有一棵树型的数据，需要实现：在外部加一个 全选和不全选的按钮，去全部勾选树结构里面每一项的选框。

当点击勾选全选的时候，树的每一项都勾选；

当取消全选的时候，树的每一项都不勾选；

当选树的其中一项时，全选按钮是半选状态；

实现效果如下：

<template>
  <div class="app-container vehContractList">
    <-- 我省略了表格。这个在这里代表表格每一行的数据：scope.row -->
    <el-button type="text" @click="handleAdd(scope.row)">配置ID</el-button>
    <el-dialog
      :title="title"
      :visible.sync="open"
      width="896px"
      append-to-body
      :close-on-click-modal="false"
    >
      <el-form
        ref="form"
        :model="form"
        :rules="rules"
        label-width="120px"
        :hide-required-asterisk="false"
      >
        <el-row :gutter="10">
          <el-col :span="16">
            <span style="position: absolute; left: 65px; top: 59px;font-size: 14px;color: #ff6d6d; margin-right: 4px;">*</span>
            <el-form-item
              label="选择树ID"
            >
              <!-- prop="itemIds" -->
              <el-row class="elRowCount">
                <el-col :span="12">
                  <el-checkbox
                    v-model="checkAll"
                    :indeterminate="isIndeternimate"
                    @change="handleCheckAll"
                  >全选/全不选
                  </el-checkbox>
                </el-col>
              </el-row>
              <el-row class="elRowTree">
                <el-tree
                  ref="itemTree"
                  v-loading="loading4"
                  :data="itemTreeData"
                  :filter-node-method="filterNode"
                  show-checkbox
                  node-key="id"
                  default-expand-all
                  :props="defaultProps"
                  :default-checked-keys="checkDefaultForItem"
                  :expand-on-click-node="false"
                  class="customDisableTree"
                  @check="handleNodeClickForItem"
                />
              </el-row>
              <div v-if="noCheck" class="el-form-item__error">请选择</div>
            </el-form-item>
          </el-col>
        </el-row>
      </el-form>
      <div
        slot="footer"
        class="dialog-footer"
      >
        <el-button
          type="primary"
          @click="submitForm"
        >
          保 存
        </el-button>
        <el-button @click="cancel">
          取 消
        </el-button>
      </div>
    </el-dialog&