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(),
        ]);
    }

}