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<head>
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<meta name="Generator" content="EditPlus®">
<meta name="Author" content="">
<meta name="Keywords" content="">
<meta name="Description" content="">
<title>Document</title>
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<body>
<p class="mainmenu">
<p class="dropdown pull-right">
<button id="dLabel" type="button" data-toggle="dropdown" aria-haspopup="true" aria-expanded="false">
Dropdown trigger
<span class="caret"></span></button>
<ul class="dropdown-menu" aria-labelledby="dLabel" style="">
<li tabindex="-1">aa</li>
<li tabindex="-1">bb</li>
<li tabindex="-1">cc</li>
<li tabindex="-1">dd</li></ul>
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比如当选中某项是 button的文本值等于选中项
https://jsfiddle.net/La5tnaqo/