微信小程序picker怎么自动弹框

微信小程序picker怎么自动弹框，不手动点击触发，怎么自动弹框

定义变量控制，要实现这种效果就不能用微信自带的，需要自定义，下面我分段贴码：
布局：

<view class="free-btns" style="margin-top: 10vh;background:none;">
  <button class="free-btn" bindtap="toggleDialog">
   选定国家：{{value}}
  </button>
 </view>

 <view class="free-dialog {{ showDialog ? 'free-dialog--show' : '' }}">
  <view class="free-dialog__mask" bindtap="toggleDialog" />
  <view class="free-dialog__container">
   <view style="padding: 5% 5% 15%;">
    <form bindsubmit='submit' bindreset="reset">
     <view bindtap='freetoBack' class="free-button free-dialog-reset">取消</view>
     <view bindtap='freeBack' class="free-button free-dialog-submit">确定</view>

     <radio-group class='free-radios' bindchange="radioChange">
      <label class="free-radio" bindtap="click" wx:for="{{items}}" wx:key="{{items}}" data-id="{{index}}" style="{{index==id?'background:#48c23d;color:#fff;':'background:#fff;color:#000;'}}">
       <radio value="{{item.name}}" name="{{item.value}}"></radio>
       <label class="free-text">{{item.value}}</label>
      </label>
     </radio-group>
    </form>

   </view>
 </view>
</view>

样式：

.free-dialog__mask {
 position: fixed;
 top: 0;
 left: 0;
 right: 0;
 bottom: 0;
 z-index: 10;
 background: rgba(0, 0, 0, 0.7);
 display: none;
}
.free-dialog__container {
 position: fixed;
 left: 0;
 bottom: 0;
 width: 750rpx;
 background: white;
 transform: translateY(150%);
 transition: all 0.4s ease;
 z-index: 11;
}
.free-dialog--show .free-dialog__container {
 transform: translateY(0);
}
.free-dialog--show .free-dialog__mask {
 display: block;
}
/*模态框中的内容*/
.free-button{
 display: inline-block;
 width:50px;
 text-align: center;
 font-size:20px;
 color:#707070;
 margin-bottom:20px;
}
.free-dialog-submit{
 float: right;
 color:#48c23d;
}
radio-group{
 margin:10rpx 0rpx;
}
radio-group>label{
 width:22.5%;
 display: inline-block;
 border:1px solid #ddd;
 padding:10px 0px;
 margin:0px 2px 2px;
}

radio-group label radio{
 width:100%;
 z-index: 3;
 display: none;
}
.checked{
 background:#48c23d;
 color:#fff;
}
radio-group label .free-text{
 width:100%;
 text-align: center;
 display: inline-block;
}

js：

data: {
    showDialog: true,//此变量就是用来控制显示和隐藏，当满足条件时为true
    items: [
      { name: '中国', value: '中国' },
      { name: '美国', value: '美国' },
      { name: '巴西', value: '巴西' },
      { name: '日本', value: '日本' },
      { name: '英国', value: '英国' },
      { name: '法国', value: '法国' },
      { name: '韩国', value: '韩国' },
      { name: '俄罗斯', value: '俄罗斯' },]
  },

  /*点击变色*/
  click: function (e) {
    var id = e.currentTarget.dataset.id
    var that = this
    that.setData({
      id: id
    })
  },
  onLoad: function (options) {
    var that = this
    that.setData({
      value: 'show'
    })
  },
  radioChange: function (e) {
    console.log('radio发生change事件，携带value值为：', e.detail.value)
    var that = this
    that.setData({
      value: e.detail.value
    })
    console.log(this.data.value)
  },
  toggleDialog() {
    this.setData({
      showDialog: !this.data.showDialog
    });
  },
  freeBack: function () {
    var that = this
    if (this.data.value == 'show') {
      wx.showModal({
        title: '提示',
        content: '你没有选择任何内容',
      })
    }
    that.setData({
      showDialog: !this.data.showDialog
    })
  },
  freetoBack: function () {
    var that = this
    wx.showModal({
      title: '提示',
      content: '你没有选择任何内容',
    })
    that.setData({
      showDialog: !this.data.showDialog,
      value: 'show',
      checked: false,
    })
  },