class EditExpressionForm

class EditExpressionForm extends FormBase {
  
  /**
   * The edited component.
   *
   * @var \Drupal\rules\Engine\RulesComponent
   */
  protected $component;
  
  /**
   * The RulesUI handler of the currently active UI.
   *
   * @var \Drupal\rules\Ui\RulesUiHandlerInterface
   */
  protected $rulesUiHandler;
  
  /**
   * The UUID of the edited expression in the rule.
   *
   * @var string
   */
  protected $uuid;
  
  /**
   * Gets the currently edited expression from the given component.
   *
   * @param \Drupal\rules\Engine\RulesComponent $component
   *   The component from which to get the expression.
   *
   * @return \Drupal\rules\Engine\ExpressionInterface|null
   *   The expression object.
   */
  protected function getEditedExpression(RulesComponent $component) {
    $rule_expression = $component->getExpression();
    return $rule_expression->getExpression($this->uuid);
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state, RulesUiHandlerInterface $rules_ui_handler = NULL, $uuid = NULL) {
    $this->rulesUiHandler = $rules_ui_handler;
    $this->component = is_object($form_state->get('component')) ? $form_state->get('component') : $this->rulesUiHandler
      ->getComponent();
    $this->uuid = $form_state->get('uuid') ?: $uuid;
    // During form rebuilds, keep track of changes using form state.
    $form_state->set('rules_ui_handler', $this->rulesUiHandler);
    $form_state->set('component', $this->component);
    $form_state->set('uuid', $this->uuid);
    $expression = $this->getEditedExpression($this->component);
    if (!$expression) {
      throw new NotFoundHttpException();
    }
    $form_handler = $expression->getFormHandler();
    $form = $form_handler->form($form, $form_state);
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'rules_expression_edit';
  }
  
  /**
   * Builds an updated component object based upon the submitted form values.
   *
   * @param array $form
   *   An associative array containing the structure of the form.
   * @param \Drupal\Core\Form\FormStateInterface $form_state
   *   The current state of the form.
   *
   * @return \Drupal\rules\Engine\RulesComponent
   *   The updated component.
   */
  protected function buildComponent(array $form, FormStateInterface $form_state) {
    $component = clone $this->component;
    // In order to update the whole component we need to invoke the submission
    // handler of the expression form. That way the expression gets changed
    // accordingly.
    $expression = $this->getEditedExpression($component);
    $form_handler = $expression->getFormHandler();
    $form_handler->submitForm($form, $form_state);
    return $component;
  }
  
  /**
   * {@inheritdoc}
   */
  public function validateForm(array &$form, FormStateInterface $form_state) {
    // Ensure the object properties are initialized, see
    // https://www.drupal.org/node/2669032.
    $this->rulesUiHandler = $form_state->get('rules_ui_handler');
    $this->component = is_object($form_state->get('component')) ? $form_state->get('component') : $this->rulesUiHandler
      ->getComponent();
    $this->uuid = $form_state->get('uuid');
    $this->rulesUiHandler
      ->validateLock($form, $form_state);
    // @todo This ignores ExpressionFormInterface::validateForm().
    $component = $this->buildComponent($form, $form_state);
    $violations = $component->checkIntegrity();
    // Only show the violations caused by the edited expression.
    foreach ($violations->getFor($this->uuid) as $violation) {
      $form_state->setError($form, $violation->getMessage());
    }
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    $this->component = $this->buildComponent($form, $form_state);
    $this->rulesUiHandler
      ->updateComponent($this->component);
    $form_state->setRedirectUrl($this->rulesUiHandler
      ->getBaseRouteUrl());
  }
  
  /**
   * Provides the page title on the form.
   */
  public function getTitle(RulesUiHandlerInterface $rules_ui_handler, $uuid) {
    $this->uuid = $uuid;
    $expression = $this->getEditedExpression($rules_ui_handler->getComponent());
    return $this->t('Edit @expression', [
      '@expression' => $expression->getLabel(),
    ]);
  }

}