Custom Twig extensions (filters, functions, tags, etc.)

Ever seen this?

Fatal error: Uncaught Twig_Error_Syntax: Unknown "SOMETHING" function.

Sometimes you need to add a custom Twig extension to Pattern Lab - particularly when using Drupal, which ships with its own. Oftentimes you don't even need Pattern Lab to do anything with it, you just need it to ignore the function or just pass a string. The good news is we have already done this by default for many of them, so you can often just copy and paste from that directory. Let's show an example:

If you need a custom url function, go to the functions directory and copy/paste one of those files (e.g., pl_kint.function.php) into a new file named pl_url.function.php in the same directory (the pl_ namespace is important - this tells Drupal to ignore this file). In the code, replace any relevant pieces with your own like so:

<?php
/**
 * @file
 * Add "url" function for Pattern Lab.
 */

$function = new Twig_SimpleFunction('url', function ($string) {
  return $string;
});

Notice this isn't doing anything but returning a string as an argument, which for most cases will be all you need. Save that file, rerun the yarn start command and voila! Now Pattern Lab will recognize that function and no more error!

Advanced

If you need to write a more advanced or custom extension, here is a good starting place for doing so.