為什么$pri用的是父類的屬性？

Group a series of buttons together on a single line with the button group, and super-power them with JavaScript.

<div class="btn-group" role="group" aria-label="Basic example">
  <button type="button" class="btn btn-secondary">Left</button>
  <button type="button" class="btn btn-secondary">Middle</button>
  <button type="button" class="btn btn-secondary">Right</button>
</div>