Flutter 底部导航栏BottomNavigationBar，并关联PageView实现滑动切换

效果图：

BottomNavigationBar

先来看一下官方的sample code：

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key}) : super(key: key);

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State {
  int _selectedIndex = 1;
  final _widgetOptions = [
    Text('Index 0: Home'),
    Text('Index 1: Business'),
    Text('Index 2: School'),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('BottomNavigationBar Sample'),
      ),
      body: Center(
        child: _widgetOptions.elementAt(_selectedIndex),
      ),
      bottomNavigationBar: BottomNavigationBar(
        items: [
          BottomNavigationBarItem(icon: Icon(Icons.home), title: Text('Home')),
          BottomNavigatio