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How to implement custom theme functions like theme_textfield or theme_form_element?

I know we can do it using hook_theme_registry_alter:

/**
 * Implements hook_theme_registry_alter().
 */
function krown_admin_theme_registry_alter(&$theme_registry) {

  //Override some Drupal defaul theme functions

  $theme_registry['textfield']['theme path'] = drupal_get_path('module', 'krown_admin');
  $theme_registry['textfield']['function'] = 'krown_admin_theme_textfield';
  $theme_registry['textarea']['theme path'] = drupal_get_path('module', 'krown_admin');
  $theme_registry['textarea']['function'] = 'krown_admin_theme_textarea';
  $theme_registry['form_element']['theme path'] = drupal_get_path('module', 'krown_admin');
  $theme_registry['form_element']['function'] = 'krown_admin_theme_form_element';
  $theme_registry['form_element_label']['theme path'] = drupal_get_path('module', 'krown_admin');
  $theme_registry['form_element_label']['function'] = 'krown_admin_theme_form_element_label';

}

The problem with this approach, is that the custom functions are applied on all website. I just want to apply them for some forms or for a specific theme.

1

Those theme functions don't get a reference to the form where the element is used. Therefore, they cannot be used to alter an element for a single form.

Every form element accepts #theme, so you can alter the #theme function used for those form fields in a single form. This is what Drupal does in node_filter_form().

  $form['filters'] = array(
    '#type' => 'fieldset',
    '#title' => t('Show only items where'),
    '#theme' => 'exposed_filters__node',
  );

theme_exposed_filters() then does its job.

  $form = $variables['form'];
  $output = '';

  if (isset($form['current'])) {
    $items = array();
    foreach (element_children($form['current']) as $key) {
      $items[] = drupal_render($form['current'][$key]);
    }
    $output .= theme('item_list', array('items' => $items, 'attributes' => array('class' => array('clearfix', 'current-filters'))));
  }

  $output .= drupal_render_children($form);

  return '<div class="exposed-filters">' . $output . '</div>';

Unfortunately, this doesn't work with theme_form_element() or theme_form_element_label().

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