Custom theme function not being called?

Question

I'm building a Drupal 7 module to create a bespoke field type. I've implemented all the required hooks. My hook_field_formatter_view() function looks like this:

function MYMODULE_field_formatter_view($entity_type, $entity, $field, $instance, $langcode, $items, $display) {
  $element = array();

  switch ($display['type']) {
    case 'default':
      foreach ($items as $delta => $item) {
                $element[$delta] = array(
                  '#theme' => 'test',
                  '#item' => $item,
                );
      }
      break;
  }

  return $element;  
}

I've then defined the theme() function below:

function theme_test($variables) {
    return '<h1>Hello World</h1>';
}

But when I view the page nothing is output. If I look at the recent log entries I see this message:

Theme key 'test' not found.

If I change my MYMODULE_field_formatter_view render array '#theme' value to a built in theme function like 'item_list' it works. But for some reason my custom theme function is not being picked up.

I've rebuilt the theme registry hundreds of times as well as cleared the caches.

Answer 1

You probably have not defined your theme hook in hook_theme().

The Using the theme layer section of the Development handbook has an in-depth discussion about this.

Answer 2

You need to define the theme function in hook_theme and then clear the cache.

 function MYMODULE_theme() {
   return array(
     'test' => array (
       'render element' => 'element',
     ),
   );
 }

Answer 3

I was facing the same issue where my custom theme function was never being called in my template, even though it was registered. When I switched to another theme, it worked.

My solution was re-naming theme into 'something_theme', because I made a stupid mistake and already had module named 'something'.

Module and theme should never have the same name, because it leads into collision which causes unexpected behavior and hook overriding.