Bootstap从v3.x版本迁移到v4.x版本时，发现dropdown组件的divider类不起作用了，下拉菜单中无法显示分割线。

查询官网手册后发现dropdown组件有如下变化：

• Switched from parent selectors to singular classes for all components, modifiers, etc.
• Simplified dropdown styles to no longer ship with upward or downward facing arrows attached to the dropdown menu.
• Dropdowns can be built with <div>s or <ul>s now.
• Rebuilt dropdown styles and markup to provide easy, built-in support for <a> and <button> based dropdown items.
• Renamed .divider to .dropdown-divider.
• Dropdown items now require .dropdown-item.
• Dropdown toggles no longer require an explicit <span class="caret"></span>; this is now provided automatically via CSS’s ::after on .dropdown-toggle.

其中可以发现，原有的"divider"类已经被重命名为"dropdown-divider"了。

用dropdown-divider替换掉divider之后，下拉菜单的分割线正常使用。