ckeditor5_upgrade_test.module

<?php
/**
 * @file
 * Provides assisting code for testing a CKEditor 4 to CKEditor 5 upgrade.
 */

/**
 * Implements hook_ckeditor5_upgrade_button_mapping_alter().
 */
function ckeditor5_upgrade_test_ckeditor5_upgrade_button_mapping_alter(&$button_mapping) {
  // Map a test button (present in ckeditor_upgrade3 text format).
  $button_mapping['TestButton'] = 'testButton';
}

/**
 * Implements hook_ckeditor5_upgrade_format_alter().
 */
function ckeditor5_upgrade_test_ckeditor5_upgrade_format_alter(&$format, $original_format) {
  if (!empty($format->filters['filter_html']->status)) {
    $allowed_tags = $format->filters['filter_html']->settings['allowed_html'];
    $toolbar = $format->editor_settings['toolbar'];
    // Add <test-tag> to the list of allowed tags if the test button is present.
    if (in_array('testButton', $toolbar) && strpos($allowed_tags, '<test-tag>') === FALSE) {
      $format->filters['filter_html']->settings['allowed_html'] = $allowed_tags . ' <test-tag>';
    }
  }
}

/**
 * Implements hook_ckeditor5_plugins().
 */
function ckeditor5_upgrade_test_ckeditor5_plugins() {
  $plugins['upgradeTest.UpgradeTest'] = array(
    'buttons' => array(
      'testButton' => array(
        'label' => 'Test Button',
        'image_alternative' => '<span class="ckeditor5-upgrade-test">Test Button</span>',
      ),
    ),
    'enabled_callback' => FALSE,
  );
  return $plugins;
}