$fields->addFieldsToTab('Root.Content.Main', array(
new BalbusAutocompleteField(
$name = 'MyField',
$title = 'Enter a value'
)
));

user_error("breakpoint:", E_USER_ERROR);

function setSourceClass($classname) {
      SS_Log::log(new Exception('Autocomplete: setSourceClass('.$className.')'), SS_Log::NOTICE);
$this->sourceClass = $classname;
}

/*
* set the sourcefield
*/
function setSourceField($fieldname) {
      SS_Log::log(new Exception('Autocomplete: setSourceField('.$fieldName.')'), SS_Log::NOTICE);
$this->sourceField = $fieldname;
}

/*
* Override the existing TextField method and add
* the jQuery requirements and the new field ettributes:
* src - url called by ajax to retrieve the suggestions
* autocomplete - flag setting autocompletion 'on' or 'off'
*/
function Field() {
      SS_Log::log(new Exception('Autocomplete: Field()'), SS_Log::NOTICE);
// Note: sometimes loading new JavaScripts in the CMS
// will not work. Looks like the assets/_CombinedFiles
// are the culprits (in that case just remove them)

// Get jQuery from SilverStripe thirdparty
Requirements::javascript(
THIRDPARTY_DIR . '/jquery/jquery.js'
);

// Get the UI from Google (the SilverStripe way)
Requirements::javascript(
'http://ajax.googleapis.com/ajax/libs/jqueryui'.
'/1.8.1/jquery-ui.min.js'
);
Requirements::css(
'http://ajax.googleapis.com/ajax/libs/jqueryui'.
'/1.8.1/themes/base/jquery-ui.css'
);

// NEW: Since inline JavaScript isn't loaded by jQueryOnDemand,
// an external file is needed to set things in motion:
Requirements::javascript(
'balbusessentials/javascript/BalbusAutocompleteField.js'
);

// altered 'text' class to 'text AutocompleteField'
$attributes = array(
'type' => 'text',
'class' => 'text AutocompleteField ' . ($this->extraClass()? $this->extraClass(): ''),
'id' => $this->id(),
'name' => $this->Name(),
'value' => $this->Value(),
'tabindex' => $this->getTabIndex(),
'maxlength' => ($this->maxLength) ? $this->maxLength : null,
'size' => ($this->maxLength) ? min( $this->maxLength, 30 ) : null
);

if($this->disabled) $attributes['disabled'] = 'disabled';

// NEW: add een attribute to enable autocompletion
$attributes['autocomplete'] = 'on';

// NEW: add a sourceClass/sourcefield, if there is one
$querystring = http_build_query(array(
'sclass' => $this->sourceClass,
'sfield' => $this->sourceField
), '', '&');

// NEW: add the current link with the Action=suggest
// parameter added as a new src parameter
$attributes['src'] =
parse_url($this->Link(),PHP_URL_PATH) . '/suggest?' . $querystring;

// Create the tag
      SS_Log::log(new Exception('Autocomplete: creating Tag: '.http_build_query($attributes)), SS_Log::NOTICE);
return $this->createTag('input', $attributes);
}

      function suggest() {
         SS_Log::log(new Exception('Autocomplete: suggest()'), SS_Log::NOTICE);
         $items = array(
         array('value' => 'London'),
         array('value' => 'Paris'),
         array('value' => 'Rome')
         );
         SS_Log::log(new Exception('Autocomplete: suggesting: '.implode($items)), SS_Log::NOTICE);
         return json_encode($items);
      }

alert("autocomplete running");
try{
   BalbusAutocompleteField = Class.create();
}catch(exception){
   alert("class create failed: "+exception);
}
try{
   BalbusAutocompleteField.applyTo('input.AutocompleteField');
}catch(exception){
   alert("class apply failed: "+exception);
}
BalbusAutocompleteField.prototype = {
   
   // initialize Autocomplete fields
   initialize: function() {
// only initialize once
if (!this.strSource){
         
// get source string src attribute
this.strSource = jQuery(this).attr('src');
         alert("autocomplete source found: "+this.strSource);
         
// remove src attribute from the textfield
jQuery(this).attr('src', '');
         try{
   // jQuery Autocomplete
   jQuery(this).autocomplete({
    source: this.strSource,
    minLength: 2
   });
         }catch(exception){
            alert("autocomplete failed: "+exception);
         }
         alert("autocomplete end");
} else {
         alert("source not found");
      }
}
};

   public static $db = array(
      'Test' => 'Varchar'
   );

   public static $has_one = array(
   );

   function getCMSFields() {
      $fields = parent::getCMSFields();

      $fields->addFieldsToTab('Root.Content.Autocomplete', array(
      new BalbusAutocompleteField (
      $name = 'Test',
      $title = 'Autocomplete testfield'
      )   
      ));

      return $fields;
   }