使用最新版本(Bootstrap 4 alpha 6).

Using the lastest version (Bootstrap 4 alpha 6).

使用 align-items-center 类在垂直方向上居中， text-center 在水平方向上居中...

Use the align-items-center class to vertically centter, and text-center to horizontal center...

<div class="row text-center align-items-center" style="height:200px;background-color:grey">
   <div class="col-md-10">
      <span>How to center this vertically/horizontally</span>
   </div>
   <div class="col-md-2">
      <span>This too</span>
   </div>
</div>

https://www.codeply.com/go/vaGYMJHA3T