在实际vue项目使用多选框时，倘若用户需要选择多个选项，那他就需要点击多次；该代码实现点击一次即选择全部，并将全部选项显示在多选框内

直接贴代码：

<div class="select-form">
  <el-form>
    <el-form-item>
      <el-select  placeholder="请选择活动区域" multiple v-model="citys">
        <el-option v-show="!all" label="选择所有" value="all"></el-option>
        <el-option v-show="all" label="取消所有" value="cancleAll"></el-option>
        <el-option v-for="item in cities" :label="item.label" :value="item":key="item.value"></el-option>
      </el-select>
    </el-form-item>
  </el-form>
  <el-button class="check-btn" type="primary" @click="check">查看</el-button>
</div>

data () {
  return {
    cities: [
      {value: 'Beijing',label: '北京'}, 
      {value: 'Shanghai',label: '上海'}, 
      {value: 'Nanjing',label: '南京'}, 
      {value: 'Chengdu',label: '成都'}, 
      {value: 'Shenzhen',label: '深圳'}, 
      {value: 'Guangzhou',label: '广州'}
    ],
    citys:[],
    all: false,
  }
},

methods: {
  check(){
    console.log(this.citys)
  },
},

watch:{
  citys(val,oldval){
    //如果新的选择里有勾选了选择所有, 则勾选整个选项
    if(val.indexOf('all')!=-1 && oldval.indexOf('all')==-1 && val.length && this.cities.length){
      this.citys=[].concat(this.cities);
      this.all = true;
    }
    //如果操作前有勾选了多项且当前也选中了取消所有且勾选数量大于等于1则取消勾选所有
    else if(val.indexOf('cancleAll')!=-1 && oldval.indexOf('cancleAll')==-1 && oldval.length>=1){
      this.all = false;
      this.citys = [];
    }
  } 
}

.select-form{
  display: flex;
  justify-content: center;
  height: 40px;
}
.check-btn{
  margin-left: 20px;
}