bootstrap-dropdown组件是个烂东西，我读后的整体感觉。
一个下拉开菜单的设计：
<ul class="nav pull-right">
<li id="fat-menu" class="dropdown">
<a href="#" class="dropdown-toggle" data-toggle="dropdown">下拉 <b class="caret"></b></a>
<ul class="dropdown-menu">
<li><a href="#">动作</a></li>
<li><a href="#">另一个动作</a></li>
<li><a href="#">其他</a></li>
<li class="divider"></li>
<li><a href="#">另一个链接</a></li>
</ul>
</li>
</ul>
首先下拉开菜单其中就是当中第二层的UL元素，类名为dropdown-menu。下拉菜单通常处于隐藏状态。我们可以在它的父元素上加个open类名，让它显示出来。它的父元素通常带个dropdown类名，其实是提供了一个相对定位的包含块。当然除了dropdown外，你还可以选择用dropup。dropdown是让下拉菜单向下显示，dropup是向上展示。要打开下拉菜单，通常我们要点击某处让它显示出来。我们称之为触发器。这个触发器带一个data-toggle=dropdown的自定义属性。通过前面的学习，你们应该隐约察觉到，data-toggle在bootstrap有特殊意义，是让目标对象表现某一类控件，什么data-toggle=button,data-toggle=buttons-checked,data-toggle=radio……不过它还有专门的类名dropdown-toggle。触发器可以通过data-target自定义属性指定目标下拉框打开，也可以通过href属性打开。不过bootstrap的下拉框有严重的排它性，一个页面只能打开一个下拉框，也无法通过套嵌组建多级下拉框。
下面是它的源码，我们发现它是支持键盘切换的，但写得很烂，乃至多数情况下失败。
!function ($) {
"use strict"; // jshint ;_;
/* DROPDOWN CLASS DEFINITION
* ========================= */
var toggle = '[data-toggle=dropdown]'
, Dropdown = function (element) {
var $el = $(element).on('click.dropdown.data-api', this.toggle)
$('html').on('click.dropdown.data-api', function () {
$el.parent().removeClass('open')
})
}
Dropdown.prototype = {
constructor: Dropdown ,
toggle: function (e) {
var $this = $(this)
, $parent
, isActive
if ($this.is('.disabled, :disabled')) return
//分别通过以下三个途径：
//1 data-target自定义属性
//2 href的属性中的hash（也是ID选择器），
//3这个按钮的直属父节点
$parent = getParent($this)
isActive = $parent.hasClass('open')
//每次页面只能有一个菜单被打开，这是个失败的设计
clearMenus()
//如果没有打开，则打开它
if (!isActive) {
$parent.toggleClass('open')
}
//让当前菜单项获得焦点
$this.focus()
return false
} ,
keydown: function (e) {
var $this
, $items
, $parent
, isActive
, index
//如果不是上下方向键或回车键,返回
if (!/(38|40|27)/.test(e.keyCode)) return
$this = $(this)
e.preventDefault()
e.stopPropagation()
//如果标识为禁止状态
if ($this.is('.disabled, :disabled')) return
//取得控件的容器
$parent = getParent($this)
//如果处于激活状态
isActive = $parent.hasClass('open')
//如果没有激活或激话了+回车,就触发其点击事件
if (!isActive || (isActive && e.keyCode == 27)) return $this.click()
//如果是菜单项(不能是作为分隔线的LI)下的A元素
$items = $('[role=menu] li:not(.divider):visible a', $parent)
if (!$items.length) return
//那么我们取得当前获得焦点的元素作为基准,通过它上下移动
index = $items.index($items.filter(':focus'))
if (e.keyCode == 38 && index > 0) index-- // up
if (e.keyCode == 40 && index
以下是它对应的less。
//
// Dropdown menus
// --------------------------------------------------
// Use the .menu class on any
.dropup,
.dropdown {
position: relative;
}
.dropdown-toggle {
// The caret makes the toggle a bit too tall in IE7
*margin-bottom: -3px;
}
.dropdown-toggle:active,
.open .dropdown-toggle {
outline: 0;
}
// Dropdown arrow/caret
// --------------------
.caret {
display: inline-block;
width: 0;
height: 0;
vertical-align: top;
border-top: 4px solid @black;
border-right: 4px solid transparent;
border-left: 4px solid transparent;
content: "";
}
// Place the caret
.dropdown .caret {
margin-top: 8px;
margin-left: 2px;
}
// The dropdown menu (ul)
// ----------------------
.dropdown-menu {
position: absolute;
top: 100%;
left: 0;
z-index: @zindexDropdown;
display: none; // none by default, but block on "open" of the menu
float: left;
min-width: 160px;
padding: 5px 0;
margin: 2px 0 0; // override default ul
list-style: none;
background-color: @dropdownBackground;
border: 1px solid #ccc; // Fallback for IE7-8
border: 1px solid @dropdownBorder;
*border-right-width: 2px;
*border-bottom-width: 2px;
.border-radius(6px);
.box-shadow(0 5px 10px rgba(0,0,0,.2));
-webkit-background-clip: padding-box;
-moz-background-clip: padding;
background-clip: padding-box;
// Aligns the dropdown menu to right
&.pull-right {
right: 0;
left: auto;
}
// Dividers (basically an hr) within the dropdown
.divider {
.nav-divider(@dropdownDividerTop, @dropdownDividerBottom);
}
// Links within the dropdown menu
li > a {
display: block;
padding: 3px 20px;
clear: both;
font-weight: normal;
line-height: @baseLineHeight;
color: @dropdownLinkColor;
white-space: nowrap;
}
}
// Hover state
// -----------
.dropdown-menu li > a:hover,
.dropdown-menu li > a:focus,
.dropdown-submenu:hover > a {
text-decoration: none;
color: @dropdownLinkColorHover;
#gradient > .vertical(@dropdownLinkBackgroundHover, darken(@dropdownLinkBackgroundHover, 5%));
}
// Active state
// ------------
.dropdown-menu .active > a,
.dropdown-menu .active > a:hover {
color: @dropdownLinkColorActive;
text-decoration: none;
outline: 0;
#gradient > .vertical(@dropdownLinkBackgroundActive, darken(@dropdownLinkBackgroundActive, 5%));
}
// Disabled state
// --------------
// Gray out text and ensure the hover state remains gray
.dropdown-menu .disabled > a,
.dropdown-menu .disabled > a:hover {
color: @grayLight;
}
// Nuke hover effects
.dropdown-menu .disabled > a:hover {
text-decoration: none;
background-color: transparent;
background-image: none; // Remove CSS gradient
.reset-filter();
cursor: default;
}
// Open state for the dropdown
// ---------------------------
.open {
// IE7's z-index only goes to the nearest positioned ancestor, which would
// make the menu appear below buttons that appeared later on the page
*z-index: @zindexDropdown;
& > .dropdown-menu {
display: block;
}
}
// 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?
.dropup,
.navbar-fixed-bottom .dropdown {
// Reverse the caret
.caret {
border-top: 0;
border-bottom: 4px solid @black;
content: "";
}
// Different positioning for bottom up menu
.dropdown-menu {
top: auto;
bottom: 100%;
margin-bottom: 1px;
}
}
// Sub menus
// ---------------------------
.dropdown-submenu {
position: relative;
}
// Default dropdowns
.dropdown-submenu > .dropdown-menu {
top: 0;
left: 100%;
margin-top: -6px;
margin-left: -1px;
.border-radius(0 6px 6px 6px);
}
.dropdown-submenu:hover > .dropdown-menu {
display: block;
}
// Dropups
.dropup .dropdown-submenu > .dropdown-menu {
top: auto;
bottom: 0;
margin-top: 0;
margin-bottom: -2px;
.border-radius(5px 5px 5px 0);
}
// Caret to indicate there is a submenu
.dropdown-submenu > a:after {
display: block;
content: " ";
float: right;
width: 0;
height: 0;
border-color: transparent;
border-style: solid;
border-width: 5px 0 5px 5px;
border-left-color: darken(@dropdownBackground, 20%);
margin-top: 5px;
margin-right: -10px;
}
.dropdown-submenu:hover > a:after {
border-left-color: @dropdownLinkColorHover;
}
// Left aligned submenus
.dropdown-submenu.pull-left {
// Undo the float
// Yes, this is awkward since .pull-left adds a float, but it sticks to our conventions elsewhere.
float: none;
// Positioning the submenu
> .dropdown-menu {
left: -100%;
margin-left: 10px;
.border-radius(6px 0 6px 6px);
}
}
// Tweak nav headers
// -----------------
// Increase padding from 15px to 20px on sides
.dropdown .dropdown-menu .nav-header {
padding-left: 20px;
padding-right: 20px;
}
// Typeahead
// ---------
.typeahead {
z-index: 1051;
margin-top: 2px; // give it some space to breathe
.border-radius(@baseBorderRadius);
}