TextFormat::processFormat

public static TextFormat::processFormat(&$element, FormStateInterface $form_state, &$complete_form)

  $values = $form_state->getValue('body');
  $values['value'] = 'foo';
  $values['format'] = 'foo';

public static function processFormat(&$element, FormStateInterface $form_state, &$complete_form) {
  $user = static::currentUser();

  // Ensure that children appear as subkeys of this element.
  $element['#tree'] = TRUE;
  $blacklist = array(
    // Make \Drupal::formBuilder()->doBuildForm() regenerate child properties.
    '#parents',
    '#id',
    '#name',
    // Do not copy this #process function to prevent
    // \Drupal::formBuilder()->doBuildForm() from recursing infinitely.
    '#process',
    // Ensure #pre_render functions will be run.
    '#pre_render',
    // Description is handled by theme_text_format_wrapper().
    '#description',
    // Ensure proper ordering of children.
    '#weight',
    // Properties already processed for the parent element.
    '#prefix',
    '#suffix',
    '#attached',
    '#processed',
    '#theme_wrappers',
  );
  // Move this element into sub-element 'value'.
  unset($element['value']);
  foreach (Element::properties($element) as $key) {
    if (!in_array($key, $blacklist)) {
      $element['value'][$key] = $element[$key];
    }
  }

  $element['value']['#type'] = $element['#base_type'];
  $element['value'] += static::elementInfo()->getInfo($element['#base_type']);
  // Make sure the #default_value key is set, so we can use it below.
  $element['value'] += array('#default_value' => '');

  // Turn original element into a text format wrapper.
  $element['#attached']['library'][] = 'filter/drupal.filter';

  // Setup child container for the text format widget.
  $element['format'] = array(
    '#type' => 'container',
    '#attributes' => array('class' => array('filter-wrapper')),
  );

  // Get a list of formats that the current user has access to.
  $formats = filter_formats($user);

  // Allow the list of formats to be restricted.
  if (isset($element['#allowed_formats'])) {
    // We do not add the fallback format here to allow the use-case of forcing
    // certain text formats to be used for certain text areas. In case the
    // fallback format is supposed to be allowed as well, it must be added to
    // $element['#allowed_formats'] explicitly.
    $formats = array_intersect_key($formats, array_flip($element['#allowed_formats']));
  }

  if (!isset($element['#format']) && !empty($formats)) {
    // If no text format was selected, use the allowed format with the highest
    // weight. This is equivalent to calling filter_default_format().
    $element['#format'] = reset($formats)->id();
  }

  // If #allowed_formats is set, the list of formats must not be modified in
  // any way. Otherwise, however, if all of the following conditions are true,
  // remove the fallback format from the list of formats:
  // 1. The 'always_show_fallback_choice' filter setting has not been activated.
  // 2. Multiple text formats are available.
  // 3. The fallback format is not the default format.
  // The 'always_show_fallback_choice' filter setting is a hidden setting that
  // has no UI. It defaults to FALSE.
  $config = static::configFactory()->get('filter.settings');
  if (!isset($element['#allowed_formats']) && !$config->get('always_show_fallback_choice')) {
    $fallback_format = $config->get('fallback_format');
    if ($element['#format'] !== $fallback_format && count($formats) > 1) {
      unset($formats[$fallback_format]);
    }
  }

  // Prepare text format guidelines.
  $element['format']['guidelines'] = array(
    '#type' => 'container',
    '#attributes' => array('class' => array('filter-guidelines')),
    '#weight' => 20,
  );
  $options = array();
  foreach ($formats as $format) {
    $options[$format->id()] = $format->label();
    $element['format']['guidelines'][$format->id()] = array(
      '#theme' => 'filter_guidelines',
      '#format' => $format,
    );
  }

  $element['format']['format'] = array(
    '#type' => 'select',
    '#title' => t('Text format'),
    '#options' => $options,
    '#default_value' => $element['#format'],
    '#access' => count($formats) > 1,
    '#weight' => 10,
    '#attributes' => array('class' => array('filter-list')),
    '#parents' => array_merge($element['#parents'], array('format')),
  );

  $element['format']['help'] = [
    '#type' => 'container',
    'about' => [
      '#type' => 'link',
      '#title' => t('About text formats'),
      '#url' => new Url('filter.tips_all'),
      '#attributes' => ['target' => '_blank'],
    ],
    '#attributes' => ['class' => ['filter-help']],
    '#weight' => 0,
  ];

  $all_formats = filter_formats();
  $format_exists = isset($all_formats[$element['#format']]);
  $format_allowed = !isset($element['#allowed_formats']) || in_array($element['#format'], $element['#allowed_formats']);
  $user_has_access = isset($formats[$element['#format']]);
  $user_is_admin = $user->hasPermission('administer filters');

  // If the stored format does not exist or if it is not among the allowed
  // formats for this textarea, administrators have to assign a new format.
  if ((!$format_exists || !$format_allowed) && $user_is_admin) {
    $element['format']['format']['#required'] = TRUE;
    $element['format']['format']['#default_value'] = NULL;
    // Force access to the format selector (it may have been denied above if
    // the user only has access to a single format).
    $element['format']['format']['#access'] = TRUE;
  }
  // Disable this widget, if the user is not allowed to use the stored format,
  // or if the stored format does not exist. The 'administer filters'
  // permission only grants access to the filter administration, not to all
  // formats.
  elseif (!$user_has_access || !$format_exists) {
    // Overload default values into #value to make them unalterable.
    $element['value']['#value'] = $element['value']['#default_value'];
    $element['format']['format']['#value'] = $element['format']['format']['#default_value'];

    // Prepend #pre_render callback to replace field value with user notice
    // prior to rendering.
    $element['value'] += array('#pre_render' => array());
    array_unshift($element['value']['#pre_render'], 'filter_form_access_denied');

    // Cosmetic adjustments.
    if (isset($element['value']['#rows'])) {
      $element['value']['#rows'] = 3;
    }
    $element['value']['#disabled'] = TRUE;
    $element['value']['#resizable'] = 'none';

    // Hide the text format selector and any other child element (such as text
    // field's summary).
    foreach (Element::children($element) as $key) {
      if ($key != 'value') {
        $element[$key]['#access'] = FALSE;
      }
    }
  }

  return $element;
}