I've replaced:
print drupal_to_js($matches);
exit();

with:
drupal_json($matches);

with good results.

Reference: http://drupal.org/update/modules/6/7#rename-drupal-to-js

This is driving me nuts. I've tried to make a custom autocomplete function as described above. Did not work. Then I copy and pasted the whole code (of course I put it into the appropriate functions). I created a table 'cities' with several entries for 'city'. Did not work.

When I use user/autocomplete, it's working fine. If I change it to cities/autocomplete then I don't get the small autocomplete circle or anything.

It simply won't work.

Am I stupid or is there something wrong with the code? Please help, I'm slowly despairing =/

I was hitting a wall about autocomplete and how I couldn't get mine to work.
Then, I found the small glitch.

This didn't work:

$form['account_sync_fields_map']['sync_'.$map_id]['remote'] = array(
	  '#type' => 'textfield',
          '#default_value' => $details['remote'],
          '#autocomplete_path' => '/admin/account_sync/autocomplete',
          '#maxlength' => 1024,
        );

This did:

$form['account_sync_fields_map']['sync_'.$map_id]['remote'] = array(
	  '#type' => 'textfield',
          '#default_value' => $details['remote'],
          '#autocomplete_path' => 'admin/account_sync/autocomplete',
          '#maxlength' => 1024,
        );

The difference is that the path had to not have a leading slash.

For Drupal 7 code is something like this.
where field_tags is taxonomy field in node with widget type autocomplete.

   $node=node_load($nid);
    $tags = array();
    foreach ($node->field_tags['und'] as $item) {
      $tags[$item['tid']] = isset($item['taxonomy_term']) ? $item['taxonomy_term'] : taxonomy_term_load($item['tid']);
    }
    $form['tags'] = array(
      '#type' => 'textfield',
      '#default_value' => taxonomy_implode_tags($tags),
      '#title' => 'Add Tags',
      '#autocomplete_path' => 'taxonomy/autocomplete/field_tags',
      '#maxlength' => 1024,
      '#element_validate' => array('taxonomy_autocomplete_validate'),page
        );

Do not start your Path with search as in "search/yourlistpath". This will not work because it triggers a Search. Obvious only to those who know ;)

I was seeing this error in firebug:

uncaught exception: Syntax error, unrecognized expression: #

I also saw a notice in Drupal referencing line 3707 of forms.inc. This is in Drupal 7.12.

If I add an id to the city field in hook_form, the errors go away...

<?php
  $form['city'] = array(
    '#id' => 'something-unique',
    '#type' => 'textfield',
    '#title' => 'City',
    '#maxlength' => 128,
    '#autocomplete_path' => 'cities/autocomplete',
  );
?>

Thanks!
Your code for autocomplete textfield worked perfectly, but how to implement that in a block ?
A block is published in various paths...

Best regards.

I added a field radio with two options: "user" or "title".
When this field is selected, the autocomplete should display its content.
How to do this?
Thanks

I am a recent convert from DotNetNuke to Drupal. So I would really appreciate some more help with this feature.

I have 2 questions -

1. $form['city'] = array(... Does this portion add a text box? I already have a text box in my code for the "Content". How can I adapt this to that textbox (input)?

2. Do the addition to hook_menu and the callback function go in system.api.php? Or should I be adding it somewhere else?

Thank you for your replies
Sai

How can the autocomplete form be modified in order to be bring the information from a remote server (e.g. accessing it via a URL) doing like a REST request, instead of getting the information from a local database? Any pointers would be really appreciated. Thank you!

Nice documentation. Saved my time.

MMMM is drupal 8 core auto complete broken or am i just wrong...

code

some_module.autocomplete:
  path: '/some-module/autocomplete'
  defaults:
    _controller: '\Drupal\some_module\Form\SomeThingForm::someCallbackFunctionName'
  requirements:
    _permission: 'access content'

in form:

 $form['some_stuff'] = array(
      '#type' => 'textfield',
      '#title' => 'enter some text ',
      '#prefix' => '<div id="div_me_up">',
      '#suffix' => '</div>',
      '#autocomplete_route_name' => 'some_module.autocomplete',
      '#autocomplete_route_parameters' => array(),
    );
// later on 
/**
   * This is a quick function for address search auto compete call back.
   *
   * @param \Symfony\Component\HttpFoundation\Request $request
   *   The current request object containing the search string.
   *
   * @return \Symfony\Component\HttpFoundation\JsonResponse
   *   A JSON response containing the autocomplete suggestions.
   */
  public static function someCallbackFunctionName(Request $request) {
    $string = $request->query->get('q');
    $matches = array('value' => $string .' (1)', 'label' => $string);
    //"value":"this (1)","label":"this" is what other working fields return 
    return new JsonResponse($matches);
  }

this gives me a strange error JSON gives correct value but
JS says ....
TypeError: suggestions.indexOf is not a function
var index = suggestions.indexOf(tagged[i]);

this is correct because suggestions is an object ..... what is a fella to do now :(

For the record, today is Christmas! And thanks to all the truth seeker before me! It's been hell of hours to have had a blade in my teeth to try to figure out wt..

"Uncaught TypeError: Cannot read property 'form' of undefined" which caused by autocomplete used in a custom form which try to render itself, then the #id must be there plus the form['xxx'] must be the same as the #id

$form['edit-keyword'] = array(
'#id' => 'edit-keyword',
'#type' => 'textfield',
'#default_value' => '',
'#autocomplete_path' => 'n-search/autocomplete',
);

Right after adding the #id, it's a merry Xmas all of a sudden!