Implementing a generateAjax method, which should work similar to the default generate method and is compatible with the Contao Ajax-Tools.

gistfile1.txt

<?php

/**
 * Ajaxify package for Contao CMS
 *
 * Developed by http://codeq.at
 *
 * @package   ajaxify
 * @author    codeQ e.U.
 * @license   GPL
 * @copyright codeQ e.U. 2013
 */


/**
 * Namespace
 */
namespace CodeQ;


/**
 * Class ModuleRegistrationAjax
 *
 * This class extends the registration module to support json requests.
 *
 * Currently this class does not support Captchas.
 * 
 * @copyright  codeQ e.U. 2013
 * @author     codeQ e.U.
 * @package    ajaxify
 */
class ModuleRegistrationAjax extends \ModuleRegistration
{
	/**
	 * Display a wildcard in the back end
	 * @return string
	 */
	public function generateAjax()
	{
		if (TL_MODE == 'BE')
		{
			header('HTTP/1.1 412 Precondition Failed');
			return 'This ajax method is only for the frontend.';
		}

		if ($this->Input->get('pageId') == null)
		{
			header('HTTP/1.1 412 Precondition Failed');
			return 'The registration module requires a pageId to determine the contect on registration.'; // PageModel jumpTo is looked up
		}

		// like normal generate, deserialize editable
		$this->editable = deserialize($this->editable);

		// Return if there are no editable fields
		if (!is_array($this->editable) || empty($this->editable))
		{
			header('HTTP/1.1 412 Precondition Failed');
			return 'This module does not have any editable fields.';
		}

		// taken from parent::generate
		$this->Template = new \FrontendTemplate($this->strTemplate);
		$this->Template->setData($this->arrData);

		$this->compileAjax();

		// instead of parsing the template now we collect the data and 
		// create an response array
		return array(
			'headline' => $this->arrData['headline'],
			'editable' => $this->arrData['editable'],
			'enctype' => $this->Template->enctype,
			'hasErrors' => $this->Template->hasError,
			'fields'  => $this->Template->fields
		);
	}

	/**
	 * Generate the module data, this is very much the same as the original compile
	 */
	protected function compileAjax()
	{
		global $objPage;

		$GLOBALS['TL_LANGUAGE'] = $objPage->language;

		\System::loadLanguageFile('tl_member');
		$this->loadDataContainer('tl_member');

		// Call onload_callback (e.g. to check permissions)
		if (is_array($GLOBALS['TL_DCA']['tl_member']['config']['onload_callback']))
		{
			foreach ($GLOBALS['TL_DCA']['tl_member']['config']['onload_callback'] as $callback)
			{
				if (is_array($callback))
				{
					$this->import($callback[0]);
					$this->$callback[0]->$callback[1]();
				}
				elseif (is_callable($callback))
				{
					$callback();
				}
			}
		}

		// Activate account
		if (\Input::get('token') != '')
		{
			$this->activateAcount();
			return;
		}

		$this->Template->fields = array(); // <---------- changed for Ajax
		$objCaptcha = null;
		$doNotSubmit = false;

		// Captcha
		if (!$this->disableCaptcha)
		{
			header('HTTP/1.1 412 Precondition Failed');
			return 'The ModuleRegistration::compileAjax does currently not support captchas.';
		}

		$arrUser = array();
		$arrFields = array();
		$hasUpload = false;
		$i = 0;

		// Build form
		foreach ($this->editable as $field)
		{
			$arrData = $GLOBALS['TL_DCA']['tl_member']['fields'][$field];

			// Map checkboxWizard to regular checkbox widget
			if ($arrData['inputType'] == 'checkboxWizard')
			{
				$arrData['inputType'] = 'checkbox';
			}

			$strClass = $GLOBALS['TL_FFL'][$arrData['inputType']];

			// Continue if the class is not defined
			if (!class_exists($strClass))
			{
				continue;
			}

			$arrData['eval']['tableless'] = $this->tableless;
			$arrData['eval']['required'] = $arrData['eval']['mandatory'];

			$objWidget = new $strClass($strClass::getAttributesFromDca($arrData, $field, $arrData['default'], '', '', $this));

			$objWidget->storeValues = true;
			$objWidget->rowClass = 'row_' . $i . (($i == 0) ? ' row_first' : '') . ((($i % 2) == 0) ? ' even' : ' odd');

			// Increase the row count if its a password field
			if ($objWidget instanceof \FormPassword)
			{
				$objWidget->rowClassConfirm = 'row_' . ++$i . ((($i % 2) == 0) ? ' even' : ' odd');
			}

			// Validate input
			if (\Input::post('FORM_SUBMIT') == 'tl_registration')
			{
				$objWidget->validate();
				$varValue = $objWidget->value;

				// Check whether the password matches the username
				if ($objWidget instanceof \FormPassword && $varValue == \Input::post('username'))
				{
					$objWidget->addError($GLOBALS['TL_LANG']['ERR']['passwordName']);
				}

				$rgxp = $arrData['eval']['rgxp'];

				// Convert date formats into timestamps (check the eval setting first -> #3063)
				if (($rgxp == 'date' || $rgxp == 'time' || $rgxp == 'datim') && $varValue != '')
				{
					try
					{
						$objDate = new \Date($varValue);
						$varValue = $objDate->tstamp;
					}
					catch (\OutOfBoundsException $e)
					{
						$objWidget->addError(sprintf($GLOBALS['TL_LANG']['ERR']['invalidDate'], $varValue));
					}
				}

				// Make sure that unique fields are unique (check the eval setting first -> #3063)
				if ($arrData['eval']['unique'] && $varValue != '' && !$this->Database->isUniqueValue('tl_member', $field, $varValue))
				{
					$objWidget->addError(sprintf($GLOBALS['TL_LANG']['ERR']['unique'], $arrData['label'][0] ?: $field));
				}

				// Save callback
				if ($objWidget->submitInput() && !$objWidget->hasErrors() && is_array($arrData['save_callback']))
				{
					foreach ($arrData['save_callback'] as $callback)
					{
						try
						{
							if (is_array($callback))
							{
								$this->import($callback[0]);
								$varValue = $this->$callback[0]->$callback[1]($varValue, null);
							}
							elseif (is_callable($callback))
							{
								$varValue = $callback($varValue, null);
							}
						}
						catch (\Exception $e)
						{
							$objWidget->class = 'error';
							$objWidget->addError($e->getMessage());
						}
					}
				}

				// Store the current value
				if ($objWidget->hasErrors())
				{
					$doNotSubmit = true;
				}
				elseif ($objWidget->submitInput())
				{
					$arrUser[$field] = $varValue;
				}
			}

			if ($objWidget instanceof \uploadable)
			{
				$hasUpload = true;
			}

			// following lines changed for Ajax
			//$this->Tempalte->fields .= $objWidget->parse();
			
			// because Template->fields is a magic function, array_push doesn't work
			$arr = $this->Template->fields;
			array_push($arr, array(
				'name' => $objWidget->name,
				'label' => $objWidget->label,
				'value' => $objWidget->value,
				'template' => $objWidget->template,
				'wizard' => $objWidget->wizard,
				'required' => $objWidget->required,
				'errors' => $objWidget->getErrors()
			));
			$this->Template->fields = $arr;

			$arrFields[$arrData['eval']['feGroup']][$field] .= $objWidget->parse(); // <---------- changed for Ajax

			++$i;
		}

		// !!!!!!!
		// Captcha part removed, not supported in Ajax

		$this->Template->rowLast = 'row_' . ++$i . ((($i % 2) == 0) ? ' even' : ' odd');
		$this->Template->enctype = $hasUpload ? 'multipart/form-data' : 'application/x-www-form-urlencoded';
		$this->Template->hasError = $doNotSubmit;

		// Create new user if there are no errors
		if (\Input::post('FORM_SUBMIT') == 'tl_registration' && !$doNotSubmit)
		{
			$this->createNewUser($arrUser);
		}

		$this->Template->loginDetails = $GLOBALS['TL_LANG']['tl_member']['loginDetails'];
		$this->Template->addressDetails = $GLOBALS['TL_LANG']['tl_member']['addressDetails'];
		$this->Template->contactDetails = $GLOBALS['TL_LANG']['tl_member']['contactDetails'];
		$this->Template->personalData = $GLOBALS['TL_LANG']['tl_member']['personalData'];
		$this->Template->captchaDetails = $GLOBALS['TL_LANG']['MSC']['securityQuestion'];

		// Add groups
		foreach ($arrFields as $k=>$v)
		{
			$this->Template->$k = $v;
		}

		$this->Template->captcha = $arrFields['captcha'];
		$this->Template->formId = 'tl_registration';
		$this->Template->slabel = specialchars($GLOBALS['TL_LANG']['MSC']['register']);
		$this->Template->action = \Environment::get('indexFreeRequest');

		// HOOK: add memberlist fields
		if (in_array('memberlist', \ModuleLoader::getActive()))
		{
			$this->Template->profile = $arrFields['profile'];
			$this->Template->profileDetails = $GLOBALS['TL_LANG']['tl_member']['profileDetails'];
		}

		// HOOK: add newsletter fields
		if (in_array('newsletter', \ModuleLoader::getActive()))
		{
			$this->Template->newsletter = $arrFields['newsletter'];
			$this->Template->newsletterDetails = $GLOBALS['TL_LANG']['tl_member']['newsletterDetails'];
		}

		// HOOK: add helpdesk fields
		if (in_array('helpdesk', \ModuleLoader::getActive()))
		{
			$this->Template->helpdesk = $arrFields['helpdesk'];
			$this->Template->helpdeskDetails = $GLOBALS['TL_LANG']['tl_member']['helpdeskDetails'];
		}
	}
}