如何从 React 中的实体化启用弹出菜单？

根据 official doc，您应该能够启用所有下拉菜单，如下所示：

document.addEventListener('DOMContentLoaded',function() {
  var elems = document.querySELEctorAll('.dropdown-trigger');
  var instances = m.Dropdown.init(elems);
});

这是一个例子（请整页打开）：

document.addEventListener('DOMContentLoaded',function() {
  var elems = document.querySELEctorAll('.dropdown-trigger');
  var instances = m.Dropdown.init(elems);
});

<!--Import Google Icon Font-->
<link href="https://fonts.googleapis.com/icon?family=Material+Icons" rel="stylesheet">

<!-- Compiled and minified CSS -->
<link rel="stylesheet" href="https://cdnjs.cloudFlare.com/ajax/libs/materialize/1.0.0/css/materialize.min.css">

<!-- Compiled and minified JavaScript -->
<script src="https://cdnjs.cloudFlare.com/ajax/libs/materialize/1.0.0/js/materialize.min.js"></script>


<!-- Dropdown Structure -->
<ul id="dropdown1" class="dropdown-content">
  <li><a href="#!">one</a></li>
  <li><a href="#!">two</a></li>
  <li class="divider"></li>
  <li><a href="#!">three</a></li>
</ul>
<nav>
  <div class="nav-wrapper">
    <a href="#!" class="brand-logo">Logo</a>
    <ul class="right hide-on-med-and-down">
      <li><a href="sass.html">Sass</a></li>
      <li><a href="badges.html">Components</a></li>
      <!-- Dropdown trigger -->
      <li><a class="dropdown-trigger" href="#!" data-target="dropdown1">Dropdown<i class="material-icons right">arrow_drop_down</i></a></li>
    </ul>
  </div>
</nav>

,

在您的第二个“li”项目中添加一个 onclick 事件，传递事件对象，通过从事件对象中获取类名来调用 dropdown() 函数。

<li>
    <a className="dropdown-trigger" href="#!" onclick{(E) => e.target.className.dropdown();} data-target="dropdown1">
       Dropdown<i className="material-icons right">arrow_drop_down</i>
    </a>
</li>

简单！只需将此代码添加到js文件中

document.addEventListener('DOMContentLoaded',function() {
  var elems = document.querySELEctorAll('.dropdown-trigger');
  var instances = m.Dropdown.init(elems);
});

您还没有初始化下拉菜单，这就是它不起作用的原因！并且不要使用这个框架，也许 MDbootstrap 会是完美的，因为他们给你一个 React 版本，而且他们有一个没有广告的漂亮文档！

如您所说，您使用的是React，并且基于the link that you left on the question，$(".dropdown-trigger").dropdown(); 是jQuery 方式，您需要找出React 方式

您可以安装 materialize-css 软件包，我想您会发现 How to use materialize-css with React? 对此很有帮助

然后像这样导入和使用包：

import React,{ useRef,useEffect } from 'react';
import M from 'materialize-css';

function DropDown() {
    const dropDowntriggerRef = useRef();
    
    useEffect(() => {
      m.Dropdown.init(dropDowntriggerRef.current);
    },[])

    return (
           // all of the other tags and other stuff
           ....
           <li>
              <a className="dropdown-trigger"
                 href="#!" data-target="dropdown1"
                 ref={dropDowntriggerRef}
              >
                  Dropdown
                  <i className="material-icons right"> arrow_drop_down </i>
              </a>
           </li>
          ....
    );
}

<!--Import Google Icon Font-->
<link href="https://fonts.googleapis.com/icon?family=Material+Icons" rel="stylesheet">

<!-- Compiled and minified CSS -->
<link rel="stylesheet" href="https://cdnjs.cloudFlare.com/ajax/libs/materialize/1.0.0/css/materialize.min.css">

<!-- Compiled and minified JavaScript -->
<script src="https://cdnjs.cloudFlare.com/ajax/libs/materialize/1.0.0/js/materialize.min.js"></script>


<!-- Dropdown Structure -->
<ul id="dropdown1" class="dropdown-content">
  <li><a href="#!">one</a></li>
  <li><a href="#!">two</a></li>
  <li class="divider"></li>
  <li><a href="#!">three</a></li>
</ul>
<nav>
  <div class="nav-wrapper">
    <a href="#!" class="brand-logo">Logo</a>
    <ul class="right hide-on-med-and-down">
      <li><a href="sass.html">Sass</a></li>
      <li><a href="badges.html">Components</a></li>
      <!-- Dropdown trigger -->
      <li><a class="dropdown-trigger" href="#!" data-target="dropdown1">Dropdown<i class="material-icons right">arrow_drop_down</i></a></li>
    </ul>
  </div>
</nav>