class RulesComponentFormBase

abstract class RulesComponentFormBase extends EntityForm {
    
    /**
     * The Rules expression manager to get expression plugins.
     *
     * @var \Drupal\rules\Engine\ExpressionManagerInterface
     */
    protected $expressionManager;
    
    /**
     * {@inheritdoc}
     */
    public static function create(ContainerInterface $container) {
        return new static($container->get('plugin.manager.rules_expression'));
    }
    
    /**
     * Creates a new object of this class.
     *
     * @param \Drupal\rules\Engine\ExpressionManagerInterface $expression_manager
     *   The expression manager.
     */
    public function __construct(ExpressionManagerInterface $expression_manager) {
        $this->expressionManager = $expression_manager;
    }
    
    /**
     * {@inheritdoc}
     */
    public function form(array $form, FormStateInterface $form_state) {
        $form['#entity_builders'][] = '::entityTagsBuilder';
        $form['settings'] = [
            '#type' => 'details',
            '#title' => $this->t('Settings'),
            '#open' => $this->entity
                ->isNew(),
        ];
        $form['settings']['label'] = [
            '#type' => 'textfield',
            '#description' => $this->t('Enter a name to be used to identify your component in the administrative interface.'),
            '#title' => $this->t('Label'),
            '#default_value' => $this->entity
                ->label(),
            '#required' => TRUE,
        ];
        $form['settings']['id'] = [
            '#type' => 'machine_name',
            '#description' => $this->t('A unique machine-readable name for your component. Can only contain lowercase letters, numbers, and underscores.'),
            '#disabled' => !$this->entity
                ->isNew(),
            '#default_value' => $this->entity
                ->id(),
            '#machine_name' => [
                'exists' => [
                    $this,
                    'exists',
                ],
                'replace_pattern' => '([^a-z0-9_]+)|(^custom$)',
                'source' => [
                    'settings',
                    'label',
                ],
                'error' => $this->t('The machine-readable name must be unique, and can only contain lowercase letters, numbers, and underscores. Additionally, it can not be the reserved word "custom".'),
            ],
        ];
        // @todo Enter a real tag field here.
        $form['settings']['keywords'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Tags'),
            '#default_value' => implode(', ', $this->entity
                ->getTags()),
            '#description' => $this->t('Enter a list of comma-separated keywords here; e.g., "notification, publishing". Tags are keywords used for filtering available components in the administration interface.'),
            '#required' => FALSE,
        ];
        $form['settings']['description'] = [
            '#type' => 'textarea',
            '#default_value' => $this->entity
                ->getDescription(),
            '#description' => $this->t('Enter a description for this component, to help document what this component is intended to do.'),
            '#title' => $this->t('Description'),
        ];
        return parent::form($form, $form_state);
    }
    
    /**
     * Callback method for the #entity_builder form property.
     *
     * Used to change format of tags from comma-separated values (as input)
     * into an array (as stored in the the configuration entity).
     *
     * @param string $entity_type
     *   The type of the entity.
     * @param \Drupal\Core\Config\Entity\ConfigEntityInterface $entity
     *   The entity whose form is being built.
     * @param array $form
     *   An associative array containing the structure of the form.
     * @param \Drupal\Core\Form\FormStateInterface $form_state
     *   The current state of the form.
     */
    public function entityTagsBuilder($entity_type, ConfigEntityInterface $entity, array $form, FormStateInterface $form_state) {
        $tags = [];
        $input_tags = $form_state->getValue('keywords');
        if (trim($input_tags) != '') {
            $tags = array_map('trim', explode(',', $input_tags));
        }
        $entity->set('tags', $tags);
    }
    
    /**
     * Machine name exists callback.
     *
     * @param string $id
     *   The machine name ID.
     *
     * @return bool
     *   TRUE if an entity with the same name already exists, FALSE otherwise.
     */
    public function exists($id) {
        $type = $this->entity
            ->getEntityTypeId();
        return (bool) $this->entityTypeManager
            ->getStorage($type)
            ->load($id);
    }

}