preprocessor

Overview

Let's take a look at a quick comparison between a traditional preprocess function and a preprocessor plugin.

Traditionally, you would preprocess a node.html.twig template using the following code:

/**
 * Implements hook_preprocess_HOOK().
 */
function MY_THEME_preprocess_node(&$variables) {
  $variables['foo'] = 'bar';
}

To achieve the same functionality as above within your theme with a Plugin, you would do the following:

Take this traditional hook function as an example. You would normally put this in a .theme file to add the foo variable to the node.html.twig template.

/**
 * Implements hook_preprocess_HOOK().
 */
function MY_THEME_preprocess_node(&$variables) {
  $variables['foo'] = 'bar';
}

With Preprocessor Files, you can instead put the preprocessing logic in a standalone file.

Just like Drupal 7, you can affect the output of certain HTML via preprocess functions. For example, if you wanted to add a class to a menu and preferred to do this at the PHP level, you can. This is a good way to alter theme-specific markup. If you want to add or edit theme-independent markup, you should create a custom module instead.