Bootstrap 4中有多种用于Navbar对齐的方法.一种方法是在内容上使用mx-auto居中(表单)... >

There are different methods for Navbar alignment in Bootstrap 4. One way is to use mx-auto on the content to center (the form)...

<nav class="navbar navbar-toggleable-xl navbar-light bg-faded">
    <div class="navbar-collapse collapse" id="mynavbar">
        <form class="form-inline mx-auto">
            <div class="input-group my-lg-0">
                <input class="form-control " type="text" placeholder="Search">
                <span class="input-group-btn">
                <button class="btn" type="submit">Search</button>
                </span>
            </div>
        </form>
        <ul class="navbar-nav">
            <li class="nav-item">
                 <div class="btn-group">
                    <a class="btn nav-link dropdown-toggle" data-toggle="dropdown"
                        href="#" role="button" aria-haspopup="true" aria-expanded="false">
                        <i class="fa fa-plus" aria-hidden="true"></i>
                    </a>
                    <div class="dropdown-menu dropdown-menu-right">
                        <a class="dropdown-item" href="#">Action</a> <a
                            class="dropdown-item" href="#">Another action</a> <a
                            class="dropdown-item" href="#">Something else here</a>
                    </div>
                </div>
            </li>
        </ul>
    </div>
</nav>

但是搜索表单将不会居中对齐，因为左侧没有内容，并且右侧的按钮会将表单略微向左推.

But the search form will not be perfectly centered because there is no content in the left, and the form will be pushed slight left by the button on the right.

因此，在这种情况下，我建议使用 flexbox实用程序类.

So in this case, I'd recommend using the flexbox utility classes..

<nav class="navbar navbar-toggleable-xl navbar-light bg-faded">
    <div class="navbar-collapse collapse" id="mynavbar">
        <span class="navbar-text w-100"> </span>
        <form class="form-inline w-100 justify-content-center">
            <div class="input-group my-lg-0">
                <input class="form-control " type="text" placeholder="Search">
                <span class="input-group-btn">
                <button class="btn" type="submit">Search</button>
                </span>
            </div>
        </form>
        <ul class="navbar-nav w-100 justify-content-end">
            <li class="nav-item">
                 <div class="btn-group">
                    <a class="btn nav-link dropdown-toggle" data-toggle="dropdown"
                        href="#" role="button" aria-haspopup="true" aria-expanded="false">
                        <i class="fa fa-plus" aria-hidden="true"></i>
                    </a>
                    <div class="dropdown-menu dropdown-menu-right">
                        <a class="dropdown-item" href="#">Action</a> <a
                            class="dropdown-item" href="#">Another action</a> <a
                            class="dropdown-item" href="#">Something else here</a>
                    </div>
                </div>
            </li>
        </ul>
    </div>
</nav>

两种居中方法的演示