How to call preprocess functions from my custom module

Question

I have created a custom module that declares a custom image formatter. Now I need to call the Core image module preprocess function 'template_preprocess_image_formatter'. How would I do that?

I tried to add it in the list of preprocess functions of my custom theme but it doesn't fire unless I clear the cache:

 /**
  * Implements hook_theme().
  */
 function foo_theme() {
    'image_formatter_foo' => [
          'variables' => ['item' => NULL, 'item_attributes' => NULL, 'image_style' => NULL],
          'file' => 'foo.field.inc',
          'preprocess functions' => [
             'template_preprocess_image',
             'template_preprocess_image_formatter_foo'
          ]
    ],
 }

Answer 1

Looking at template_preprocess_image_formatter, I would try the following code below.

Replace mymodule with the machine name of the module. This assume foo is the machine name of your image formatter.

mymodule_preprocess_image(&$variables) {
  /* processing logic */
}

mymodule_preprocess_image_formatter_foo(&$variables) {
  /* processing logic */
}

I also recommend reviewing Drupal 8's Core Image module as it has some of the files that call the hooks you need.

1. image.field.inc contains template_preprocess_image_formatter
2. template_preprocess_image