class AddExpressionForm

class AddExpressionForm extends EditExpressionForm {
    
    /**
     * The Rules expression manager to get expression plugins.
     *
     * @var \Drupal\rules\Engine\ExpressionManagerInterface
     */
    protected $expressionManager;
    
    /**
     * The expression ID that is added, example: 'rules_action'.
     *
     * @var string
     */
    protected $expressionId;
    
    /**
     * Creates a new object of this class.
     */
    public function __construct(ExpressionManagerInterface $expression_manager) {
        $this->expressionManager = $expression_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('plugin.manager.rules_expression'));
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildForm(array $form, FormStateInterface $form_state, RulesUiHandlerInterface $rules_ui_handler = NULL, $uuid = NULL, $expression_id = NULL) {
        $this->expressionId = $expression_id;
        $this->uuid = $uuid;
        // When initially adding the expression, we have to initialize the object
        // and add the expression.
        if (!$this->uuid) {
            // Before we add our edited expression to the component's expression,
            // we clone it such that we do not change the source component until
            // the form has been successfully submitted.
            $component = clone $rules_ui_handler->getComponent();
            $this->uuid = $this->getEditedExpression($component)
                ->getUuid();
            $form_state->set('component', $component);
            $form_state->set('uuid', $this->uuid);
        }
        return parent::buildForm($form, $form_state, $rules_ui_handler, $this->uuid);
    }
    
    /**
     * {@inheritdoc}
     */
    protected function getEditedExpression(RulesComponent $component) {
        $component_expression = $component->getExpression();
        if (!$component_expression instanceof ExpressionContainerInterface) {
            throw new LogicException('Cannot add expression to expression of type ' . $component_expression->getPluginId());
        }
        if ($this->uuid && ($expression = $component_expression->getExpression($this->uuid))) {
            return $expression;
        }
        else {
            $expression = $this->expressionManager
                ->createInstance($this->expressionId);
            $rule_expression = $component->getExpression();
            $rule_expression->addExpressionObject($expression);
            return $expression;
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public function submitForm(array &$form, FormStateInterface $form_state) {
        parent::submitForm($form, $form_state);
        $form_state->setRedirectUrl($this->rulesUiHandler
            ->getBaseRouteUrl());
    }
    
    /**
     * Provides the page title on the form.
     */
    public function getTitle(RulesUiHandlerInterface $rules_ui_handler, $expression_id) {
        $this->expressionId = $expression_id;
        $expression = $this->expressionManager
            ->createInstance($this->expressionId);
        return $this->t('Add @expression', [
            '@expression' => $expression->getLabel(),
        ]);
    }

}