遵循教程，效果很好。我只是不明白update方法是如何从Select标记在数据库中保存$instance数组的？您可以在管理区域中选择默认或完全、平滑保存，并将其保存在db中。

   public function widget( $args, $instance ) {
      echo $args[\'before_widget\'];

          echo \'<div class="g-ytsubscribe"       
              data-layout="\'.$instance[\'data_layout\'].\'" 
              data-count="default">
            </div>\';
      
      echo $args[\'after_widget\'];
    }

    public function form( $instance ) {
      $data_layout = ! empty( $instance[\'data_layout\'] ) ? $instance[\'data_layout\'] : esc_html__( \'default\', \'yts_domain\' );
      ?>
      
      
      <select class="widefat" 
        <option value="default" <?php echo ($data_layout == \'default\' ? \'selected\' : \'\'); ?>>Default</option>
        <option value="full" <?php echo ($data_layout == \'full\' ? \'selected\' : \'\'); ?>>Full</option>
      </select>
      
      }
      
    public function update( $new_instance, $old_instance ) {
      $instance = array();

      $instance[\'data_layout\'] = ( ! empty( $new_instance[\'data_layout\'] ) ) ? sanitize_text_field( $new_instance[\'data_layout\'] ) : \'\';

      return $instance;
    }