点击三角形，展开或者收起内容

<template>
  <div>
    <div class="zhankai" @click="btn()">
      展开 <span :class="{'sanjiao':true,'rotate':flag}"></span>
    </div>
    <!-- 展示或者收起 -->
    <el-collapse-transition>
      <div v-show="flag">
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      </div>
    </el-collapse-transition>
  </div>
</template>

<script>
export default {
  data () {
    return {
      flag: true
    }
  },
  methods: {
    btn () {
      this.flag = !this.flag
    }
  }
}
</script>

<style lang="less" scoped>
.zhankai {
  margin-left: 100px;
  position: relative;
  .sanjiao {
    width: 0;
    height: 0;
    border-style: solid;
    border-width: 0 4px 6px 4px;
    border-color: transparent transparent #333 transparent;
    position: absolute;
    left: 36px;
    top: 50%;
    transform: translate(-50%, -50%);
    transition: transform 0.3s ease-in-out;
  }
  .rotate {
    transform: translate(-50%, -50%) rotate(180deg);
  }
  &:hover {
    color: #1890ff;
    .sanjiao {
      border-color: transparent transparent #1890ff transparent;
    }
    // 只要滑过三角形就旋转，放开这个代码
    // .rotate {
    //   transform: translate(-50%, -50%) rotate(180deg);
    // }
  }
}
</style>