bootstrap之dropdown

一、前言

这节介绍下dropdown(下拉菜单)模块的源码实现。

二、源码

1、dropdowns.less

 // // Dropdown menus(下拉菜单) // -------------------------------------------------- // Dropdown arrow/caret(下拉菜单箭头) .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: 下拉菜单容器) .dropup, .dropdown {   position: relative; } // Prevent the focus on the dropdown toggle when closing dropdowns .dropdown-toggle:focus {   outline: 0; } // The dropdown menu (ul: 下拉菜单) .dropdown-menu {   position: absolute;   top: 100%;   left: 0;   z-index: @zindex-dropdown;   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;   font-size: @font-size-base;   text-align: left; // Ensures proper alignment if parent has it changed (e.g., modal footer)   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(下拉菜单右对齐)   //   // Deprecated as of 3.1.0 in favor of `.dropdown-menu-[dir]`   &.pull-right {     right: 0;     left: auto;   }   // Dividers (basically an hr) within the dropdown(下拉分割线)   .divider {     .nav-divider(@dropdown-divider-bg);   }   // Links within the dropdown menu(下拉菜单链接)   > li > a {     display: block;     padding: 3px 20px;     clear: both;     font-weight: normal;     line-height: @line-height-base;     color: @dropdown-link-color;     white-space: nowrap; // prevent links from randomly breaking onto new lines   } } // Hover/Focus state(菜单项状态: 悬浮状态、焦点状态) .dropdown-menu > li > a {   &:hover,   &:focus {     text-decoration: none;     color: @dropdown-link-hover-color;     background-color: @dropdown-link-hover-bg;   } } // Active state(当前状态) .dropdown-menu > .active > a {   &,   &:hover,   &:focus {     color: @dropdown-link-active-color;     text-decoration: none;     outline: 0;     background-color: @dropdown-link-active-bg;   } } // Disabled state(禁用状态) // // Gray out text and ensure the hover/focus state remains gray .dropdown-menu > .disabled > a {   &,   &:hover,   &:focus {     color: @dropdown-link-disabled-color;   }   // Nuke hover/focus effects   &:hover,   &:focus {     text-decoration: none;     background-color: transparent;     background-image: none; // Remove CSS gradient     .reset-filter();     cursor: @cursor-disabled;   } } // Open state for the dropdown(展开下拉菜单) .open {   // Show the menu   > .dropdown-menu {     display: block;   }   // Remove the outline when :focus is triggered   > a {     outline: 0;   } } // Menu positioning(下拉菜单对齐方式) // // Add extra class to `.dropdown-menu` to flip the alignment of the dropdown // menu with the parent.(右对齐) .dropdown-menu-right {   left: auto; // Reset the default from `.dropdown-menu`   right: 0; } // With v3, we enabled auto-flipping if you have a dropdown within a right // aligned nav component. To enable the undoing of that, we provide an override // to restore the default dropdown menu alignment. // // This is only for left-aligning a dropdown menu within a `.navbar-right` or // `.pull-right` nav component.(左对齐) .dropdown-menu-left {   left: 0;   right: auto; } // Dropdown section headers(下拉菜单标题) .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 } // Backdrop to catch body clicks on mobile, etc.(背景遮罩层) .dropdown-backdrop {   position: fixed;   left: 0;   right: 0;   bottom: 0;   top: 0;   z-index: (@zindex-dropdown - 10); } // Right aligned dropdowns(右对齐下拉菜单) .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: @caret-width-base dashed;     border-bottom: @caret-width-base solid ~"\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();     }   } }

2、dropdown.js

 /* ========================================================================  * Bootstrap: dropdown.js v3.3.7(下拉菜单)  * 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) {     $(element).on('click.bs.dropdown', this.toggle)   }   Dropdown.VERSION = '3.3.7'   // 获取触发按钮或链接的目标元素   function getParent($this) {     var selector = $this.attr('data-target')     // 如果不存在data-target，那么获取href属性并将格式处理为CSS选择器     if (!selector) {       selector = $this.attr('href')       selector = selector && /#[A-Za-z]/.test(selector) && selector.replace(/.*(?=#[^\s]*$)/, '') // strip for ie7     }     // 如果存在目标元素，那么直接选中     var $parent = selector && $(selector)     // 否则默认使用触发元素的父元素     return $parent && $parent.length ? $parent : $this.parent()   }   // 隐藏下拉菜单   function clearMenus(e) {     // 如果鼠标右键点击不做任何处理     if (e && e.which === 3) return     // 移除背景     $(backdrop).remove()     $(toggle).each(function () {       var $this         = $(this)       var $parent       = getParent($this)       var relatedTarget = { relatedTarget: this }       // 如果不是下拉菜单已经展开菜单了，那么不做任何处理       if (!$parent.hasClass('open')) return       // 如果事件发生在目标元素中的input/textarea元素中，那么不做任何处理       if (e && e.type == 'click' && /input|textarea/i.test(e.target.tagName) && $.contains($parent[0], e.target)) return       // 触发开始隐藏菜单事件       $parent.trigger(e = $.Event('hide.bs.dropdown', relatedTarget))       if (e.isDefaultPrevented()) return       $this.attr('aria-expanded', 'false')       // 触发已经隐藏菜单事件       $parent.removeClass('open').trigger($.Event('hidden.bs.dropdown', relatedTarget))     })   }   // 切换下拉菜单显示状态   Dropdown.prototype.toggle = function (e) {     var $this = $(this)     if ($this.is('.disabled, :disabled')) return     var $parent  = getParent($this)     var isActive = $parent.hasClass('open')     clearMenus()     // 显示下拉菜单     if (!isActive) {       // 如果是在移动设备上，那么点击后添加遮罩层       if ('ontouchstart' in document.documentElement && !$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))     }     // 返回false，阻止冒泡和默认事件发生；如果是用JS直接触发，而且JS绑定的元素是document的子元素就可以避免绑定触发多次的问题，因为这里返回了false     return false   }   // 键盘事件处理函数   Dropdown.prototype.keydown = function (e) {     // 38:Up Arrow; 40:Down Arrow; 27:Esc; 32:Spacebar     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) {       // 如果菜单处于展开状态而且点击esc键盘按钮，那么让触发元素获取焦点       if (e.which == 27) $parent.find(toggle).trigger('focus')       // 触发元素上触发点击事件       return $this.trigger('click')     }     var desc = ' li:not(.disabled):visible a'     var $items = $parent.find('.dropdown-menu' + desc)     // 如果$items.length == false，说明拓展菜单隐藏，不执行下面操作；否则执行下拉菜单选择，选中菜单触发focus事件     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)     // 如果点击发生在document上，那么隐藏拓展开的菜单     .on('click.bs.dropdown.data-api', clearMenus)     // 如果点击事件发生在下拉菜单元素上，那么阻止冒泡到document上     .on('click.bs.dropdown.data-api', '.dropdown form', function (e) { e.stopPropagation() })     // 如果点击在触发元素上，触发Dropdown.prototype.toggle事件     .on('click.bs.dropdown.data-api', toggle, Dropdown.prototype.toggle)     // 在触发元素上设置键盘监听函数     .on('keydown.bs.dropdown.data-api', toggle, Dropdown.prototype.keydown)     // 如果是展开状态(只有展开状态才有.dropdown-menu)，那么则在展开菜单上也设置监听函数     .on('keydown.bs.dropdown.data-api', '.dropdown-menu', Dropdown.prototype.keydown) }(jQuery);

三、应用 & 源码分析

1、应用

 <nav class="navbar navbar-default" role="navigation">     <div class="container-fluid">      <div class="navbar-header">         <a class="navbar-brand" href="#">菜鸟教程</a>     </div>     <div>         <ul class="nav navbar-nav">             <li class="active"><a href="#">iOS</a></li>             <li><a href="#">SVN</a></li>             <li class="dropdown">                 <a href="#" class="dropdown-toggle" data-toggle="dropdown">                     Java                      <b class="caret"></b>                 </a>                 <ul class="dropdown-menu pull-right">                     <li role="presentation" class="dropdown-header">下拉菜单标题</li>                     <li><a href="#">jmeter</a></li>                     <li><a href="#">EJB</a></li>                     <li><a href="#">Jasper Report</a></li>                     <li class="divider"></li>                     <li><a href="#">分离的链接</a></li>                     <li class="divider"></li>                     <li><a href="#">另一个分离的链接</a></li>                 </ul>             </li>         </ul>     </div>     </div> </nav>

2、源码分析

2.1、dropdowns.less

1、可通过向 .dropdown-menu 添加 class .pull-right 或 .dropdown-menu-right 来向右对齐下拉菜单，默认左对齐
2、可使用 class dropdown-header 向下拉菜单的标签区域添加标题
3、可使用 class disabled 向下拉菜单的标签区域设置禁用项
4、可使用 class divider 向下拉菜单的标签区域设置分割线

2.2、dropdown.js

1、如果点击事件发生在 document ，那么执行 clearMenus 隐藏下拉框
2、监听属性 data-toggle=”dropdown” 的点击事件，触发下拉菜单显示隐藏切换，并返回false，阻止事件冒泡到document上
3、在 data-toggle=”dropdown” 和 class dropdown-menu 元素上监听键盘事件，执行相应操作