BasicCondition.php

<?php
  /**
   * Implements ConditionInterface::getElement().
   *
   * Gets the form for the condition.
   *
   * @see sps_condition_form_validate_callback
   * @see sps_condition_form_submit_callback
   */
  public function getElement($element, &$form_state) {
    $sub_state = $form_state;
    $sub_state['values'] = isset($form_state['values']['widget']) ? $form_state['values']['widget'] : array();
    $element['widget'] = $this->widget->getPreviewForm(array(), $sub_state);
    $element['widget']['#tree'] = TRUE;

    $element['preview'] = array(
      '#type' => 'submit',
      '#value' => 'Preview',
    );

    $element['#sps_validate'] = array($this, 'validateElement');
    $element['#validate'] = 'sps_condition_form_validate_callback';

    $element['#sps_submit'] = array($this, 'submitElement');
    $element['#submit'] = 'sps_condition_form_submit_callback';

    return $element;
  }

sps.module

<?php


/**
 * A validate callback for forms which will subsequently call the forms
 * #sps_validate function.
 *
 * This allows us to call class methods as form validators.  To use #sps_validate,
 * should be set to a callable item, and #submit set to this function.
 */
function sps_condition_form_validate_callback($form, &$form_state) {
  if (!empty($form['#sps_validate']) && is_callable($form['#sps_validate'])) {
    $form['#sps_validate']($form, $form_state);
  }
}

/**
 * A submit callback for forms which will subsequently call the forms
 * #sps_submit function.
 *
 * This allows us to call class methods as form submitters.  To use #sps_submit,
 * should be set to a callable item, and #submit set to this function.
 */
function sps_condition_form_submit_callback($form, &$form_state) {
  if (!empty($form['#sps_submit']) && is_callable($form['#sps_submit'])) {
    $form['#sps_submit']($form, $form_state);
  }
}