实例中有3个Tab标签Home, School,Self;在School页面添加功能跳转到Home.
需要注意的地方:

1. CupertinoTabBar中如果需要指定currentIndex参数选中tab标签，那么就必须在onTap中实现更新选中的tab标签，示例中就要有

onTap: (index){
   selectedIndex = index;
}

没有的话再次设置selectedIndex不会刷新UI，是一个比较特殊的地方.

2. CupertinoPageScaffold的child需要加SafeArea，不然child的垂直坐标会是从屏目顶部开始,加了SafeArea后是从NvBar下方开始

import 'package:flutter/cupertino.dart';
import 'package:flutter/material.dart';

class CupertinoTabWidget extends StatefulWidget{
  @override
  State<StatefulWidget> createState() {
    return _CupertinoTabWidgetState();
  }
}
class _CupertinoTabWidgetState extends State<CupertinoTabWidget>{
  List<BottomNavigationBarItem> _bottomBarItems = [
    BottomNavigationBarItem(icon: Icon(Icons.home), title: Text("Home")),
    BottomNavigationBarItem(icon: Icon(Icons.school), title: Text("School")),
    BottomNavigationBarItem(icon: Icon(Icons.people), title: Text("Self"))
  ];
  Widget getPages(int index){
    switch(index){
      case 0:
        return _HomeCupertinoPage();
        break;
      case 1:
        return _SchoolCupertinoPage(toHome: (){
          print("to home callback.");
          this.selectedIndex = 0;
        },);
      case 2:
        return _SelfCupertinoPage();
        break;
      default:
        return null;
        break;
    }
  }
  Map<String, WidgetBuilder> _routes = {};

  int _selectedIndex = 2;
  set selectedIndex(int value){
    if(this.mounted && _selectedIndex != value) {
      setState(() {
        print("to index: $value");
        _selectedIndex = value;
      });
    }
  }

  @override
  Widget build(BuildContext context) {
    print("current index: $_selectedIndex");
    return CupertinoTabScaffold(
      tabBar: CupertinoTabBar(
        items: _bottomBarItems,
        currentIndex: _selectedIndex,
        onTap: (index){
          //当有指定选中tab时(_selectedIndex)，需要这个设置
          //否则再次设置_selectedIndex不会刷新
          selectedIndex = index;
          print("selected index: $index");
        },
      ),
      tabBuilder: (ctx, index){
        return CupertinoTabView(
          builder: (ctx){
            return this.getPages(index);
          },
          routes: _routes,
        );
      },
    );
  }
}

class _HomeCupertinoPage extends StatelessWidget{
  @override
  Widget build(BuildContext context) {
    return CupertinoPageScaffold(
      //这里需要加SafeArea，不然child的垂直会是从屏目顶部开始
      //加了SafeArea后是从NvBar下方开始
      child: SafeArea(child: Text("Home")),
//              child: Text("Home"),
      navigationBar: CupertinoNavigationBar(
        middle: Text("Home"),
      ),
    );
  }
}

class _SchoolCupertinoPage extends StatelessWidget{
  _SchoolCupertinoPage({@required this.toHome}):super();
  VoidCallback toHome;

  @override
  Widget build(BuildContext context) {
    return CupertinoPageScaffold(
      child: SafeArea(
          child: Center(
            child: RaisedButton(
                onPressed: (){
                  this.toHome();
                },
              child: Text("To Home"),
            ),
          )
      ),
      navigationBar: CupertinoNavigationBar(
        middle: Text("School"),
        leading: RaisedButton(
          onPressed: (){
            this.toHome();
          },
          child: Icon(Icons.home),
        ),
      ),
    );
  }
}

class _SelfCupertinoPage extends StatelessWidget{
  double size = 40.0;
  @override
  Widget build(BuildContext context) {
    return CupertinoPageScaffold(
      child: SafeArea(
        child: Center(
          child: Container(
            width: size,
            height: size,
            decoration: BoxDecoration(
              borderRadius: BorderRadius.circular(size / 2),
              color: Colors.green
            ),
            child: Center(
              child: Text(
                  "30",
                  style: TextStyle(
                    color: Colors.white,
                    fontSize: 20
                  ),
              ),
            ),
          ),
        ),
      ),
    );
  }
}