Element中select多数据加载优化的实现

<template>
  <div class="app">
    <el-dialog title="标题" :visible.sync="relatedOpen" :append-to-body="true" width="500px">
      <el-row :gutter="16">
        <el-col :span="20">
          <el-select
            v-model="value"
            filterable
            clearable
            style="width:100%"
            placeholder="请选择"
            :loading="searchLoad"
            :filter-method="filterMethod"
            v-el-select-loadmore="loadMore(rangeNumber)"
            @visible-change="visibleChange"
          >
            <el-option v-for="item in options.slice(0, rangeNumber)" :key="item.key" :label="item.value" :value="item.key"></el-option>
          </el-select>
        </el-col>
        <el-col :span="4">
          <el-button type="primary" @click="submit">确定</el-button>
        </el-col>
      </el-row>
    </el-dialog>
  </div>
</template>

  // 自定义指令
  directives: {
    "el-select-loadmore": (el, binding) => {
      // 获取element UI定义好的scroll元素
      const SELECTWRAP_ROM = el.querySelector(".el-select-dropdown .el-select-dropdown__wrap");
      if (SELECTWRAP_ROM) {
    // 添加scroll事件
        SELECTWRAP_ROM.addEventListener("scroll", function() {
      // 判断滚动
          const condition = this.scrollHeight - this.scrollTop <= this.clientHeight;
          condition && binding.value();
        });
      }
    }
  }

export default {
  data() {
    return {
      relatedOpen: false,
      options: [] /* 选择下拉框的值 */,
      courseList: [],
      rangeNumber: 10,
      searchLoad: false /* 下拉框的loading状态 */,
      value: "",
      timer: null
    };
  },
  created() {
    this.getOptions();
  },
  methods: {
    // 按需加载
    loadMore(n) {
      return () => (this.rangeNumber += 5);
    },
    // 过滤课件
    filterMethod(query) {
      if (this.timer != null) clearTimeout(this.timer);
      !this.searchLoad && (this.searchLoad = true);
      this.timer = setTimeout(() => {
        this.options = !!query ? this.courseList.filter(el => el.value.toLowerCase().includes(query.toLowerCase())) : this.courseList;
        clearTimeout(this.timer);
        this.searchLoad = false;
        this.rangeNumber = 10;
        this.timer = null;
      }, 500);
    },
    // 监听select下拉框的显示和隐藏
    visibleChange(flag) {
      // 显示时初始化列表
      flag && this.filterMethod("");
      // 初始化默认值
      this.rangeNumber = 10;
    },
    // 获取选项
    async getOptions() {
      await searchCourseware().then(res => {
        let list = res.data || [];
        // 默认展示的数据
        this.options = list;
        // 原始数据
        this.courseList = list;
      });
    }
  }
}