Theme Hook Suggestions

This hook allows any module or theme to provide alternative theme function or template name suggestions and reorder or remove suggestions provided by hook_theme_suggestions_HOOK() or by earlier invocations of this hook.

These theme suggestions will show up in the Twig debug output (when enabled). This allows to create a more complex logic to decide which template file should be used to render the data.

Don't get confused with the double hook. The first hook stands for the name of the theme or module, the second stands for the template.

Drupal 7:

* Implements hook_preprocess_HOOK() for node templates.
function MYTHEME_preprocess_node(&$variables) {
  $variables['theme_hook_suggestions'][] = 'node__' . 'first';
  $variables['theme_hook_suggestions'][] = 'node__' . 'second';

Drupal 8:

* Implements hook_theme_suggestions_HOOK_alter() for node templates.
function MYTHEME_theme_suggestions_node_alter(array &$suggestions, array $variables) {
  $suggestions[] = 'node__' . 'first';
  $suggestions[] = 'node__' . 'second';

To add a template file suggestion for the page hook, you would implement hook_theme_suggestions_page_alter().

hook_preprocess for suggestions

Each theme hook suggestion will automatically get it's own preprocess function. For the example above, the following two preprocess functions will be available:

/* for $suggestions[] = 'node__' . 'first'; */
function hook_preprocess_node__first(&$variables);

/* for $suggestions[] = 'node__' . 'second'; */
function hook_preprocess_node__second(&$variables);