Adding template in Drupal module for theming

Part III of the practical guide to creating basic Drupal 8 modules
From .info to tests, just the basics

loremipsum.module

/**
 * Implements hook_theme().
 */
function loremipsum_theme($existing, $type, $theme, $path) {
  $variables = array(
    'loremipsum' => array(
      'variables' => array(
        'source_text' => NULL,
      ),
      'template' => 'loremipsum',
    ),
  );
  return $variables;
}

Another reason not to abandon the .module file is that it’s where hook_theme() goes. It works almost the same as in D7: you define an array containing your variables and the template file, which must be stored in the correct location (the templates folder) with a .html.twig extension.

Then, before passing the render array to Twig, you can perform some preprocessing. The following hook inserts random punctuation at the end of each sentence:

/**
 * Template preprocess function for Lorem ipsum.
 *
 * @param array $variables
 *   An associative array containing:
 *   - source_text
 */
function template_preprocess_loremipsum(&$variables) {
  $punctuation = array('. ', '! ', '? ', '... ', ': ', '; ');
  for ($i = 0; $i < count($variables['source_text']); $i++) {
    $big_text = explode('. ', $variables['source_text'][$i]);
    for ($j = 0; $j < count($big_text) - 1; $j++) {
      $big_text[$j] .= $punctuation[floor(mt_rand(0, count($punctuation) - 1))];
    }
    $variables['source_text'][$i] = implode('', $big_text);
  }
}

/templates/loremipsum.html.twig

{#
/**
 * @file
 * Default theme implementation to print Lorem ipsum text.
 *
 * Available variables:
 *   - source_text
 *
 * @see template_preprocess_loremipsum()
 *
 * @ingroup themeable
 */
#}
<div class="loremipsum">
{% for item in source_text %}
  <p>{{ item }}</p>
{% endfor %}
</div>

Now the $source_text array is processed using a simple for loop inside our Twig template, wrapped with <p> tags.

Note the correspondence between hook_theme(), template_preprocess_hook(), and our Twig file: