code: function setExtraStyle(el) {
    const $el = el;
    const $currStyle = el.getAttribute('style') || '';
    const extraStyle = $el.dragConfig.style;
    const extraClass = $el.dragConfig.class;
    // 如果指定了额外的样式
    if (extraStyle) {
      $el.setAttribute('style', `${$currStyle}${extraStyle}`);
    }
    // 如果指定了额外的 css 类
    if (extraClass) {
      $el.classList.add(extraClass);
    }
  }
docs: 额外样式和类