Support core #autocomplete_path

Actually we can intercept the AJAX request via the /js.js file. There's already a beforeSend callback that is invoked via a global AJAX event.

We could detect if #autocomplete_path is set (should just be set to js, IMO) and then set an additional attribute data-autocomplete="true" or whatever. In the beforeSend callback, we can check if (options.$trigger.is('[data-autocomplete]')). If it has that attribute, then we can change the method of the AJAX request to POST (a GET JS module request fully bootstraps Drupal) and add in the data-js-module, data-js-callback and data-js-token values to the data being sent, which thus invokes a JS module AJAX request.

I'm not entirely sure if what I said above is 100% accurate, but I think it's more or less in the right ballpark of what needs to be accomplished.

Ok, I will have to take a look then. I may be missing something... but it really shouldn't need these kind of verbose conditions as there are only two that really need to be checked: if (element.is('[data-js-module][data-autocomplete]')) as we would only add these attributes if both #js_callback and #autocomplete_path are set on the form element (then we'd know it's ours to mess with).

Just realized my suggestion in #1 about simply setting the #autocomplete_path to just js won't work. This then requires a module to explicitly define this one as a dependency.

In order to keep this module optional, we'd also likely need to lookup the menu item associated with the #autocomplete_path at some point in the request to extract the components like access callback/arguments if set. Not entirely sure how that might work yet, but just making a note for when I actually get around to looking at this in more detail.

I'm not yet entirely sure what the best way to handle this is TBH.

That being said, it also includes the patch from this related issue which essentially nullifies the purpose of a token, so definitely setting back to CNW.

I've actually got this working quite well with the patch I'm about to commit.

Replace all MODULENAME instances with the machine name of the module implementing this support:

/**
 * Implements hook_menu().
 */
function MODULENAME_menu() {
  // Provide a non JS Callback fallback menu entry.
  $items['my_module/autocomplete'] = array(
    'title' => 'My Module Autocomplete',
    'page callback' => 'MODULE_js_callback_autocomplete',
    'access callback' => 'user_access',
    'access arguments' => array('access content'),
    'type' => MENU_CALLBACK,
    // Most autocompletes invoke drupal_json_output() in their callback.
    // However, to get this working with both core and JS callback, the
    // return value from the callback needs to be the same for both.
    'delivery callback' => 'drupal_json_output',
  );
  return $items;
}

/**
 * Implements hook_js_info().
 */
function MODULENAME_js_info() {
  $info['autocomplete'] = array(
    // Optional, but highly recommended since tokens are disabled.
    'access callback' => 'user_access',
    'access arguments' => array('access content'),
    'bootstrap' => DRUPAL_BOOTSTRAP_SESSION,

    // Bypass the JS Callback module's normal delivery callback. This is
    // important so it works with what core autocomplete.js expects.
    'delivery callback' => 'drupal_json_output',

    // Add dependencies (depends what is used to generate the autocomplete).
    'dependencies' => array('user'),

    // Core autocomplete.js uses GET, not POST.
    'methods' => array('GET'),

    // Disable token checks since this is just a simple DB query
    // and it will likely be needed for anonymous users too.
    'token' => FALSE,

  );
  return $info;
}

/**
 * Implements MODULE_js_callback_CALLBACK().
 */
function MODULENAME_js_callback_autocomplete() {
  $json = array();

  // Core autocomplete.js appends the query string to the end of the
  // menu hook entry path above. So, in this case, the query string
  // should be the last argument, 2:
  // $_GET['q'] = array(
  //   0 => 'my_module',
  //   1 => 'autocomplete',
  //   2 => 'abc' // <-- query string
  // );
  if ($query = arg(2)) {
    // Do your querying code here with the query string the user provided.
    // The following example is from user_autocomplete().
    $result = db_select('users')->fields('users', array('name'))->condition('name', db_like($query) . '%', 'LIKE')->range(0, 10)->execute();
    foreach ($result as $user) {
      $json[$user->name] = check_plain($user->name);
    }
  }

  // Just return the JSON array, do not print it out.
  return $json;
}

Then, in some form somewhere, add the following to your textfield:

$form['text'] = array(
  '#type' => 'textfield',

  // Core fallback path.
  '#autocomplete_path' => 'my_module/autocomplete',

  // Enabled JS Callback support.
  '#js_callback' => array('MODULENAME' => 'autocomplete'),
);