The manager getForm function

Manager.php

<?php
 /**
  * Passthrough from Drupal form to the correct condition for building the preview form
  *
  * @param $form
   *   The form array used in hook_form
   * @param $form_state
   *   The form_state array as used in hook_form
   *
   * @return
   *   A drupal form array created but the root condition
   */
  public function getPreviewForm() {
    $root_condition = $this->getRootCondition();
    $getForm = function($form, &$form_state) {
      return $root_condition->getElement($form, $form_state);
    }
    drupal_get_form('sps_condition_preview_form', $getForm);
  }

sps.module

<?php
/**
 * A form callback for use by drupal_get_form().
 *
 * When called like:
 * drupal_get_form('sps_condition_preview_form', array($this, 'getPreviewForm'))
 * by the manager or a condition, it will return that form.
 *
 * @return
 *  A FAPI array for drupal_get_form if a callback is provided, or NULL
 *
 * @see Manager::getPreviewForm();
 */
function sps_condition_preview_form($form, &$form_state) {
  if (!empty($form_state['build_info']['args'][0])) {
    $form_function = $form_state['build_info']['args'][0];
    if(is_callable($form_function)) {
      $form_function($form, $form_state);
    }
  }
}