what is the hook name in order to alter theme function of specific element

Question

imagine that we have a list if items in $items array that it's theme function is list_item in specific page.

code is

theme('item_list', array('items' => $items));

but in my module, I want to alter the parameter 'item_list' and I want to use my custom render function 'custom_item_list'

I know that I must register and define custom_item_list function but I don't know in what hook I am able to alter the theme_function parameter of specific element.

Answer 1

You can use hook_theme() function to use custom item list.

Hook theme:

function mymodule_theme($existing, $type, $theme, $path)
{
  if ($type == 'module')
  {
    return array(
      'custom_item_list' => array(
        'variables' => array('var_name'=>NULL),
        'template' => 'custom-item-list',
      ),
    );
  }
  return array(); //will raise fatal error if void
}

Call it as usual:

theme('custom_item_list', array('var_name' => mymodule_variable()));

Answer 2

You need to implement hook_theme_registry_alter() which is the hook modules can use to alter a theme function implemented by other modules. hook_theme() is for theme functions your module defines itself.

Just set $theme_registry['item_list']['function'] to 'theme_custom_item_list', and theme_custom_item_list() will be called instead of theme_item_list().
Keep in mind that, if you need to alter the content of the $variables parameter passed to a theme function, and not its output, you can implement hook_preprocess_item_list(), since every theme function has its own preprocess functions.

function mymodule_preprocess_item_list(&$variables) {
  // Change the title for the list.
  $variables['title'] = 'My own list title';
}