/core/lib/Drupal/Core/Render/Element/RenderElement.php
index c3a74d8..553496c 100644
--- a/core/lib/Drupal/Core/Render/Element/RenderElement.php
+++ b/core/lib/Drupal/Core/Render/Element/RenderElement.php
@@ -123,10 +123,10 @@ public static function preRenderGroup($element) {
    * @return array
    *   The processed element.
    *
-   * @see ajax_pre_render_element()
+   * @see self::preRenderAjaxForm()
    */
   public static function processAjaxForm(&$element, FormStateInterface $form_state, &$complete_form) {
-    $element = ajax_pre_render_element($element);
+    $element = static::preRenderAjaxForm($element);
     if (!empty($element['#ajax_processed'])) {
       $form_state->setCached();
     }
@@ -134,6 +134,174 @@ public static function processAjaxForm(&$element, FormStateInterface $form_state
   }
 
   /**
+   * Adds Ajax information about an element to communicate with JavaScript.
+   *
+   * If #ajax['path'] is set on an element, this additional JavaScript is added
+   * to the page header to attach the Ajax behaviors. See ajax.js for more
+   * information.
+   *
+   * @param array $element
+   *   An associative array containing the properties of the element.
+   *   Properties used:
+   *   - #ajax['event']
+   *   - #ajax['prevent']
+   *   - #ajax['path']
+   *   - #ajax['options']
+   *   - #ajax['wrapper']
+   *   - #ajax['parameters']
+   *   - #ajax['effect']
+   *   - #ajax['accepts']
+   *
+   * @return array
+   *   The processed element with the necessary JavaScript attached to it.
+   */
+  public static function preRenderAjaxForm($element) {
+    // Skip already processed elements.
+    if (isset($element['#ajax_processed'])) {
+      return $element;
+    }
+    // Initialize #ajax_processed, so we do not process this element again.
+    $element['#ajax_processed'] = FALSE;
+
+    // Nothing to do if there are no Ajax settings.
+    if (empty($element['#ajax'])) {
+      return $element;
+    }
+
+    // Add a reasonable default event handler if none was specified.
+    if (isset($element['#ajax']) && !isset($element['#ajax']['event'])) {
+      switch ($element['#type']) {
+        case 'submit':
+        case 'button':
+        case 'image_button':
+          // Pressing the ENTER key within a textfield triggers the click event of
+          // the form's first submit button. Triggering Ajax in this situation
+          // leads to problems, like breaking autocomplete textfields, so we bind
+          // to mousedown instead of click.
+          // @see http://drupal.org/node/216059
+          $element['#ajax']['event'] = 'mousedown';
+          // Retain keyboard accessibility by setting 'keypress'. This causes
+          // ajax.js to trigger 'event' when SPACE or ENTER are pressed while the
+          // button has focus.
+          $element['#ajax']['keypress'] = TRUE;
+          // Binding to mousedown rather than click means that it is possible to
+          // trigger a click by pressing the mouse, holding the mouse button down
+          // until the Ajax request is complete and the button is re-enabled, and
+          // then releasing the mouse button. Set 'prevent' so that ajax.js binds
+          // an additional handler to prevent such a click from triggering a
+          // non-Ajax form submission. This also prevents a textfield's ENTER
+          // press triggering this button's non-Ajax form submission behavior.
+          if (!isset($element['#ajax']['prevent'])) {
+            $element['#ajax']['prevent'] = 'click';
+          }
+          break;
+
+        case 'password':
+        case 'textfield':
+        case 'number':
+        case 'tel':
+        case 'textarea':
+          $element['#ajax']['event'] = 'blur';
+          break;
+
+        case 'radio':
+        case 'checkbox':
+        case 'select':
+          $element['#ajax']['event'] = 'change';
+          break;
+
+        case 'link':
+          $element['#ajax']['event'] = 'click';
+          break;
+
+        default:
+          return $element;
+      }
+    }
+
+    // Attach JavaScript settings to the element.
+    if (isset($element['#ajax']['event'])) {
+      $element['#attached']['library'][] = 'core/jquery.form';
+      $element['#attached']['library'][] = 'core/drupal.ajax';
+
+      $settings = $element['#ajax'];
+
+      // Assign default settings. When 'path' is set to NULL, ajax.js submits the
+      // Ajax request to the same URL as the form or link destination is for
+      // someone with JavaScript disabled. This is generally preferred as a way to
+      // ensure consistent server processing for js and no-js users, and Drupal's
+      // content negotiation takes care of formatting the response appropriately.
+      // However, 'path' and 'options' may be set when wanting server processing
+      // to be substantially different for a JavaScript triggered submission.
+      // One such substantial difference is form elements that use
+      // #ajax['callback'] for determining which part of the form needs
+      // re-rendering. For that, we have a special 'system/ajax' route.
+      $settings += array(
+        'path' => isset($settings['callback']) ? 'system/ajax' : NULL,
+        'options' => array(),
+        'accepts' => 'application/vnd.drupal-ajax'
+      );
+
+      // @todo Legacy support. Remove in Drupal 8.
+      if (isset($settings['method']) && $settings['method'] == 'replace') {
+        $settings['method'] = 'replaceWith';
+      }
+
+      // Change path to URL.
+      $settings['url'] = isset($settings['path']) ? url($settings['path'], $settings['options']) : NULL;
+      unset($settings['path'], $settings['options']);
+
+      // Add special data to $settings['submit'] so that when this element
+      // triggers an Ajax submission, Drupal's form processing can determine which
+      // element triggered it.
+      // @see _form_element_triggered_scripted_submission()
+      if (isset($settings['trigger_as'])) {
+        // An element can add a 'trigger_as' key within #ajax to make the element
+        // submit as though another one (for example, a non-button can use this
+        // to submit the form as though a button were clicked). When using this,
+        // the 'name' key is always required to identify the element to trigger
+        // as. The 'value' key is optional, and only needed when multiple elements
+        // share the same name, which is commonly the case for buttons.
+        $settings['submit']['_triggering_element_name'] = $settings['trigger_as']['name'];
+        if (isset($settings['trigger_as']['value'])) {
+          $settings['submit']['_triggering_element_value'] = $settings['trigger_as']['value'];
+        }
+        unset($settings['trigger_as']);
+      }
+      elseif (isset($element['#name'])) {
+        // Most of the time, elements can submit as themselves, in which case the
+        // 'trigger_as' key isn't needed, and the element's name is used.
+        $settings['submit']['_triggering_element_name'] = $element['#name'];
+        // If the element is a (non-image) button, its name may not identify it
+        // uniquely, in which case a match on value is also needed.
+        // @see _form_button_was_clicked()
+        if (!empty($element['#is_button']) && empty($element['#has_garbage_value'])) {
+          $settings['submit']['_triggering_element_value'] = $element['#value'];
+        }
+      }
+
+      // Convert a simple #ajax['progress'] string into an array.
+      if (isset($settings['progress']) && is_string($settings['progress'])) {
+        $settings['progress'] = array('type' => $settings['progress']);
+      }
+      // Change progress path to a full URL.
+      if (isset($settings['progress']['path'])) {
+        $settings['progress']['url'] = url($settings['progress']['path']);
+        unset($settings['progress']['path']);
+      }
+
+      $element['#attached']['js'][] = array(
+        'type' => 'setting',
+        'data' => array('ajax' => array($element['#id'] => $settings)),
+      );
+
+      // Indicate that Ajax processing was successful.
+      $element['#ajax_processed'] = TRUE;
+    }
+    return $element;
+  }
+
+  /**
    * Arranges elements into groups.
    *
    * This method is useful for non-input elements that can be used in and
diff --git a/core/modules/system/core.api.php b/core/modules/system/core.api.php
index 8852c31..e88a3b5 100644
--- a/core/modules/system/core.api.php
+++ b/core/modules/system/core.api.php
@@ -1619,3 +1619,159 @@ function hook_display_variant_plugin_alter(array &$definitions) {
 /**
  * @} End of "addtogroup hooks".
  */
+
+/**
+ * @defgroup ajax Ajax API
+ * @{
+ * Overview for Drupal's Ajax API.
+ *
+ * @section sec_overview Overview of Ajax
+ * Ajax is the process of dynamically updating parts of a page's HTML based on
+ * data from the server. When a specified event takes place, a PHP callback is
+ * triggered, which performs server-side logic and may return updated markup or
+ * JavaScript commands to run. After the return, the browser runs the JavaScript
+ * or updates the markup on the fly, with no full page refresh necessary.
+ *
+ * Many different events can trigger Ajax responses, including:
+ * - Clicking a button
+ * - Pressing a key
+ * - Moving the mouse
+ *
+ * @section sec_framework Ajax responses in forms
+ * Forms that use the Drupal Form API (see the
+ * @link form_api Form API topic @endlink for more information about forms) can
+ * trigger AJAX responses. Here is an outline of the steps:
+ * - Add property '#ajax' to a form element in your form array, to trigger an
+ *   Ajax response.
+ * - Write an Ajax callback to process the input and respond.
+ * See sections below for details on these two steps.
+ *
+ * @subsection sub_form Adding Ajax triggers to a form
+ * As an example of adding Ajax triggers to a form, consider editing a date
+ * format, where the user is provided with a sample of the generated date output
+ * as they type. To accomplish this, typing in the text field should trigger an
+ * Ajax response. This is done in the text field form array element
+ * in \Drupal\config_translation\FormElement\DateFormat::getFormElement():
+ * @code
+ * '#ajax' => array(
+ *   'callback' => 'Drupal\config_translation\FormElement\DateFormat::ajaxSample',
+ *   'event' => 'keyup',
+ *   'progress' => array(
+ *     'type' => 'throbber',
+ *     'message' => NULL,
+ *   ),
+ * ),
+ * @endcode
+ *
+ * As you can see from this example, the #ajax property for a form element is
+ * an array. Here are the details of its elements, all of which are optional:
+ * - callback: The callback to invoke to handle the server side of the
+ *   Ajax event. More information on callbacks is below in @ref sub_callback.
+ * - path: The URL path to use for the request. If omitted, defaults to
+ *   'system/ajax', which invokes the default Drupal Ajax processing (this will
+ *   call the callback supplied in the 'callback' element). If you supply a
+ *   path, you must set up a routing entry to handle the request yourself and
+ *   return output described in @ref sub_callback below. See the
+ *   @link menu Routing topic @endlink for more information on routing.
+ * - wrapper: The HTML 'id' attribute of the area where the content returned by
+ *   the callback should be placed. Note that callbacks have a choice of
+ *   returning content or JavaScript commands; 'wrapper' is used for content
+ *   returns.
+ * - method: The jQuery method for placing the new content (used with
+ *   'wrapper'). Valid options are 'replaceWith' (default), 'append', 'prepend',
+ *   'before', 'after', or 'html'. See
+ *   http://api.jquery.com/category/manipulation/ for more information on these
+ *   methods.
+ * - effect: The jQuery effect to use when placing the new HTML (used with
+ *   'wrapper'). Valid options are 'none' (default), 'slide', or 'fade'.
+ * - speed: The effect speed to use (used with 'effect' and 'wrapper'). Valid
+ *   options are 'slow' (default), 'fast', or the number of milliseconds the
+ *   effect should run.
+ * - event: The JavaScript event to respond to. This is selected automatically
+ *   for the type of form element; provide a value to override the default.
+ * - prevent: A JavaScript event to prevent when the event is triggered. For
+ *   example, if you use event 'mousedown' on a button, you might want to
+ *   prevent 'click' events from also being triggered.
+ * - progress: An array indicating how to show Ajax processing progress. Can
+ *   contain one or more of these elements:
+ *   - type: Type of indicator: 'throbber' (default) or 'bar'.
+ *   - message: Translated message to display.
+ *   - url: For a bar progress indicator, URL path for determining progress.
+ *   - interval: For a bar progress indicator, how often to update it.
+ *
+ * @subsection sub_callback Setting up a callback to process Ajax
+ * Once you have set up your form to trigger an Ajax response (see @ref sub_form
+ * above), you need to write some PHP code to process the response. If you use
+ * 'path' in your Ajax set-up, your route controller will be triggered with only
+ * the information you provide in the URL. If you use 'callback', your callback
+ * method is a function, which will receive the $form and $form_state from the
+ * triggering form. You can use $form_state to get information about the
+ * data the user has entered into the form. For instance, in the above example
+ * for the date format preview,
+ * \Drupal\config_translation\FormElement\DateFormat\ajaxSample() does this to
+ * get the format string entered by the user:
+ * @code
+ * $format_value = \Drupal\Component\Utility\NestedArray::getValue(
+ *   $form_state->getValues(),
+ *   $form_state->getTriggeringElement()['#array_parents']);
+ * @endcode
+ *
+ * Once you have processed the input, you have your choice of returning HTML
+ * markup or a set of Ajax commands. If you choose to return HTML markup, you
+ * can return it as a string or a renderable array, and it will be placed in
+ * the defined 'wrapper' element (see documentation above in @ref sub_form).
+ * In addition, any messages returned by drupal_get_messages(), themed as in
+ * status-messages.html.twig, will be prepended.
+ *
+ * To return commands, you need to set up an object of class
+ * \Drupal\Core\Ajax\AjaxResponse, and then use its addCommand() method to add
+ * individual commands to it. In the date format preview example, the format
+ * output is calculated, and then it is returned as replacement markup for a div
+ * like this:
+ * @code
+ * $response = new AjaxResponse();
+ * $response->addCommand(new ReplaceCommand(
+ *   '#edit-date-format-suffix',
+ *   '<small id="edit-date-format-suffix">' . $format . '</small>'));
+ * return $response;
+ * @endcode
+ *
+ * The individual commands that you can return implement interface
+ * \Drupal\Core\Ajax\CommandInterface. Available commands provide the ability
+ * to pop up alerts, manipulate text and markup in various ways, redirect
+ * to a new URL, and the generic \Drupal\Core\Ajax\InvokeCommand, which
+ * invokes an arbitrary jQuery command.
+ *
+ * As noted above, status messages are prepended automatically if you use the
+ * 'wrapper' method and return HTML markup. This is not the case if you return
+ * commands, but if you would like to show status messages, you can add
+ * @code
+ * array('#theme' => 'status_messages')
+ * @endcode
+ * to a render array, use drupal_render() to render it, and add a command to
+ * place the messages in an appropriate location.
+ *
+ * @section sec_other Other methods for triggering Ajax
+ * Here are some additional methods you can use to trigger Ajax responses in
+ * Drupal:
+ * - Add class 'use-ajax' to a link. The link will be loaded using an Ajax
+ *   call. When using this method, the href of the link can contain '/nojs/' as
+ *   part of the path. When the Ajax JavaScript processes the page, it will
+ *   convert this to '/ajax/'. The server is then able to easily tell if this
+ *   request was made through an actual Ajax request or in a degraded state, and
+ *   respond appropriately.
+ * - Add class 'use-ajax-submit' to a submit button in a form. The form will
+ *   then be submitted via Ajax to the path specified in the #action.  Like the
+ *   ajax-submit class on links, this path will have '/nojs/' replaced with
+ *   '/ajax/' so that the submit handler can tell if the form was submitted in a
+ *   degraded state or not.
+ * - Add property '#autocomplete_route_name' to a text field in a form. The
+ *   route controller for this route must return an array of options for
+ *   autocomplete, as a \Symfony\Component\HttpFoundation\JsonResponse object.
+ *   See the @link menu Routing topic @endlink for more information about
+ *   routing.
+ */
+
+/**
+ * @} End of "defgroup ajax".
+ */