jQuery自定义多选下拉框效果实现方法

<p id="demo" class="dropdown-container">
 <p class="dropdown-display">
 <span></span>
 <input type="text" class="iptdiplay" placeholder="请选择" readonly="readonly" />
 </p>
 <p class="dropdown-panel">
 <p class="dropdown-search">
 <span></span>
 <input type="text" class="iptsearch" placeholder="关键字搜索" />
 </p>
 <ul class="dd-select">
 <!-- area for dropdown items -->
 </ul>
 <p class="dropdown-opt">
 <button class="dd-btn ok">确定</button>
 <button class="dd-btn cancel">取消</button>
 </p>
 </p>
 </p>

.dropdown-container{
 display: block;
 width: 200px;
 height: 30px;
 padding: 0px;
 position: relative;
 margin: 0px auto;
}

.dropdown-display{
 display: block;
 height: 30px;
 position: absolute;
 top: 0;
 width:100%;
 margin: 0px;
 border:1px solid steelblue;
}
.dropdown-display span{
 background: url(../static/choose_down_icon_01.png) no-repeat left 8px;
 display: block;
 height: 25px;
 width: 10px;
 position: absolute;
 right: 5px;
 top: 3px;
}
.dropdown-display input[class='iptdiplay']{
 border: none;
 border-color: transparent;
 background: transparent;
 border-radius: 0px;
 box-shadow: none;
 height: 30px;
 width: 100%;
 margin: 0px;
 box-sizing: border-box;
 box-shadow: none;
 padding-left: 10px;
 padding-right: 18px;
 outline: none;
 cursor: pointer;
 text-overflow: ellipsis;
}
.dropdown-panel {
 position: absolute;
 top: 32px;
 width:100%;
 padding: 0px;
 display: none;
 border-left: 1px solid steelblue;
 border-bottom: 1px solid steelblue;
 border-right: 1px solid steelblue;
}

.dropdown-panel .dropdown-search{
 display: block;
 width: 100%;
 height: 30px;
}
.dropdown-search span{
 background: url(../static/chosen-sprite.png) no-repeat 100% -20px,linear-gradient(#eee 1%,#fff 15%);
 display: block;
 height: 25px;
 width: 20px;
 position: absolute;
 right: 0px;
 top: 3px;
}
.dropdown-search input[class='iptsearch']{
 border: none;
 border-color: transparent;
 background: transparent;
 border-radius: 0px;
 box-shadow: none;
 height: 30px;
 width: 100%;
 margin: 0px;
 box-sizing: border-box;
 box-shadow: none;
 padding-left: 10px;
 outline: none;
}
.dropdown-panel .dd-select{
 display: block;
 width:100%;
 position: relative;
 height: auto;
 margin: 0px;
 padding:0px !important;
 box-sizing: border-box;
 list-style-type: none;
 text-align: left;
 max-height: 300px;
 overflow-y: scroll;
 overflow-x: hidden;
}
.dd-item{
 display: block;
 height: 30px;
 line-height: 30px;
 padding-left: 5px;
 border-bottom: 1px solid steelblue;
 font-size: 13px;
 z-index: 8;
 overflow: hidden;
}
.dd-item .dd-v{
 width: 0px;
 height: 0px;
 display: none;
}
.dd-item .dd-k{
 z-index: 8;
 padding-left: 15px;
}
.dd-item:first-child{
 border-top: 1px solid steelblue;
}
.dd-item:last-child{
 border-bottom: none;
}
.dd-select .on{
 background-color: steelblue\9;
}
.dd-item:hover::before,.dd-item:hover before{
 content:'\2714';
 position: absolute;
 left: 0px;
 color: #79a979
 z-index: 9;
 font-size: 16px;
 padding-right: 3px;
 padding-left: 2px;
 background-color: #fff;
}
.dd-select .on::before,.dd-select .on before{
 content:'\2714';
 position: absolute;
 left: 0px;
 color: green;
 z-index: 9;
 font-size: 16px;
 padding-right: 3px;
 padding-left: 2px;
 background-color: #fff;
}
.dd-item:hover{
 cursor: pointer;
 background-color: #ccc;
}
.dropdown-container .dropdown-opt{
 width:100%;
 text-align: center;
 position: absolute;
 left: -1px;
 bottom: -30px;
 padding: 0px;
 border-left: 1px solid steelblue;
 border-bottom: 1px solid steelblue;
 border-right: 1px solid steelblue;
 box-sizing: content-box;
}
.dropdown-container .dd-btn{
 color: #333;
 height: 28px !important;
 display: inline-block;
 background-color: #e6e6e6;
 border-color: #adadad;
 user-select: none;
 background-image: none;
 border: 1px solid transparent;
 border-radius: 4px;
 margin: 0px 15px;
}
.dropdown-container .dd-btn:hover{
 color:#fff;
 background-color: steelblue;
}
/* scrollbar */
.dropdown-container ::-webkit-scrollbar { width: 5px; height: 10px; } 
.dropdown-container ::-webkit-scrollbar-track, 
.dropdown-container ::-webkit-scrollbar-thumb { border-radius: 999px; border: 5px solid transparent; } 
.dropdown-container ::-webkit-scrollbar-track { box-shadow: 1px 1px 5px rgba(0,0,0,.2) inset; } 
.dropdown-container ::-webkit-scrollbar-thumb { min-height: 20px; background-clip: content-box; box-shadow: 0 0 0 5px rgba(0,0,0,.2) inset; } 
.dropdown-container ::-webkit-scrollbar-corner { background: transparent; }