Override theme template from module without implementing a theme

Question

I have a project on D8 where I don't use any custom themes, I use default seven theme for everything because there is no need of any visual theming.

So My question: Is it possible to override theme template using module? without implementing theme at all?

I have a case where I want to add custom third party generated value in node's template.

This is what I have done:

Implemented custom template:

function MY_MODULE_theme($existing, $type, $theme, $path) {
  return array(
    'video_teaser' => [
      'template' => 'video_teaser',
      'variables' => NULL,
    ]
  );
}

Implemented hook_theme_suggestions_alter:

function MY_MODULE_theme_suggestions_alter(array &$suggestions, array $variables, $hook) {
  if(in_array('node__teaser', $suggestions)){
    $suggestions[] = 'video_teaser';
  }
} 

And this, works, instead of default template, my video_teser template is used.

BUT: I have a problem passing default node variables to my custom template, they aren't passed automatically and I couldn't find the way to do this using current approach.

Answer 1

You can do this in hook_theme() with the base hook 'node':

function MY_MODULE_theme($existing, $type, $theme, $path) {
  return [
    'node__video_teaser' => [
      'template' => 'node--video-teaser',
      'base hook' => 'node',
    ],
  ];
}

You don't need to define the base hook. Drupal can discover this automatically. What's important is that you use the double underscore, because this works exactly like the theme suggestions. After Drupal found the base hook it will clone the preprocess functions of this hook and will provide the variables for the node.

Answer 2

Yes, but for some cases it is not clear, I find out that for every cases you can do something like that. You need to do something like that in your custom_module_preprocess

/**
 * Implements hook_preprocess().
 */
function MY_MODULE_preprocess(&$variables, $hook, &$info) {
  $route_name = \Drupal::routeMatch()->getRouteName();
  if ($route_name == 'custom_route_name') {
    $module_path = drupal_get_path('module', 'MY_MODULE');
    if ($hook == 'html' && $variables['theme_hook_original'] == 'html') {
      $info['theme path'] = $module_path;
      $info['path'] = $module_path . '/templates';
    }
    if ($hook == 'page' && $variables['theme_hook_original'] == 'page') {
      $info['theme path'] = $module_path;
      $info['path'] = $module_path . '/templates';
    }
  }
}

It tells drupal to suggest html.html.twig and page.html.twig templates for exact route from your custom module's /templates directory.

Additionally MY_MODULE_theme_registry_alter(&$theme_registry) will work only once, only after clear caches and will not be dynamic, so you shouldn't use it!