Adding CSS Classes to Blocks in Drupal 8

block.html.twig

<div class="block block--fancy">
  {{ title_prefix }}
  {% if label %}
    <h2  class="block__title block__title--fancy">{{ label }}</h2>
  {% endif %}
  {{ title_suffix }}
  {% block content %}
    {{ content }}
  {% endblock %}
</div>

This works in a lot of cases, but may not be flexible enough. The second approach utilizes the new attributes object – the successor to Drupal 7’s attributes array. The attribute object encapsulates all the attributes for a given tag. It also includes a number of methods which enable you to add, remove and alter those attributes before printing. For now we’ll just focus on the attributes.addClass() method. You can learn more about available methods in the official Drupal 8 documentation.

block.html.twig

{%
  set classes = [
    'block',
    'block--fancy'
  ]
%}
 
{%
  set title_classes = [
    'block__title',
    'block__title--fancy'
  ]
%}
 
<div{{ attributes.addClass(classes) }}>
  {{ title_prefix }}
  {% if label %}
    <h2{{ title_attributes.addClass(title_classes) }}>{{ label }}</h2>
  {% endif %}
  {{ title_suffix }}
  {% block content %}
    {{ content }}
  {% endblock %}
</div>

Alternatively, we can add our class directly to the class attribute with the existing classes from the attribute.class then print the remaining attributes. To prevent the class attribute from printing twice, we exclude it using the without Twig filter. Either way works.

block.html.twig

<div class="block--fancy {{ attributes.class }}"{{attributes|without('class') }}>
  {{ title_prefix }}
  {% if label %}
    <h2  class="block--fancy {{ title_attributes.class }}" {{title_attributes|without('class') }}>{{ label }}</h2>
  {% endif %}
  {{ title_suffix }}
  {% block content %}
    {{ content }}
  {% endblock %}
</div>

In any case, all our blocks on the site now look fancy as hell (assuming we’ve styled .block--fancy as such)

Template Suggestions

The above examples work. In reality if all our blocks look fancy, no blocks will look fancy. We need to apply this class only to our special blocks that truly deserve to be fancy. This introduces my second favorite DX improvement to Drupal 8 – hook_theme_suggestions_HOOK_alter.

If you wanted to make a custom template available for use to a certain block In Drupal 7, you had to do so in a preprocess function. Altering theme hook suggestions (the list of possible templates) in the Drupal 8 is delegated to its very own hook. The concept is pretty straight forward. Before Drupal renders an element, it looks at an array of possible template file names (a.k.a. suggestions) one-by-one. For each template file, it looks in the file system to see if that file exists in our theme, its base theme or core themes. Once it finds a match, it stops looking and renders the element using the matching template.

We’ll use this hook to add our new template file to the list of suggestions. In the case of blocks, the function we’ll define is hook_theme_suggestions_block_alter. It takes two arguments, the first is the array of suggestions which are passed by reference (by prefixing the parameter with a & so we can alter them directly. The second is the variables from our element that we can use to determine which templates we want to include.

Lets assume we renamed one of our templates above to block--fancy.html.twig and saved it to our theme. We then add the following function to my_theme.theme where “my_theme” is the name of our theme.

my_theme.theme

<?php
 
/**
 * Implements hook_theme_suggestions_HOOK_alter() for block templates.
 */
function my_theme_theme_suggestions_block_alter(array &$suggestions, array $variables) {
  $block_id = $variables['elements']['#id'];
 
  /* Uncomment the line below to see variables you can use to target a block */
  // print $block_id . '<br/>';
 
  /* Add classes based on the block id. */
  switch ($block_id) {
    /* Account Menu block */
    case 'account_menu':
      $suggestions[] = 'block__fancy';
      break;
  }
}

Now the account menu block on our site will use block--fancy.html.twig as we can see from the output of twig debug

This is just one example of the improvements in D8 theming. I’m really excited for the clarity that the new Twig templates bring to Drupal 8 and the simplicity of managing template suggestions through hook_theme_suggestions_HOOK_alter.