<!-- 组件：下拉菜单 -->
<div class="dropdown">
 <!-- 触发按钮 -->
 <button class="btn btn-default dropdown-toggle" type="button" data-toggle="dropdown">
 Dropdown
 <span class="caret"></span>
 </button>
 <!-- 下拉菜单 -->
 <ul class="dropdown-menu">
 <li><a href="#">Action</a></li>
 <li><a href="#">Another action</a></li>
 <li><a href="#">Something else here</a></li>
 </ul>
</div>

// Dropdown arrow/caret
.caret {
 display: inline-block;
 width: 0;
 height: 0;
 margin-left: 2px;
 vertical-align: middle;
 border-top: @caret-width-base dashed;
 border-top: @caret-width-base solid ~"\9"; // IE8
 border-right: @caret-width-base solid transparent;
 border-left: @caret-width-base solid transparent;
}

// The dropdown wrapper (div)
.dropup,
.dropdown {
 position: relative; // 父元素相对定位
}

// Prevent the focus on the dropdown toggle when closing dropdowns
.dropdown-toggle:focus {
 outline: 0;
}

// The dropdown menu (ul)
.dropdown-menu {
 position: absolute; //子元素绝对定位
 top: 100%; // 下拉菜单紧贴父元素下边沿
 left: 0;
 z-index: @zindex-dropdown;
 display: none; //默认隐藏，当触发按钮显示(display:block)
 float: left;
 min-width: 160px;
 padding: 5px 0;
 margin: 2px 0 0; // override default ul
 list-style: none;
 font-size: @font-size-base;
 text-align: left; 
 background-color: @dropdown-bg;
 border: 1px solid @dropdown-fallback-border; // IE8 fallback
 border: 1px solid @dropdown-border;
 border-radius: @border-radius-base;
 .box-shadow(0 6px 12px rgba(0,0,0,.175));
 background-clip: padding-box;

 // Aligns the dropdown menu to right
 &.pull-right {
 right: 0;
 left: auto;
 }

 // 高度为1px的水平分隔线
 .divider {
 .nav-divider(@dropdown-divider-bg);
 }

 // Links within the dropdown menu
 > li > a {
 display: block;
 padding: 3px 20px;
 clear: both;
 font-weight: normal;
 line-height: @line-height-base;
 color: @dropdown-link-color;
 white-space: nowrap; // 防止链接换行
 }
}

// Hover/Focus state
.dropdown-menu > li > a {
 &:hover,
 &:focus {
 text-decoration: none;
 color: @dropdown-link-hover-color;
 background-color: @dropdown-link-hover-bg;
 }
}

// Active state
.dropdown-menu > .active > a {
 &,
 &:hover,
 &:focus {
 color: @dropdown-link-active-color;
 text-decoration: none;
 outline: 0;
 background-color: @dropdown-link-active-bg;
 }
}

// 显示下拉菜单
.open { 
 > .dropdown-menu {
 display: block; // 显示
 }

 // Remove the outline when :focus is triggered
 > a {
 outline: 0;
 }
}

// Menu positioning
.dropdown-menu-right {
 left: auto; // Reset the default from `.dropdown-menu`
 right: 0;
}
// `.pull-right` nav component.
.dropdown-menu-left {
 left: 0;
 right: auto;
}

// Dropdown section headers
.dropdown-header {
 display: block;
 padding: 3px 20px;
 font-size: @font-size-small;
 line-height: @line-height-base;
 color: @dropdown-header-color;
 white-space: nowrap; // as with > li > a
}

// 非下拉菜单区域
.dropdown-backdrop {
 position: fixed;
 left: 0;
 right: 0;
 bottom: 0;
 top: 0;
 z-index: (@zindex-dropdown - 10); //确保点击下拉菜单时，不会关闭下拉菜单
}

// Right aligned dropdowns
.pull-right > .dropdown-menu {
 right: 0;
 left: auto;
}

// Allow for dropdowns to go bottom up (aka, dropup-menu)
//
// Just add .dropup after the standard .dropdown class and you're set, bro.
// TODO: abstract this so that the navbar fixed styles are not placed here"\9"; // IE8
 content: "";
 }
 // Different positioning for bottom up menu
 .dropdown-menu {
 top: auto;
 bottom: 100%;
 margin-bottom: 2px;
 }
}


// Component alignment
//
// Reiterate per navbar.less and the modified component alignment there.

@media (min-width: @grid-float-breakpoint) {
 .navbar-right {
 .dropdown-menu {
  .dropdown-menu-right();
 }
 // Necessary for overrides of the default right aligned menu.
 // Will remove come v4 in all likelihood.
 .dropdown-menu-left {
  .dropdown-menu-left();
 }
 }
}

/* ========================================================================
 * Bootstrap: dropdown.js v3.3.6
 * http://getbootstrap.com/javascript/#dropdowns
 * ========================================================================
 * Copyright 2011-2016 Twitter, Inc.
 * Licensed under MIT (https://github.com/twbs/bootstrap/blob/master/LICENSE)
 * ======================================================================== */


+function ($) {
 'use strict';

 // DROPDOWN CLASS DEFINITION
 // =========================

 var backdrop = '.dropdown-backdrop'
 var toggle = '[data-toggle="dropdown"]'
 var Dropdown = function (element) {
 $(element).on('click.bs.dropdown', this.toggle)
 }

 Dropdown.VERSION = '3.3.6'

 function getParent($this) {
 var selector = $this.attr('data-target')

 if (!selector) {
  selector = $this.attr('href')
  selector = selector && /#[A-Za-z]/.test(selector) && selector.replace(/.*(?=#[^\s]*$)/, '') // strip for ie7
 }

 var $parent = selector && $(selector)

 return $parent && $parent.length ? $parent : $this.parent()
 }

 function clearMenus(e) {
 if (e && e.which === 3) return
 $(backdrop).remove()
 $(toggle).each(function () {
  var $this   = $(this)
  var $parent  = getParent($this)
  var relatedTarget = { relatedTarget: this }

  if (!$parent.hasClass('open')) return

  if (e && e.type == 'click' && /input|textarea/i.test(e.target.tagName) && $.contains($parent[0], e.target)) return

  $parent.trigger(e = $.Event('hide.bs.dropdown', relatedTarget))

  if (e.isDefaultPrevented()) return

  $this.attr('aria-expanded', 'false')
  $parent.removeClass('open').trigger($.Event('hidden.bs.dropdown', relatedTarget))
 })
 }

 Dropdown.prototype.toggle = function (e) {
 var $this = $(this)

 if ($this.is('.disabled, :disabled')) return

 var $parent = getParent($this)
 var isActive = $parent.hasClass('open')

 clearMenus()

 if (!isActive) {
  if ('ontouchstart' in document.documentElement && !$parent.closest('.navbar-nav').length) {
  // if mobile we use a backdrop because click events don't delegate
  $(document.createElement('div'))
   .addClass('dropdown-backdrop')
   .insertAfter($(this))
   .on('click', clearMenus)
  }

  var relatedTarget = { relatedTarget: this }
  $parent.trigger(e = $.Event('show.bs.dropdown', relatedTarget))

  if (e.isDefaultPrevented()) return

  $this
  .trigger('focus')
  .attr('aria-expanded', 'true')

  $parent
  .toggleClass('open')
  .trigger($.Event('shown.bs.dropdown', relatedTarget))
 }

 return false
 }

 Dropdown.prototype.keydown = function (e) {
 if (!/(38|40|27|32)/.test(e.which) || /input|textarea/i.test(e.target.tagName)) return

 var $this = $(this)

 e.preventDefault()
 e.stopPropagation()

 if ($this.is('.disabled, :disabled')) return

 var $parent = getParent($this)
 var isActive = $parent.hasClass('open')

 if (!isActive && e.which != 27 || isActive && e.which == 27) {
  if (e.which == 27) $parent.find(toggle).trigger('focus')
  return $this.trigger('click')
 }

 var desc = ' li:not(.disabled):visible a'
 var $items = $parent.find('.dropdown-menu' + desc)

 if (!$items.length) return

 var index = $items.index(e.target)

 if (e.which == 38 && index > 0)     index--   // up
 if (e.which == 40 && index < $items.length - 1) index++   // down
 if (!~index)         index = 0

 $items.eq(index).trigger('focus')
 }


 // DROPDOWN PLUGIN DEFINITION
 // ==========================

 function Plugin(option) {
 return this.each(function () {
  var $this = $(this)
  var data = $this.data('bs.dropdown')

  if (!data) $this.data('bs.dropdown', (data = new Dropdown(this)))
  if (typeof option == 'string') data[option].call($this)
 })
 }

 var old = $.fn.dropdown

 $.fn.dropdown    = Plugin
 $.fn.dropdown.Constructor = Dropdown


 // DROPDOWN NO CONFLICT
 // ====================

 $.fn.dropdown.noConflict = function () {
 $.fn.dropdown = old
 return this
 }


 // APPLY TO STANDARD DROPDOWN ELEMENTS
 // ===================================

 $(document)
 .on('click.bs.dropdown.data-api', clearMenus)
 .on('click.bs.dropdown.data-api', '.dropdown form', function (e) { e.stopPropagation() })
 .on('click.bs.dropdown.data-api', toggle, Dropdown.prototype.toggle)
 .on('keydown.bs.dropdown.data-api', toggle, Dropdown.prototype.keydown)
 .on('keydown.bs.dropdown.data-api', '.dropdown-menu', Dropdown.prototype.keydown)

}(jQuery);