如何使用css或javascript使多级下拉菜单在触摸屏设备上工作？

<ul class="dropdown-menu">
  <li>
    <a href="#">Menu Item 1</a>
    <ul class="sub-menu">
      <li><a href="#">Sub Menu Item 1.1</a></li>
      <li><a href="#">Sub Menu Item 1.2</a></li>
      <li>
        <a href="#">Sub Menu Item 1.3</a>
        <ul class="sub-menu">
          <li><a href="#">Sub Sub Menu Item 1.3.1</a></li>
          <li><a href="#">Sub Sub Menu Item 1.3.2</a></li>
        </ul>
      </li>
    </ul>
  </li>
  <li><a href="#">Menu Item 2</a></li>
  <li><a href="#">Menu Item 3</a></li>
</ul>

.dropdown-menu {
  list-style: none;
  padding: 0;
  margin: 0;
}

.dropdown-menu li {
  position: relative;
}

.dropdown-menu a {
  display: block;
  padding: 10px;
  text-decoration: none;
  color: #333;
}

.sub-menu {
  display: none;
  position: absolute;
  top: 100%;
  left: 0;
  background-color: #fff;
  border: 1px solid #ccc;
  padding: 0;
  margin: 0;
}

.sub-menu li {
  width: 100%;
}

.sub-menu .sub-menu {
  top: 0;
  left: 100%;
}

document.addEventListener('DOMContentLoaded', function() {
  const menuItems = document.querySelectorAll('.dropdown-menu > li');

  menuItems.forEach(item => {
    item.addEventListener('touchstart', function(event) {
      event.preventDefault();
      const subMenu = this.querySelector('.sub-menu');
      if (subMenu) {
        subMenu.style.display = subMenu.style.display === 'block' ? 'none' : 'block';
      }
    });
  });
});