Drupal 7 Module Development

Theming a Module
After all, if you choose an existing theme hook, you only have to build the data.
You don't have to worry about building the implementation, registering it with
hook_theme, building any helper pre-render functions, deciding on the best HTML
to use, and building any supporting CSS and jQuery. What's not to like about
skipping extra work?
However, laziness isn't the only reason you'll want to reuse existing theme hooks.
Not only is it the easy way out, you'll also discover interesting integrations with
other modules. As we learned in Chapter 3, Drupal's Theme Layer, modules can alter
the way a theme implementation works by using preprocess/process functions.
Those alterations aren't linked to only Drupal core's use of the theme hook; they are
run even when your module uses the theme hook.
For example, when writing the menu_block module, I could have used my own
theme hooks to theme the various parts of the menu trees that it displays. However,
I decided to reuse core's menu tree-related theme hooks and just pump the data
with lots of extra meta-data that core's usage lacked. I later discovered that another
module was designed to modify core's menu trees and make them expand and
collapse dynamically. That module did this by altering core's theme hooks. Since
my menu_block module used the same hooks, the two modules were instantly
interoperable. Neither module developer had to write any integration code. Score!
The hardest part to reusing existing theme hooks is simply finding the right one
for you. You can browse all of Drupal core's default theme implementations at:
http://api.drupal.org/api/group/themeable/7
Any hook implemented as a theme function is listed first. The template files are listed
next, under the Files section.
In Drupal 7, there are 184 theme hooks that you could use in your own code.
Many of those theme hooks are specific to a core module's usage, like theming an
administration form, but it's still useful to go through the entire list to find hooks
you could reuse. To make it even easier, we've included a list of the most commonly
reused theme hooks:
Common theme hooks
file_link Returns HTML for a link to a file.
html_tag Returns HTML for a generic HTML tag with attributes. This can often be
too generic a theme hook to use, but is really useful for adding a tag to
the of a document or for theming a tag inside a render element.
image
Returns HTML for an image.
image_style Returns HTML for an image using a specific image style.
item_list Returns HTML for a list of items which can optionally be nested.
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