引入ngx-bootstrap之後

在父元件的module.ts中匯入
BsDropdownModule.forRoot() 匯入必須寫在父元件中，否則不生效

import {BsDropdownModule} from 'ngx-bootstrap/dropdown';

BsDropdownModule.forRoot()

<div class="btn-group" dropdown>
    <button id="button-basic" dropdownToggle type="button" class="btn btn-primary dropdown-toggle"
 

            aria-controls="dropdown-basic">
      下拉選單 <span class="caret"></span>
    </button>
    <ul id="dropdown-basic" *dropdownMenu class="dropdown-menu"
        role="menu" aria-labelledby="button-basic">
      <li role="menuitem"><a class="dropdown-item" href
 
="#">選單項1</a></li>
      <li role="menuitem"><a class="dropdown-item" href="#">選單項2</a></li>
      <li role="menuitem"><a class="dropdown-item" href="#">選單項3</a></li>
    </ul>
  </div>