metatag-8.x-1.x-dev/tests/modules/metatag_test_integration/metatag_test_integration.module

<?php

/**
 * @file
 * Contains metatag_test_integration.module.
 */

/**
 * Implements hook_metatags_attachments_alter().
 */
function metatag_test_integration_metatags_attachments_alter(array &$attachments) {
  $title = "This is the title I want | [site:name] | Yeah!";
  _metatag_test_integration_replace_tag('title', \Drupal::token()->replace($title), $attachments);
}

/**
 * Replaces meta tag in html head with given content.
 *
 * @param string $name
 *   The name of the tag to replace.
 * @param string $content
 *   The content to use.
 * @param array $attachments
 *   The array of attachments to act on.
 */
function _metatag_test_integration_replace_tag($name, $content, array &$attachments): void {
  if (empty($attachments['#attached'])) {
    $attachments['#attached'] = [];
  }

  if (empty($attachments['#attached']['html_head'])) {
    $attachments['#attached']['html_head'] = [];
  }

  $index = _metatag_test_integration_find_tag($name, $attachments);

  if ($index > -1) {
    $attachments['#attached']['html_head'][$index][0]['#attributes']['content'] = $content;
  }
  else {
    $attachments['#attached']['html_head'][] = [
      0 => [
        '#attributes' => ['name' => $name, 'content' => $content],
        '#tag' => 'meta',
      ],
      1 => 'description',
    ];
  }
}

/**
 * Finds the index of a meta tag in the html head.
 *
 * @param string $name
 *   The name of the tag to find.
 * @param array $attachments
 *   The array of attachments to search.
 *
 * @return int
 *   The position in the attachment array that the tag is found in. -1 if the
 *   tag is not set.
 */
function _metatag_test_integration_find_tag($name, array $attachments): int {
  foreach ($attachments['#attached']['html_head'] as $index => $attachment) {
    if ($attachment[1] == $name) {
      return $index;
    }
  }
  return -1;
}