Dividing Drupal's process and preprocess functions into separate files

If you use a lot of process and preprocess functions within your Drupal theme, then your template.php can get very long and it can become difficult to find a certain piece of code. Following the example of the Omega theme, I've started separating my process and preprocess functions into their own files. For example, mytheme_preprocess_node can be placed within a preprocess/node.inc file, and mytheme_process_page can be placed within process/page.inc.

The first step is to use the default mytheme_process() and mytheme_preprocess() functions to utilise my custom function. So within my template.php file:

<?php

/**
* Implements hook_preprocess().
*
* Initialises the mytheme_invoke() function for the preprocess hook.
*/
function mytheme_preprocess(&$variables, $hook) {
mytheme_invoke('preprocess', $hook, $variables);
}

/**
* Implements hook_process().
*
* Initialises the mytheme_invoke() function for the process hook.
*/
function mytheme_process(&$variables, $hook) {
mytheme_invoke('process', $hook, $variables);
}

Now, to write the mytheme_invoke() function:

<?php

/**
* Invokes custom process and preprocess functions.
*
* @param string $type
* The type of function we are trying to include (i.e. process or preprocess).
*
* @param array $variables
* The variables array.
*
* @param string $hook
* The name of the hook.
*
* @see mytheme_preprocess()
* @see mytheme_process()
*/
function mytheme_invoke($type, $hook, &$variables) {
global $theme_key;

// The name of the function to look for (e.g. mytheme_process_node).
$function = $theme_key . '_' . $type . '_' . $hook;

// If the function doesn't exist within template.php, look for the
// appropriate include file.
if (!function_exists($function)) {
// The file to search for (e.g. process/node.inc).
$file = drupal_get_path('theme', $theme_key) . '/' . $type . '/' . $type . '-' . str_replace('_', '-', $hook) . '.inc';

// If the file exists, include it.
if (is_file($file)) {
include($file);
}
}

// Try to call the function again.
if (function_exists($function)) {
$function($variables);
}
}

As mytheme_invoke() checks to see if the function already exists before searching for checking the include files, I could still add the functions into template.php as normal and this would override any corresponding include file.