Using hook_theme_registry_alter to only modify specific pages instead of all pages

Question

I have a custom module which invokes the hook_theme_registry_alter() to modify the presentation of some input fields.

However, I've discovered that the overridden styling is affecting ALL input fields on my entire site.

This is not good!

How to I make it so that my implementation of the hook_theme_registry_alter() is only seen on my module's pages?

Here is my function, and it is applying the theming functions to ALL radio and form elements on our site.

function MY_MODULE_theme_registry_alter(&$theme_registry) {
  if (isset($theme_registry['radio'])) {
    $theme_registry['radios']['function'] = "MY_MODULE_radios";
    $theme_registry['radio']['function'] = "MY_MODULE_radio";
    $theme_registry['form_element']['function'] = "MY_MODULE_element";
  }
}

Answer 1

For simple alterations, instead of implementing hook_elements(), you could simply implement hook_form_alter() or it's cousin hook_form_FORM_ID_alter(). Both of these functions will effectively allow you to (as their name suggests) alter a form's input elements, and they are much easier to implement than hook_elements(). The former gets called for every function in the website, while the later only gets called for the FORM_ID that you specified in your hook implementation.

If the ID of the form you wish to modify is my_form, then hook_form_FORM_ID_alter() would be written as hook_form_my_form_alter(). If you want a more thorough explanation of these two hooks, I highly recommend reading Lullabot's writeup on modifying Drupal 6 Forms.

Answer 2

Instead of implementing hook_theme_registry_alter() that changes the theme implementations for every module, you could define new form fields using hook_elements(). In this way you can implement a new radio field that is used only from your module, and that render a radio field in a customized way.
The default field definitions implemented by Drupal are contained in system_elements(), which defines the form fields you are interested to using the following arrays:

  $type['radios'] = array(
    '#input' => TRUE,
    '#process' => array('expand_radios'),
  );
  $type['radio'] = array(
    '#input' => TRUE,
    '#default_value' => NULL,
    '#process' => array('form_expand_ahah'),
  );

Using your form fields, you would just replace code similar to the first snippet with code similar to the second snippet.

$form['posting_settings']['comment_preview'] = array(
  '#type' => 'radios', 
  '#title' => t('Preview comment'), 
  '#default_value' => variable_get('comment_preview', 1), 
  '#options' => array(t('Optional'), t('Required')),
);

$form['posting_settings']['comment_preview'] = array(
  '#type' => 'mymodule_radios', 
  '#title' => t('Preview comment'), 
  '#default_value' => variable_get('comment_preview', 1), 
  '#options' => array(t('Optional'), t('Required')),
);

As only your module would use the "mymodule_radios" form field type, only your module would render the "radios" form field in a different way.

In Drupal 7, the hook has been renamed to hook_element_info().

For more information, see Creating Custom Elements Using Drupal 6.x.