My talk given at DrupalCamp Utah 2016 about theming in Drupal 8. Complementary material located loganfarr.com

1. 8 Things to Know About
Theming in Drupal 8
Or, “Lessons I Learned when Building a D8 Subtheme”
Logan Farr || LoganFarr.com || @logan_farr || @loganfarr

2. Site building
Object oriented programming
Module building
Hooks and API functions not
normally in theming.
JavaScript frameworks
Things we will not
be covering.
Constructing a Drupal 8 (sub)theme
How a Drupal 8 theme works
Templating and Twig*
Template preprocessing and theme functions
What the (theme) differences are between D7 and D8
Tools to debug a theme
Things we will be covering

3. 1.File structures have changed
D8 Base Path
D8 Theme

4. 2. File types have changed
THEME_NAME.info -> THEME_NAME.info.yml
template.php -> THEME_NAME.theme
hook_template.tpl.php -> hook_template.html.twig

5. dcu.info
name = DCU
description = An example theme for Drupal Camp Utah
base theme = Omega
screenshot = screenshot.png
engine = phptemplate
core = 7.x
stylesheets[all][] = css/dcu.styles.css
scripts[] = js/dcu.behaviors.js
regions[content] = Content
...

6. dcu.info.yml
name: DCU
type: theme
description: An example theme for Drupal Camp Utah
base theme: Omega
engine = phptemplate
screenshot: screenshot.png
core: 8.x
libraries:
- dcu/global-styling
#Regions
regions:
navigation: Navigation
header: Header
content: Content
machine_readable: Human Readable
stylesheets-remove:
- core/assets/vendor/normalize-css/normalize.css

7. 3. stylesheets-remove
You can prevent stylesheets from rendering on your page!
In THEME_NAME.info.yml:
stylesheets-remove:
- (path to stylesheet here)
- core/assets/.../normalize.css
Note: You need the FULL path of the CSS stylesheet (from the root directory).
Especially useful if:
● Trying to remove default module
styling
● Trying to remove unnecessary
system stylesheets
● Trying to remove a base theme
stylesheet

8. 4. Libraries
Probably the most notable change in Drupal 8.
Define library:
A collection of CSS stylesheets and JS scripts
Can be used globally, or just as needed.
Library Naming (NOT file naming):
THEME_NAME/LIB_NAME
dcu/global-styling

9. dcu.libraries.yml - Overview
global-styling: #LIBRARY_NAME
version: 1.x
css:
theme:
styles/css/style.css {}
styles/css/print.css { media: print }
js:
js/script.js {}
header:
js/header.js {}
dependencies:
core/jquery
sessions: #LIBRARY_NAME
...
(Yes, jQuery is now included in Core!)

10. dcu.libraries.yml - CSS
D8 dcu.info.yml:
global-styling:
version: 1.x
css:
theme:
styles/css/style.css {}
styles/css/print.css { media: print }
D7 dcu.info:
stylesheets[all][] = css/global.styles.css
stylesheets[all][] = css/responsive.styles.css
stylesheets[all][] = css/print.css

11. dcu.libraries.yml - JS
D8 dcu.info.yml:
global-styling:
...
# By default, all JS is loaded in the footer
js:
js/script.js: {}
# But it can be overridden!
js-header:
header: true
js:
js/header.js: {}
js-footer:
js:
js/footer.js: {}
D7 dcu.info:
scripts[] = js/script.js

12. Attaching a Library to a Page
Options for loading a library:
● Every page
● A subset of pages
● Individual templates
(Remember, THEME_NAME/LIBRARY_NAME is how you reference a library)
To load a library on every page, place in your THEME_NAME.info.yml file.
To load a library on a subset of pages:
<?php
function dcu_preprocess_maintenance_page(&$variables) {
$variables['#attached']['library'][] = dcu/sessions';
}
?>
To load on a Twig template:
{{ attach_library('dcu/sessions') }}

13. You can override a library and remove a library with the commands
library-override
library-remove
Another note about libraries...

14. 5. Don’t mess up your YML.
If you are developing a theme and you get a WSOD or a 500 error, it’s because you have messed up
YML code.
Use spaces, not tabs
Don’t forget your colons (also forget your semi-colons)
Use colons not equal signs
Follow indentation

15. 6. The Themer’s Best Friend
Twig debugging!
To turn on, edit settings.php and services.yml
sites/default/settings.php
$settings['twig_debug'] = TRUE;
$settings['container_yamls'][] = __DIR__ . '/services.yml';
sites/default/services.yml
parameters:
twig.config:
debug: true
Thanks to Derek Walker (@derekawalker) for helping me out with this.
Also Devel - although a note: the
dpm() function is now kint()

16. Twig Debug, Turned OFF

17. Twig Debug, Turned ON
<!-- THEME DEBUG -->
<!-- THEME HOOK: ‘page’ →
<!-- FILE NAME SUGGESTIONS:
x page--page.html.twig
* page--front.html.twig
* page.html.twig
-->
<!-- BEGIN OUTPUT FROM
‘themes/dcu/templates/page--page.html.twig’ -->
(HTML HERE)

18. Functions now done in THEME_NAME.theme!
Some functions are different though.
For example, to add template suggestions to a page, use
hook_theme_suggestions_HOOK_alter().
Preprocess functions, like in D7, are used to pass variables to the page (Twig) templates.
7. Preprocess Functions are still a thing

19. Example Functions in dcu.theme
/**
* Implements hook_preprocess_HOOK().
*/
function dcu_preprocess_page(&$variables) {
$variables[‘title’] = ‘Come to the dark side.’;
$variables[‘page_class’] = ‘page-dark-side’;
kint();
}
/**
* Implements hook_theme_suggestions_HOOK_alter().
*/
function dcu_theme_suggestions_page_alter(array &$suggestions, array $variables) {
if ($node = Drupal::routeMatch()->getParameter('node')) {
$suggestions[] = 'page__' . $node->getType();
}
return $suggestions;
}

20. 8. Templates are now done in Twig (not PHP)
<section id="content-outer-wrapper" class="outer-wrapper clearfix">
<div id="main-layout" class="{{ region_classes.main }}
inner-wrapper clearfix">
{% if page.content %}
{{ page.content }}
{% endif %}
{% if page.sidebar_first %}
{{ page.sidebar_first }}
{% endif %}
{% if page.sidebar_second %}
{{ page.sidebar_second }}
{% endif %}
</div><!-- /#main -->
</section><!-- /#main-outer-wrapper -->
<div class="l-main-wrapper">
<div class="l-main">
<div class="l-content" role="main">
<a id="main-content"></a>
<?php print render($tabs); ?>
<?php print render($page['help']); ?>
<?php if ($action_links): ?>
<ul class="action-links">
<?php print render($action_links); ?>
</ul>
<?php endif; ?>
<?php print render($page['content']); ?>
</div>
<?php print render($page['sidebar_first']); ?>
<?php print render($page['sidebar_second']); ?>
</div>
</div>

21. Twig Reference & PHP Equivalent
{{ twig_variable }}
{% if twig_variable %}
{% elseif twig_variable %}
{% else %}
{% endif %}
{% for i in 0..10 %}
{{ i }}
{% endfor %}
echo $php_variable;
if($boolean)
else if($boolean)
else
for($i = 0; $i < 10; $i++) {
echo $i;
}
Twig reference to come soon…

22. Twig Variables & Preprocess Functions
<body class=”{{ page_class }} main”>
<h1>{{ title }} It’s awesome.</h1>
...
</body>
<body class=”page_dark_side main”>
<h1>Come to the dark side. It’s
awesome.</h1>
...
</body>

23. Twig Loops and PHP Arrays
<?php
function dcu_preprocess_page(&$variables) {
$variables[‘sessions’] = array(
‘auditorium’ => array(
‘title’ => ‘Drupal 8 Theming’,
‘time => ‘11:15 AM’,
),
‘room_b’ => array(
‘title’ => ‘Drulenium’,
‘time’ => ‘11:15 AM’,
),
);
}
?>
{% for session in sessions %}
{{ session.title }} at {{ session.time }}
{% endfor %}
Helpful note: {{ variable_name.key }} for array
elements

24. Drupal 8 Theming Fundamentals, Part 1
https://www.lullabot.com/articles/drupal-8-theming-fundamentals-part-1
Drupal 8 Assets Theming Guide
https://www.drupal.org/theme-guide/8/assets
Classy Theming by Morten DK
https://drupalwatchdog.com/volume-5/issue-1/classy-theming
My Drupal 8 Omega Subtheme “Resistor” (as reference material)
https://github.com/loganfarr/Resistor
Barcelona DrupalCon - Drupal 8 Theming
https://events.drupal.org/barcelona2015/sessions/drupal-8-theming-0
MortenDK’s Los Angeles DrupalCon Session - Drupal 8 Theming with <3
https://events.drupal.org/losangeles2015/sessions/drupal-8-theming
Twig Reference (Coming Soon)
https://loganfarr.com/twig
This presentation + extended examples (Also coming soon)
https://loganfarr.com/d8-theming
Additional Reading Material

25. Now go build a
Drupal 8 theme.