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/**
 * Implements hook_form_FORM_ID_alter().
 *
 * Modifies the content form for page manager variants to add an "edit content"
 * link on blocks that are entities.
 *
 * @param $form
 * @param $form_state
 */
function caxy_form_page_manager_block_page_content_alter(&$form, &$form_state) {
  // Identify the form elements that refer to blocks. These children all have a
  // key that is a UUID. Filter out non-UUIDs. The array key stays the same.
  /** @var array $uuids */
  $uuids = array_filter(Element::children($form['blocks']), [Uuid::class, 'isValid']);
  if (!empty($uuids)) {
    /** @var EntityRepositoryInterface $repository */
    $repository = \Drupal::service('entity.repository');

    // Load all of the entities in this page variant. The array keys will be
    // the same as in the source array $uuids.
    /** @var EntityInterface[] $entities */
    $entities = array_filter(array_map(function ($uuid) use ($repository, $form) {
      $args = explode(':', $form['blocks'][$uuid]['id']['#markup'], 2);
      if (count($args) !== 2) {
        return FALSE;
      }

      return call_user_func_array([$repository, 'loadEntityByUuid'], $args);
    }, $uuids));

    // Loop through each UUID and use the entity at the same index in $entities
    // to generate an `edit-form` link. This link opens in a new window because
    // modal entity forms are hard.
    foreach (array_intersect_key($uuids, $entities) as $k => $v) {
      /** @var EntityInterface $entity */
      $entity = $entities[$k];
      // Using `array_merge` allows the edit content link to be first in the
      // operations list.
      $form['blocks'][$v]['operations']['#links'] = array_merge([
        'edit-content' => [
          'title' => 'Edit content',
          'url' => $entity->toUrl('edit-form'),
          'attributes' => [
            'rel' => 'noopener noreferrer',
            'target' => '_blank',
          ],
        ]
      ], $form['blocks'][$v]['operations']['#links']);
    }
  }
}