How to split the code of a module into separated files

Question

I am very curious to know how a Drupal module can be dis-integrated into multiple include files. A number of hook support to link include components, like hook_menu, hook_theme etc.

Once I planned to simplify one of my complex module that have reached to 2.3K of lines with half of its feature set. I have to roll back all those steps due to lack of knowledge about the scope of inclusion.

Help me in this regard if there is some detailed information.

Answer 1

What Nikit said is correct.
I will add there are some hooks that are allowed to define which files need to be loaded. Examples of such hooks are hook_theme(), and hook_menu().

A module should never unconditionally load a file it needs calling module_load_include() from outside a function.

function book_menu() {
  $items['admin/content/book'] = array(
    'title' => 'Books',
    'description' => "Manage your site's book outlines.",
    'page callback' => 'book_admin_overview',
    'access arguments' => array('administer book outlines'),
    'file' => 'book.admin.inc',
  );
  $items['admin/content/book/list'] = array(
    'title' => 'List',
    'type' => MENU_DEFAULT_LOCAL_TASK,
  );
  $items['admin/content/book/settings'] = array(
    'title' => 'Settings',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('book_admin_settings'),
    'access arguments' => array('administer site configuration'),
    'type' => MENU_LOCAL_TASK,
    'weight' => 8,
    'file' => 'book.admin.inc',
  );
  // …
}

function user_theme() {
  return array(
    'user_picture' => array(
      'arguments' => array('account' => NULL),
      'template' => 'user-picture',
    ),
    'user_profile' => array(
      'arguments' => array('account' => NULL),
      'template' => 'user-profile',
      'file' => 'user.pages.inc',
    ),
    // …
    'user_admin_perm' => array(
      'arguments' => array('form' => NULL),
      'file' => 'user.admin.inc',
    ),
    // …
  );
}

Answer 2

Using more files, is just a matter of grouping similar things together in the same file, to keep it more managed. Typical files used is

• .admin.inc for all administration stuff, menu callbacks, forms etc.
• .pages.inc for frontend menu callbacks.
• .theme.inc for theme functions, preprocess hooks etc.
• .forms.inc for non admin forms and their handlers.

This is more a coding style that anything else. So this is just to help yourself maintain the code you have written.