el-table获取全选的勾选状态

el-table开启多选模式时， 有时候需要获取当前页的全选checkbox是否勾选的状态，勾选dom标签肯定是有is-checked这个类的， 但是获取dom再判断dom的className的值就有点low, el-table肯定也是有对应的状态中心去管理其内部的checkbox的

<el-table
        ref="multiTableInst"
        class="avue-crud multi-table"
        :data="dataList"
        style="width: 100%"
        :row-key="option.rowKey"
        v-loading="loading"
        header-row-class-name="custom-menu-list-header"
        @select="handleSelect"
        @row-click="handleRowClick"
        @selection-change="handleSelectionChange"
        @select-all="handleSelectAll"
    >
     <el-table-column
          v-if="option.isMulti"
          class-name="selection_wrapper"
          type="selection"
          :reserve-selection="true"
          width="50"
          align="center"></el-table-column>
      <el-table-column
          v-if="option.index"
          type="index"
          label="序号"
          width="50"
          :index="getIndex"
      ></el-table-column>
      
//....other-column

</el-table>


//methods
handleSelectAll(){
	if（this.getIsAllChecked()){
	//全选逻辑
	//.....
}else {
//取消逻辑
//....
}

}
getIsAllChecked() {
//下面这行为获取table组件中的全选checkbox的勾选状态
        return this.$refs.multiTableInst.store.states.isAllSelected
  }