如何设置TabBarView的索引？

import 'package:flutter/material.dart';

TabController _tabController;

@override
void initState() {
  super.initState();
  _tabController = TabController(vsync: this, length: 3, initialIndex: 0); // length为TabBar选项卡的数量，initialIndex为初始索引值
}

TabBarView(
  controller: _tabController,
  children: [
    // 页面1的内容
    Page1(),
    // 页面2的内容
    Page2(),
    // 页面3的内容
    Page3(),
  ],
)

TabBar(
  controller: _tabController,
  tabs: [
    // TabBar的选项卡
    Tab(
      text: "Page 1",
    ),
    Tab(
      text: "Page 2",
    ),
    Tab(
      text: "Page 3",
    ),
  ],
)

import 'package:flutter/material.dart';

class TabBarViewExample extends StatefulWidget {
  @override
  _TabBarViewExampleState createState() => _TabBarViewExampleState();
}

class _TabBarViewExampleState extends State<TabBarViewExample> with SingleTickerProviderStateMixin {
  TabController _tabController;

  @override
  void initState() {
    super.initState();
    _tabController = TabController(vsync: this, length: 3, initialIndex: 0);
  }

  @override
  void dispose() {
    _tabController.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("TabBarView Example"),
      ),
      body: TabBarView(
        controller: _tabController,
        children: [
          Page1(),
          Page2(),
          Page3(),
        ],
      ),
      bottomNavigationBar: TabBar(
        controller: _tabController,
        tabs: [
          Tab(
            text: "Page 1",
          ),
          Tab(
            text: "Page 2",
          ),
          Tab(
            text: "Page 3",
          ),
        ],
      ),
    );
  }
}

class Page1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: Text("Page 1"),
    );
  }
}

class Page2 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: Text("Page 2"),
    );
  }
}

class Page3 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(
      child: Text("Page 3"),
    );
  }
}