hook library

gistfile1.txt

inside MYMODULE.info :

/**
 * Implements hook_library().
 */
function MYMODULE_library(){
   $libraries=array();
  // UI checkbox
  $libraries['ui.checkbox'] = array(
    'title' => 'jQuery UI: Checkbox',
    'website' => 'http://view.jqueryui.com/formcontrols/tests/visual/checkbox/checkbox.html',
    'version' => '1.8.7',
    'js' => array(
      '/misc/ui/jquery.ui.checkbox.js' => array(),
    ),
    'css' => array(
      '/misc/ui/jquery.ui.checkbox.css' => array(),
    ),
    'dependencies' => array(
      array('system', 'ui.widget'),
    )
  );
 return $libraries;
 }


function MYMODULE_preprocess_page(&$variables) { 
    drupal_add_library('system','ui.checkbox');
    dpm(drupal_get_library('system'));
} 
 
The array returned by dpm show that ui.checkbox is FALSE. Why ?