fences-8.x-2.0-rc1/modules/fences_presets/fences_presets.module

<?php

/**
 * @file
 * Alter wrapping markup of fields.
 */

use Drupal\Core\Url;

/**
 * Implements hook_fences_field_formatter_third_party_settings_form_alter().
 */
function fences_presets_fences_field_formatter_third_party_settings_form_alter(array &$settingsForm) {
  $presets = \Drupal::entityTypeManager()->getStorage('fences_preset')->loadByProperties(['status' => TRUE]);
  $presetsArray = [];
  $selectOptions = [];
  foreach ($presets as $preset) {
    $presetsArray[$preset->id()] = $preset->toArray();
    $selectOptions[$preset->id()] = $preset->label();
  }

  $settingsForm['fences']['fences_presets'] = [
    '#type' => 'details',
    '#open' => TRUE,
    '#title' => t('Fences Presets'),
    '#weight' => -99,
  ];

  // We are rendering the ui only select through an inline template to avoid the
  // element value getting implicitly saved. We only need this select, to
  // apply the preset to the fields:
  $settingsForm['fences']['fences_presets']['fences_preset_selector'] = [
    '#type' => 'inline_template',
    '#template' => '
    <div class="js-form-item form-item">
      <select class="form-select form-element form-element--type-select fences-preset-selector">
        <option value="" selected="selected">{{ "- Select Preset -"|t }}</option>
        {% for presetId, presetLabel in selectOptions %}
          <option value={{ presetId|escape("html_attr") }}>{{ presetLabel|escape }}</option>
        {% endfor %}
      </select>
      <div class="form-item__description">{{ description|t }}</div>
    </div>',
    '#context' => [
      'selectOptions' => $selectOptions,
      'description' => 'Select a preset to apply to the fences configuration. Note, that the selected preset is not persistent and will only modify the fences fields.',
    ],
  ];

  $settingsForm['fences']['fences_presets']['fences_preset_link'] = [
    '#type' => 'link',
    '#title' => t('Manage Fences Presets'),
    '#url' => Url::fromRoute('entity.fences_preset.collection'),
    '#attributes' => ['target' => '_blank'],
  ];

  $settingsForm['fences']['#attached']['library'][] = 'fences_presets/fences_preset_selector';
  $settingsForm['fences']['#attached']['drupalSettings']['fencesPresets'] = $presetsArray;
}