页面保持状态，不重绘，在app中很重要，比如看着朋友圈/热门微博，看到一半，需要切换到其他tab设置点东西，再切换回来，这个时候肯定不需要页面重绘，然后从头开始看吧，而是希望直接接着前面的状态继续看。
这样我们就需要用到页面保持状态，使他不销毁不重绘。

关键词：

BottomNavigationBar+PageView+AutomaticKeepAliveClientMixin

要点

子页面的class

class _HomePageState with AutomaticKeepAliveClientMixin{//要点1


  @override
  bool get wantKeepAlive => true;//要点2

  Widget build(BuildContext context){
    super.build(context);//要点3
  }
}

光上面其实还不够，还需要搭配其他Widget使用，例如BottomNavigationBar+PageView活着TabBar+TabbarView，这样页面才不会销毁。

//PageView是重点的重点
Scaffold(
        body:PageView.builder(
          physics: NeverScrollableScrollPhysics(),//禁止页面左右滑动切换
          controller: _pageController,
          onPageChanged: _pageChanged,
          itemCount: _pages.length,
          itemBuilder: (context,index)=>_pages[index]
        )
        bottomNavigationBar:...
)

完整代码

frame.dart

class MyApp extends StatefulWidget{
  @override
  _MyAppState createState() => _MyAppState();
}

class _MyAppState extends State<MyApp>{
  var _pages = [HomePage(),ListViewDemo(),DemoPage(),UserListPage()]; 
  int _tabIndex = 0;

  var _pageController =PageController();

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

  Widget build(BuildContext context){
    return MaterialApp(
      theme: ThemeData(
        primaryColor: Colors.pinkAccent
      ),
      
      home:Scaffold(
        body:PageView.builder(//要点1
          physics: NeverScrollableScrollPhysics(),//禁止页面左右滑动切换
          controller: _pageController,
          onPageChanged: _pageChanged,//回调函数
          itemCount: _pages.length,
          itemBuilder: (context,index)=>_pages[index]
        ),
        bottomNavigationBar:BottomNavigationBar(
          items: <BottomNavigationBarItem>[
            BottomNavigationBarItem(title:Text('主页'),icon: Icon(Icons.home)),
            BottomNavigationBarItem(title:Text('商城'),icon: Icon(Icons.shopping_basket)),
            BottomNavigationBarItem(title:Text('测试'),icon: Icon(Icons.pageview)),
            BottomNavigationBarItem(title:Text('我的'),icon: Icon(Icons.account_box)),
          ],
          onTap: (index){
            print('onTap');
            _pageController.jumpToPage(index);
  
          },      
          type:BottomNavigationBarType.fixed,
          currentIndex: _tabIndex,  
        ),
       
        
      )    
      );
  }

  void _pageChanged(int index){
    print('_pageChanged');
    setState(() {
      if(_tabIndex != index)
        _tabIndex = index;
    });
  }
  
}

demo_page.dart

class DemoPage extends StatefulWidget {
  _DemoPageState createState() => _DemoPageState();
}

class _DemoPageState extends State<DemoPage> with AutomaticKeepAliveClientMixin{//要点1
  int _count = 0;

  @override
  bool get wantKeepAlive => true;//要点2

  @override
  Widget build(BuildContext context) {
    super.build(context);//要点3
    return Scaffold(
      appBar: AppBar(title:Text('Demo Page')),
      body: Center(child:Text('当前计数:$_count')),
      floatingActionButton: FloatingActionButton(
        child: Icon(Icons.add),
        onPressed: (){
          setState(() {
            _count++;
          });
        },
      ),
    );
  }
}