file @BootstrapPreprocess

<!-- @file Documentation for the @BootstrapPreprocess annotated plugin. -->
<!-- @defgroup -->
<!-- @ingroup -->
# @BootstrapPreprocess

- [Create a plugin](#create)
- [Rebuild the cache](#rebuild)

---

## Create a plugin {#create}

We'll use `Page` implemented by this base theme as an example of how to add
custom classes for the `page.html.twig` template that should only be added
under certain conditions.

Replace all following instances of `THEMENAME` with the actual machine name of
your sub-theme.

Create a file at `./themes/THEMENAME/src/Plugin/Preprocess/Page.php` with the
following contents:

```php
<?php

namespace Drupal\THEMENAME\Plugin\Preprocess;

use Drupal\bootstrap\Plugin\Preprocess\Page as BootstrapPage;
use Drupal\bootstrap\Utility\Element;
use Drupal\bootstrap\Utility\Variables;

/**
 * Pre-processes variables for the "page" theme hook.
 *
 * @ingroup plugins_preprocess
 *
 * @BootstrapPreprocess("page")
 */
class Page extends BootstrapPage {
  /*
   * It should be noted that you do not need all three methods here.
   * This is to just show you the different examples of how this plugin
   * works and how they can be tailored to your needs.
   */

  /**
   * {@inheritdoc}
   */
  public function preprocess(array &$variables, $hook, array $info) {
    $value = isset($variables['element']['child']['#value']) ? $variables['element']['child']['#value'] : FALSE;
    if (_some_module_condition($value)) {
      $variables['attributes']['class'][] = 'my-theme-class';
      $variables['attributes']['class'][] = 'another-theme-class';
      $key = array_search('page', $variables['attributes']['class']);
      if ($key !== FALSE) {
        unset($variables['attributes']['class'][$key]);
      }
    }

    // If you are extending and overriding a preprocess method from the base
    // theme, it is imperative that you also call the parent (base theme) method
    // at some point in the process, typically after you have finished with your
    // preprocessing.
    parent::preprocess($variables, $hook, $info);
  }

  /**
   * {@inheritdoc}
   */
  public function preprocessVariables(Variables $variables) {
    // This method is almost identical to the one above, but it introduces the
    // Variables utility class in the base theme. This class has a plethora of
    // helpful methods to quickly modify common tasks when you're in a
    // preprocess function. It also acts like the normal $variables array when
    // you need it to in instances of accessing nested content or in loop
    // structures like foreach.
    $value = isset($variables['element']['child']['#value']) ? $variables['element']['child']['#value'] : FALSE;
    if (_some_module_condition($value)) {
      $variables->addClass(['my-theme-class', 'another-theme-class'])->removeClass('page');
    }
    parent::preprocessVariables($variables);
  }

  /**
   * {@inheritdoc}
   */
  protected function preprocessElement(Element $element, Variables $variables) {
    // This method is only ever invoked if either $variables['element'] or
    // $variables['elements'] exists. These keys are usually only found in forms
    // or render arrays when there is a #type being used. This introduces the
    // Element utility class in the base theme. It too has a bucket-load of
    // features, specific to the unique characteristics of render arrays with
    // their "properties" (keys starting with #). This will quickly allow you to
    // access some of the nested element data and reduce the overhead required
    // for commonly used logic.
    $value = $element->child->getProperty('value', FALSE);
    if (_some_module_condition($value)) {
      $variables->addClass(['my-theme-class', 'another-theme-class'])->removeClass('page');
    }
    parent::preprocessElement($element, $variables);
  }

}
?>
```

## Rebuild the cache {#rebuild}

Once you have saved, you must rebuild your cache for this new plugin to be
discovered. This must happen anytime you make a change to the actual file name
or the information inside the `@BootstrapPreprocess` annotation.

To rebuild your cache, navigate to `admin/config/development/performance` and
click the `Clear all caches` button. Or if you prefer, run `drush cr` from the
command line.

Voilà! After this, you should have a fully functional `@BootstrapPreprocess`
plugin!