我在自定义组件中使用了input做为搜索框，但是在定义placeholder-class时遇到了问题。

其中wxml文件中定义

<input id="search_input" class="input input-class" type="text"
  focus="{{focus}}" confirm-type="search" value="{{value}}" disabled="{{disabled}}" maxlength="{{maxlength}}"
  placeholder="{{placeholder}}" placeholder-class="placeholder-class" placeholder-style="{{placeholderStyle}}"
  bind:input="handleInput" bind:focus="handleFocus" bind:blur="handleBlur" bind:confirm="handleConfirm"/>

并且在js文件中含有

externalClasses: ['input-class', 'placeholder-class']

但是在页面中调用此组件时

<search-bar placeholder-class="search-placeholder" input-class="search-input"/>

input-class可以被外部类替换，placeholder-class却不能。placeholder-class="placeholder-class"在开发者工具中没看到有任何变化