The vue table contains a column of multiple checkboxes. How can I get the data for the selected row?

1. Imitate the demo,table table posted on the official website of element-ui to select multiple selections, and use Checkbox when selecting multiple rows of data. But cannot get the value of this.multipleSelection. The related code is as follows:
< el-table ref= "studentAllTable": data= "studentAllData" stripe border: default-sort = "{prop: "sgrade", order:" ascending"}" @ selection-change= "handleSelectionChange21" @ row-click= "handleclick21": row-class-name = "tableRowClassName21" style= "position:relative;top:5px;" height= "600" >

                    <el-table-column type="selection" width="55"></el-table-column>
                    <el-table-column prop="sid" label="Id" sortable show-overflow-tooltip v-if="false"></el-table-column>
                    <el-table-column prop="sname" label="" sortable show-overflow-tooltip></el-table-column>
                    <el-table-column prop="sgender" label="" sortable width="80px">
                        <template scope="scope">
                            <span v-if="scope.row.sgender === 0"></span>
                            <span v-else></span>
                        </template>
                    </el-table-column>
                    <el-table-column prop="sbirth" label="" sortable width="120px"></el-table-column>
                    <el-table-column prop="sorganization" label="" sortable width="90px">
                        <template scope="scope">
                            <span v-if="scope.row.sorganization === 0"></span>
                            <span v-else></span>
                        </template>
                    </el-table-column>
                    <el-table-column prop="sperson" label="" sortable width="90px"></el-table-column>
                    <el-table-column prop="sfield" label="" sortable width="120px"></el-table-column>
                    <el-table-column prop="sprocess" label="" sortable width="90px"></el-table-column>
                    <el-table-column prop="stimes" label="" sortable width="120px"></el-table-column>
                    <el-table-column prop="sgrade" label="" sortable width="95px"></el-table-column>
                    <el-table-column label="" width="120">
                        <template slot-scope="scope">
                            <el-popover trigger="hover" placement="top">
                                : {{ scope.row.stel }}
                                : {{ scope.row.smail }}
                                : {{ scope.row.sadd }}
                                <div slot="reference" class="name-wrapper">
                                    <el-tag size="medium">{{ scope.row.stel }}</el-tag>
                                </div>
                            </el-popover>
                        </template>
                    </el-table-column>
                    <el-table-column label="" width="120">
                        <template slot-scope="scope">
                            <el-popover trigger="hover" placement="top">
                                <p style="width: 500px;">: {{ scope.row.sdescription }}
                                <div slot="reference" class="name-wrapper">
                                    <el-tag size="medium"></el-tag>
                                </div>
                            </el-popover>
                        </template>
                    </el-table-column>
                    <el-table-column label="">
                        <template scope="scope">
                            <el-button size="small" type="primary" @click="handleSendMessage(scope.$index, scope.row)"></el-button>
                            <el-button size="small" type="success" @click="handleLike(scope.$index, scope.row)"></el-button>
                        </template>
                    </el-table-column>
                </el-table>

2. MultipleSelection: [],
3 has also been added to the data area. Response function: handleSelectionChange21:function (val) {/ / is used to listen to whether the current line is selected

            this.multipleSelection = val;
        },

4. The extra button in the table is responsible for getting the information of the selected row

        sendMessageSelected:function () {//
            //sid
                alert(this.multipleSelection);
        },

5. At this point, I don"t know how to get the value of this.multipleSelection. I want to get each row of data. The screenshot is as follows:

Vue.js

Feb.28,2021