elementui select组件下拉框底部增加自定义按钮

el-select组件的visible-change 事件（下拉框出现/隐藏时触发）

<el-selectref="select":value="value"placeholder="请选择"@visible-change="visibleChange"></el-select>visibleChange(visible) {// 下拉框显示隐藏if (visible) {const ref = this.$refs.selectlet popper = ref.$refs.popperif (popper.$el) popper = popper.$el// 判断是否有添加按钮if (!Array.from(popper.children).some(v => v.className === 'btn-box')) {const el = document.createElement('div')el.className = 'btn-box'el.innerHTML = `<a class="btn" style="font-size:16px;display:block;line-height:38px;text-align:center;"><i class="el-icon-plus"></i>添加</a>`popper.appendChild(el)el.onclick = () => {// todo}}}},