Drupal 7 Module Development

Chapter 3
Here's an example that should make this concept a bit clearer. This is an actual code
snippet from Drupal's block preprocess function. In each page region, all of the
blocks in the region get a variable whose value alternates between "odd" and "even".
These values can be used to create zebra-striped styling, that is, alternate styling on
every other block in a region.
function template_preprocess_block(&$variables) {
// We store all block counters using drupal_static().
$block_counter = &drupal_static(__FUNCTION__, array());
// All blocks get an independent counter for each region.
if (!isset($block_counter[$variables['block']->region])) {
$block_counter[$variables['block']->region] = 1;
}
// Generate the zebra striping variable.
$variables['block_zebra'] = ($block_counter[$variables['block']-
>region] % 2) ? 'odd' : 'even';
}
// Increment the region's block count.
$block_counter[$variables['block']->region]++;
The PHP logic in this function is directly related to the display of the block and
not to the general business logic of this data. So, it doesn't make sense that the
block module would calculate that meta data before calling theme(); the meta data
clearly belongs to the display logic, which is why it's placed in the block module's
preprocess function.
Multi-hook preprocess functions
In some rare circumstances, you may need to alter or provide some variables for
all theme hooks. In fact, Drupal's theme system does provide some variables to all
templates; the preprocess function that provides these variables is both a "template_"
preprocess function and a multi-hook preprocess function. Multi-hook preprocess
functions are simply functions that don't have a _HOOK suffix added to their name
and are run for every single template file. Their name is of the following form:
[module]_preprocess(&$variables, $hook)
Obviously, there can be a big performance hit if a module needlessly implements
a multi-hook preprocess function. If you're contemplating writing one, if at all
possible, consider writing several preprocess functions that target the specific
hooks you need instead, rather then hit all hooks.
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