Drupal 7 Module Development

Chapter 3
Theme registry
So all of the wonderful things that theme() does, take a considerable amount of
work. It's extremely inefficient for theme() to determine all of this information
about theme hooks on the fly each time it's called. So to help its load and improve
performance, Drupal uses a theme registry.
The theme registry holds a list of all the theme hooks known by the system, and for
each theme hook, it also stores the following:
• Whether it's a theme function or a template
• The list of preprocess functions to run
• The list of process functions to run
• The path to the template file (which includes whether the original module
template file is used or a theme version of it.)
• The theme function name (which indicates if it's the original module theme
function or one overridden by a theme.)
• The list of variables that should be passed to theme() when this theme hook
is called and a default value for each variable
• Whether, instead of a list of variables, the theme function expects a
single render element as its parameter, and what that render element
should be named
While this theme registry does improve performance for the website user, it does
cause some inconvenience for module and theme developers. Since Drupal is caching
data about theme hooks, if you are actively writing or altering a theme hook, you'll
need to make sure that you rebuild the theme registry before testing your changes.
Fortunately, this can easily by accomplished by clicking the Clear all caches button
on the Performance page found in the Configuration admin (admin/config/
development/performance). The devel module also has a handy Rebuild the
theme registry on every page load option in its settings.
Variable default values
Earlier when we talked about theme functions and the single $variables parameter,
you may have noticed one short-coming of that parameter. When you have a normal
list of parameters in a function definition, you specify the default values for parameters
that are optional. This allows code to only list a few parameters when calling the
function and have the additional parameters just get sensible default values.
[ 85 ]