Bootstrap3.3.7导航栏下拉菜单鼠标滑过展开效果

下面是详细讲解“Bootstrap3.3.7导航栏下拉菜单鼠标滑过展开效果”的完整攻略：

准备工作

首先，你需要在HTML文件中引入Bootstrap库：

<link rel="stylesheet" href="https://cdn.bootcdn.net/ajax/libs/twitter-bootstrap/3.3.7/css/bootstrap.min.css">
<script src="https://cdn.bootcdn.net/ajax/libs/jquery/1.12.4/jquery.min.js"></script>
<script src="https://cdn.bootcdn.net/ajax/libs/twitter-bootstrap/3.3.7/js/bootstrap.min.js"></script>

然后，在导航菜单中添加下拉菜单的HTML代码，如下所示：

<nav class="navbar navbar-default" role="navigation">
    <div class="container-fluid">
        <!-- Brand and toggle get grouped for better mobile display -->
        <div class="navbar-header">
            <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="#">Brand</a>
        </div>

        <!-- Collect the nav links, forms, and other content for toggling -->
        <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
            <ul class="nav navbar-nav">
                <li class="active"><a href="#">Link <span class="sr-only">(current)</span></a></li>
                <li><a href="#">Link</a></li>
                <li class="dropdown">
                    <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
                    <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>
                        <li role="separator" class="divider"></li>
                        <li><a href="#">Separated link</a></li>
                        <li role="separator" class="divider"></li>
                        <li><a href="#">One more separated link</a></li>
                    </ul>
                </li>
            </ul>
        </div><!-- /.navbar-collapse -->
    </div><!-- /.container-fluid -->
</nav>

实现鼠标滑过展开效果

接下来，你需要添加一些CSS代码来实现鼠标滑过下拉菜单展开的效果：

.dropdown:hover .dropdown-menu {
    display: block;
}

这段CSS代码的意思是，当下拉菜单所在的 .dropdown 元素被鼠标滑过时，其子元素 .dropdown-menu 的 display 属性将变成 block，从而展开下拉菜单。

示例说明

示例一

我们可以通过修改上述代码中的导航菜单，来看看实现效果。代码如下：

<nav class="navbar navbar-default" role="navigation">
    <div class="container-fluid">
        <!-- Brand and toggle get grouped for better mobile display -->
        <div class="navbar-header">
            <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="#">Brand</a>
        </div>

        <!-- Collect the nav links, forms, and other content for toggling -->
        <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
            <ul class="nav navbar-nav">
                <li class="active"><a href="#">Home <span class="sr-only">(current)</span></a></li>
                <li><a href="#">About</a></li>
                <li class="dropdown">
                    <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Services <span class="caret"></span></a>
                    <ul class="dropdown-menu">
                        <li><a href="#">Service one</a></li>
                        <li><a href="#">Service two</a></li>
                        <li><a href="#">Service three</a></li>
                        <li role="separator" class="divider"></li>
                        <li><a href="#">Service four</a></li>
                        <li role="separator" class="divider"></li>
                        <li><a href="#">Service five</a></li>
                    </ul>
                </li>
                <li><a href="#">Contact</a></li>
            </ul>
        </div><!-- /.navbar-collapse -->
    </div><!-- /.container-fluid -->
</nav>

这里我们将导航菜单的内容修改为了一个网站导航。你可以在这个网站https://getbootstrap.com/docs/4.6/components/navbar/中找到这个完整的例子，并在此基础上修改。

当你运行这段代码并鼠标滑过 Services 时，你会发现下拉菜单展开了。

示例二

下面，我们来看一个稍微复杂一些的例子。代码如下：

<nav class="navbar navbar-default" role="navigation">
    <div class="container-fluid">
        <!-- Brand and toggle get grouped for better mobile display -->
        <div class="navbar-header">
            <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1">
                <span class="sr-only">Toggle navigation</span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
                <span class="icon-bar"></span>
            </button>
            <a class="navbar-brand" href="#">Brand</a>
        </div>

        <!-- Collect the nav links, forms, and other content for toggling -->
        <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
            <ul class="nav navbar-nav">
                <li class="active"><a href="#">Home <span class="sr-only">(current)</span></a></li>
                <li><a href="#">About</a></li>
                <li class="dropdown">
                    <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Products <span class="caret"></span></a>
                    <ul class="dropdown-menu">
                        <li><a href="#">Product one</a></li>
                        <li><a href="#">Product two</a></li>
                        <li class="dropdown-submenu">
                            <a tabindex="-1" href="#">Product three <span class="caret"></span></a>
                            <ul class="dropdown-menu">
                                <li><a tabindex="-1" href="#">Product three one</a></li>
                                <li class="dropdown-submenu">
                                    <a href="#">Product three two <span class="caret"></span></a>
                                    <ul class="dropdown-menu">
                                        <li><a href="#">Product three two one</a></li>
                                        <li><a href="#">Product three two two</a></li>
                                    </ul>
                                </li>
                                <li><a tabindex="-1" href="#">Product three three</a></li>
                            </ul>
                        </li>
                        <li role="separator" class="divider"></li>
                        <li><a href="#">Product four</a></li>
                        <li role="separator" class="divider"></li>
                        <li><a href="#">Product five</a></li>
                    </ul>
                </li>
                <li><a href="#">Contact</a></li>
            </ul>
        </div><!-- /.navbar-collapse -->
    </div><!-- /.container-fluid -->
</nav>

这里，我们添加了一个多级的下拉菜单。你可以在 Product three 中看到另一个下拉菜单。在这个例子中，你需要注意，当你鼠标滑过某个菜单项时，它的下一级菜单会展开，但在你离开它时，它的下一级菜单会收起，直到你又重复鼠标滑过该菜单项时才重新展开。

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