使用微信小程序如何实现动态设置placeholder提示文字

<view class="retrieve-list">
  <p class="option">
    <span bindtap="placeholder" data-num='1' data-key='' data-val='' class="{{num==1?'active':''}}">全部</span>
    <span bindtap="placeholder" data-num='2' data-key='标题' data-val='ti' class="{{num==2?'active':''}}">标题</span>
    <span bindtap="placeholder" data-num='3' data-key='摘要' data-val='ab' class="{{num==3?'active':''}}">摘要</span>
    <span bindtap="placeholder" data-num='4' data-key='申请人' data-val='ap' class="{{num==4?'active':''}}">申请人</span>
    <span bindtap="placeholder" data-num='5' data-key='申请号' data-val='an' class="{{num==5?'active':''}}">申请号</span>
    <span bindtap="placeholder" data-num='6' data-key='代理人' data-val='ag' class="{{num==6?'active':''}}">代理人</span>
  </p>
  <p class="option">
    <span bindtap="placeholder" data-num='7' data-key='代理机构' data-val='ac' class="{{num==7?'active':''}}">代理机构</span>
    <span bindtap="placeholder" data-num='8' data-key='公开号' data-val='pn' class="{{num==8?'active':''}}">公开号</span>
    <span bindtap="placeholder" data-num='9' data-key='发明人' data-val='in' class="{{num==9?'active':''}}">发明人</span>
    <span bindtap="placeholder" data-num='10' data-key='专利权人' data-val='pa' class="{{num==10?'active':''}}">专利权人</span>
  </p>
</view>

var app = getApp()
Page({
 data: {
  placeholder:'请输入',
  ph:'',
  num:1,
  key:'',
 },
 placeholder:function(e){
  var num = e.currentTarget.dataset.num
  this.setData({
   ph: this.data.placeholder+e.currentTarget.dataset.key
  })
  this.setData({
   num: num
  })
})