Theming in Drupal 8 with Twig (Part 1)
Besides fundamental changes in the backend of Drupal 8 (i.e usage of several Symfony 2 components) a new theme engine called Twig is also introduced to Drupal.
In the course of further development of Drupal it is necessary to look beyond our community boundaries and to be open to new ideas and technologies. This is the only way to ensure a sustained and stable growth of the community. With the introduction of the Symfony 2 components Drupal took a major step forward to the PHP community.
In addition to PHPTemplate themers can use Twig as a new theme engine in Drupal 8. This helps lowering the entry barrier for new developers since learning PHP for theming Drupal is no longer necessary.
In our multi-part blog post on Twig we want to explain the fundamental changes of the new theme engine and show new possibilities in theming introduced by its implementation in Drupal Core.
Why Twig?
Twig has its origin in the world of Django and was developed by Armin Ronacher as an alternative theme engine for writing templates directly in PHP. Since 2009 Twig has been an integral part of the Symfony framework and has proven to be a reliable and fast theme engine. Since then the further development of Twig is promoted by Fabien Potencier, the leader of the Symfony project. Further information on his thoughts about templating engines in PHP can be found in his blog post Templating Engines in PHP.
The following list provides an overview on PHP frameworks that also support Twig:
Drupal & Twig
Apart from an easier tag-based syntax, Twig offers a lot more security on theming level. The fact can be described very briefly by a quote of John Albin at DrupalCon Denver.
We hand themers a loaded gun and tell them to hammer in a nail with it. Oh and be careful!
For instance, it would be possible to access the Drupal database via PHPTemplate and delete individual tables or access the file system of your server.
<?php db_query('DROP TABLE {users}'); unlink('sites/default/files/myfilename.pdf') ?>
Advantages using Twig
- more security on theme level due to limited possibilities given by tag-based syntax
- no more PHP/HTML mix in template files
- simplification of theming system due to the reduction of preprocess functions
- consistent and easy-to-learn tag-based syntax, that is limited to the most common use cases in template 'development'
- Reusability of template files (via Twig includes)
- IDE integration (Netbeans, vim, PHPStorm, Eclipse, Textmate, Sublime)
- good documentation - Twig documentation
- will bring Symfony and Drupal community closer, because more components can be shared
Unfortunately there is not so much documentation on drupal.org yet but the latest coding standards can be found here: Twig Coding Standards.
Disadvantages
- learning a new template syntax
- performance and memory issues caused by an extra theme engine
- differing update cycles of Drupal Core and Twig regarding to the code freeze of Drupal 8
In my opinion only the first issue should be considered as a real disadvantage - in addition to the fundamentely changed backend in Drupal 8, Drupalistas also have to get used to a new template syntax. But if you have a look at other Content Management Systems or Frameworks you'll also find custom syntax for templating, unless they are using PHP as their main language for templating.
The performance issue will be adressed in one of our following blog posts on Twig. The already mentioned update cycles are a well-known problem in the Drupal community (jQuery is a good example in Drupal 7) - but I don't think that Twig will change dramatically in future releases.
Twig is part of Drupal Core
In the latest Drupal 8 release Twig is available as an additional theme engine and is ready to use in custom themes (Integrate Twig into core: Implementation issue). Currently the built-in themes are still using PHPTemplate as their theme engine - but you can try the Drupal 8 Twig Sandbox instead. The goal should be to reduce all Drupal Core templates and their related theme functions to just one Twig template and to simplify the theming system. The Stark theme located in the sandbox is a good example for the new approach - besides the module templates all parts of administrational pages are provided as Twig templates and can be easily overriden.
We won't get rid of the big "render array" (of doom) in Drupal 8, but the usage of Twig as our new templating engine will be much easier.
In the second part of our blog post series we want to talk about the syntax of Twig and show some simple examples to explain how we can benefit from these changes.
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