为了获取到tree组件，添加ref：

ref=“tree”

<el-tree
        ref="tree"
        :data="rights"
        show-checkbox
        node-key="id"
        :default-expanded-keys="defaultExpandedKeys"
        :default-checked-keys="defaultCheckedKeys"
        :props="defaultProps"
      ></el-tree>

代码：

let arr=this.$refs.tree.getCheckedKeys().concat(this.$refs.tree.getHalfCheckedKeys());