Twig custom functions and filters

Since Drupal has got template engine like Twig we can't use custom php code directly in the templates and it is fine.

But not worry, we have a posability how to do it in right way. Bay the way, there are two approaches to create custom php code like Functions and Filters.

Function way, probably is usual php function:

{{ a_function_to_print_hello_string('Drupal') }} {# "Hello Drupal" #}

Filter way is almost usual php function, but it is different syntax to call it:

{{ 'DRUPAL'|lower }} {# "drupal" #}

Generally speaking, a Filter is a way to transform the displayed data, but a Function is used when you need to compute things to render the result.

Well, let's create a simple Drupal module to see how Functions & Filters looks in details.

There is a file tree for the simple module:

dw_twig_extension
├── src
│   └── Twig
│       └── Extension
│           ├── Functions.php
│           └── Filters.php
├── dw_twig_extension.info.yml
└── dw_twig_extension.services.yml

File dw_twig_extension.info.yml

name: 'Twig Extension'
type: module
description: 'This module provides example of custom Twig functions and filters.'
core_version_requirement: ^9
package: 'Drupal Way'

This is the description of the services in dw_twig_extension.services.yml:

services:
  dw_twig_extension.twig.extension.functions:
    class: Drupal\dw_twig_extension\Twig\Extension\Functions
    tags:
      - { name: twig.extension }
  dw_twig_extension.twig.extension.filters:
    class: Drupal\dw_twig_extension\Twig\Extension\Filters
    tags:
      - { name: twig.extension }

And this is the service with Twig functions Functions.php:

<?php

namespace Drupal\dw_twig_extension\Twig\Extension;

use Twig\Extension\AbstractExtension;
use Twig\TwigFunction;

/**
 * Provides functions for Twig.
 */
class Functions extends AbstractExtension {

  /**
   * {@inheritdoc}
   */
  public function getFunctions(): array {
    return [
      new TwigFunction('hello', [$this, 'getHello']),
    ];
  }

  /**
   * Get hello.
   *
   * @param string $name
   *   The name value.
   *
   * @return string
   *   The return value.
   */
  public function getHello(string $name): string {
    return 'Hello' . ' ' . $name . '!';
  }

}

And this is the service with Twig filters Filters.php:

<?php

namespace Drupal\dw_twig_extension\Twig\Extension;

use Twig\Extension\AbstractExtension;
use Twig\TwigFilter;

/**
 * Provides filters for Twig.
 */
class Filters extends AbstractExtension {

  /**
   * {@inheritdoc}
   */
  public function getFilters(): array {
    return [
      new TwigFilter('striptags', [$this, 'getStripTags']),
    ];
  }

  /**
   * Strips HTML from a string.
   *
   * @param string $string
   *   The HTML content.
   *
   * @return string
   *   The cleaned string.
   */
  public function getStripTags(string $html): string {
    return strip_tags($html);
  }

}

And how to use this "hello" function and "striptags" filter in a template

{{ hello('Drupal') }} {{ '<h1>Title</h1>'|striptags }}

Also, we can use Dependency Injection in the services like this.

First of all, let's add a service to the file dw_twig_extension.services.yml:

services:
  dw_twig_extension.twig.extension.functions:
    class: Drupal\dw_twig_extension\Twig\Extension\Functions
    tags:
      - { name: twig.extension }
  dw_twig_extension.twig.extension.filters:
    arguments: ['@theme_handler']
    class: Drupal\dw_twig_extension\Twig\Extension\Filters
    tags:
      - { name: twig.extension }

And let's use it in the code.

<?php

namespace Drupal\dw_twig_extension\Twig\Extension;

use Drupal\Core\Extension\ThemeHandlerInterface;
use Twig\Extension\AbstractExtension;
use Twig\TwigFilter;

/**
 * Provides functions for Twig.
 */
class Functions extends AbstractExtension {
  
  /**
   * The theme handler service.
   *
   * @var \Drupal\Core\Extension\ThemeHandlerInterface
   */
  protected ThemeHandlerInterface $themeHandler;

  /**
   * Constructor.
   *
   * @param \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler
   */
  public function __construct(ThemeHandlerInterface $theme_handler) {
    $this->themeHandler = $theme_handler;
  }

  /**
   * {@inheritdoc}
   */
  public function Functions(): array {
    return [
      new TwigFilter('get_default_theme', [$this, 'getDefaultTheme']),
    ];
  }

  /**
   * Get default theme.
   *
   * @return string
   *   The default theme.
   */
  public function getDefaultTheme(): string {
    return $this->themeHandler->getDefault();
  }

}

And now we can use this function in any Twig template.

{{ get_default_theme() }} {# "a_theme" #}