class Fixed

class Fixed extends ArgumentDefaultPluginBase implements CacheableDependencyInterface {
    
    /**
     * {@inheritdoc}
     */
    protected function defineOptions() {
        $options = parent::defineOptions();
        $options['argument'] = [
            'default' => '',
        ];
        return $options;
    }
    
    /**
     * {@inheritdoc}
     */
    public function buildOptionsForm(&$form, FormStateInterface $form_state) {
        parent::buildOptionsForm($form, $form_state);
        $form['argument'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Fixed value'),
            '#default_value' => $this->options['argument'],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getArgument() {
        return $this->options['argument'];
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheMaxAge() {
        return Cache::PERMANENT;
    }
    
    /**
     * {@inheritdoc}
     */
    public function getCacheContexts() {
        return [];
    }

}