微信小程序下拉框

<view class='top'>
    <view class='top-text'> 选择接收班级</view>
    <!-- 下拉框 -->
    <view class='top-selected' bindtap='bindShowMsg'>
      <text>{{grade_name}}</text>
      <image src='/images/icon/down.png'></image>
    </view>
    <!-- 下拉需要显示的列表 -->
    <view class="select_box" wx:if="{{select}}">
      <view wx:for="{{grades}}" wx:key="unique">
        <view class="select_one" bindtap="mySelect" data-name="{{item}}">{{item}}</view>
      </view>
    </view>
  </view>

/* 顶部 */
.top{
  width: 100vw;
  height: 80rpx;
  padding: 0 20rpx;
  line-height: 80rpx;
  font-size: 34rpx;
  border-bottom: 1px solid #000;
}
.top-text{
  float: left
}
/* 下拉框 */
.top-selected{
    width: 50%;
    display: flex;
    float: right;
    align-items: center;
    justify-content: space-between;
    border: 1px solid #ccc;
    padding: 0 10rpx;
    font-size: 30rpx;
}
/* 下拉内容 */
.select_box {
    background-color: #fff;
    padding: 0 20rpx;
    width: 50%;
    float: right;
    position: relative;
    right: 0;
    z-index: 1;
    overflow: hidden;
    text-align: left;
    animation: myfirst 0.5s;
    font-size: 30rpx;
}
.select_one {
    padding-left: 20rpx;
    width: 100%;
    height: 60rpx;
    position: relative;
    line-height: 60rpx;
    border-bottom: 1px solid #ccc;
}
/* 下拉过度效果 */
@keyframes myfirst {
    from {
        height: 0rpx;
    }
 
    to {
        height: 210rpx;
    }
}
/* 下拉图标 */
.top-selected image{
  height:50rpx;
  width:50rpx;
  position: absolute;
  right: 0rpx;
  top: 20rpx;
}

 /**
   * 页面的初始数据
   */
  data: {
    select:false,
    grade_name:'--请选择--',
    grades: [
        '猛犸机器人1班',
        '猛犸机器人2班',
        '口才1班',
      ]
  },/**
 *  点击下拉框
 */
  bindShowMsg() {
    this.setData({
      select: !this.data.select
    })
  },
/**
 * 已选下拉框
 */
  mySelect(e) {
    console.log(e)
    var name = e.currentTarget.dataset.name
    this.setData({
      grade_name: name,
      select: false
    })
  },