底部导航 bottomNavigationBar

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import 'views/Index.dart';
import 'views/Me.dart';
import 'views/News.dart';
import 'views/Products.dart';

import 'package:flutter/material.dart';

class App extends StatefulWidget {
  @override
  _AppState createState() => _AppState();
}

class _AppState extends State<App> {
  var _currentIndex = 0;

  Index index;
  Me me;
  News news;
  Products products;

  currentPage() {
    switch (_currentIndex) {
      case 0:
        if (index == null) {
          index = Index();
        }
        return index;
      case 1:
        if (news == null) {
          news = News();
        }
        return news;
      case 2:
        if (products == null) {
          products = Products();
        }
        return products;
      case 3:
        if (me == null) {
          me = Me();
        }
        return me;
    }
  }

  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('First Route'),
      ),
      body: currentPage(),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        onTap: ((index) {
          setState(() {
            _currentIndex = index;
          });
        }),
        items: [
          BottomNavigationBarItem(
              title: Text('shouye'), icon: Icon(Icons.home)),
          BottomNavigationBarItem(
              icon: Icon(Icons.search), title: Text('search'))
        ],
      ),
    );
  }
}