如何在ngx-treeview中选中不确定复选框

<ngx-treeview [items]="treeData" [config]="treeConfig" (selectedChange)="onSelectedChange($event)"></ngx-treeview>

import { Component } from '@angular/core';

@Component({
  selector: 'app-treeview-example',
  templateUrl: './treeview-example.component.html',
  styleUrls: ['./treeview-example.component.css']
})
export class TreeviewExampleComponent {
  treeData = [
    {
      text: 'Node 1',
      checked: false,
      children: [
        {
          text: 'Node 1.1',
          checked: false,
          children: [
            {
              text: 'Node 1.1.1',
              checked: false
            },
            {
              text: 'Node 1.1.2',
              checked: false
            }
          ]
        },
        {
          text: 'Node 1.2',
          checked: false
        }
      ]
    },
    {
      text: 'Node 2',
      checked: false
    }
  ];

  treeConfig = {
    hasAllCheckBox: false,
    hasFilter: false,
    hasCollapseExpand: false,
    decoupleChildFromParent: false,
    maxHeight: 500
  };

  onSelectedChange(event) {
    // 处理复选框的选中状态
    // 更新节点的checked属性
    // 更新子节点的选中状态
  }
}