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'),];@overrideWidget build(BuildContext context) {return Scaffold(appBar: AppBar(title: Text('BottomNavigationBar Sample'),),body: Center(child: _widgetOptions.elementAt(_selectedIndex),),bottomNavigationBar: BottomNavigationBar(items: [BottomNavigationBarIte