class AddPatientView extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
    return _AddPatientViewState();
  }
}

class _AddPatientViewState extends State<AddPatientView> {
  int _currentIndex = 0;
  List<Widget> _children;
  List<File> _imageFileList = new List<File>();

  @override
  void initState() {
    super.initState();
    _children = [
      AddPatientInfo(savePatient),AddPatientimages(_imageFileList)
    ];
  }


  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("New Patient Record"),),body: _children[_currentIndex],bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,items: [
          BottomNavigationBarItem(icon: new Icon(Icons.create),title: new Text('Info')),BottomNavigationBarItem(icon: new Icon(Icons.camera_alt),title: new Text('Images')),],onTap: (int index) {
          setState(() {
            _currentIndex = index;
          });
        },);
  }
}

class AddPatientInfo extends StatefulWidget {
  final Function savePatient;

  AddPatientInfo(this.savePatient){
  }

  @override
  State<StatefulWidget> createState() {
    return _AddPatientInfoState();
  }
}

class _AddPatientInfoState extends State<AddPatientInfo> with AutomaticKeepAliveClientMixin<AddPatientInfo> {

  Function _savePatient;
  String _firstName,_lastName,_gender,_phone,_email,_diabetesMeds,_hypertensionMeds,_others;
  int _age,_diabetesYears,_hypertensionYears,_smokesPerDay,_smokerYears;
  bool _diabetes = false,_hypertension = false,_smoker = false,_chestPain = false,_cva = false,_ckd = false,_breathlessness = false,_syncope = false,_sweating = false,_sweatingFeet = false;

  List<String> _genderList = new List<String>();
  List<String> _yesNoList = new List<String>();
  List<File> _imageFileList = new List<File>();

  @override
  void initState() {
    super.initState();
    _savePatient = widget.savePatient;
    _genderList.addAll(['Male','Female','Other']);
    _yesNoList.addAll(['Yes','No']);
    _gender = _genderList.elementAt(0);
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      resizetoAvoidBottomPadding: false,body: Container(
        margin: EdgeInsets.all(10.0),child: Form(
          child: new ListView(
            children: <Widget>[
              TextField(
                decoration: Inputdecoration(
                    labelText: 'Patient First Name',labelStyle: TextStyle(
                        color: Colors.black
                    ),hintText: 'Enter patients first name'
                ),onChanged: (String value) {
                  setState(() {
                    _firstName = value;
                  });
                },TextField(
                decoration: Inputdecoration(
                    labelText: 'Patient Last Name',hintText: 'Enter patients last name'
                ),onChanged: (String value) {
                  setState(() {
                    _lastName = value;
                  });
                },//other textfield widgets below
            ],)
      ),);
  }

  @override
  bool get wantKeepAlive => true;
}

解决方法

/// A mixin with convenience methods for clients of [AutomaticKeepAlive]. Used
/// with [State] subclasses.
///
/// Subclasses must implement [wantKeepAlive],and their [build] methods must
/// call `super.build` (the return value will always return null,and should be
/// ignored).

Widget build(BuildContext context) {
    super.build(context);
    return Scaffold(...);
  }

底栏切换每次都重新请求是一件非常恶心的事，flutter 中提供了AutomaticKeepAliveClientMixin 帮我们完成页面状态保存效果。

1、AutomaticKeepAliveClientMixin

AutomaticKeepAliveClientMixin 这个 Mixin 是 Flutter 为了保持页面设置的。哪个页面需要保持页面状态，就在这个页面进行混入。

不过使用使用这个 Mixin 是有几个先决条件的：

• 使用的页面必须是 StatefulWidget，如果是 StatelessWidget 是没办法办法使用的。
• 其实只有两个前置组件才能保持页面状态：PageView 和 IndexedStack。
• 重写 wantKeepAlive 方法，如果不重写也是实现不了的。

2、修改index_page.dart

明白基本知识之后，就可以修改 index_page.dart，思路就是增加一个 IndexedStack 包裹在 tabBodies 外边。

整体代码如下：

import ''package:flutter/cupertino.dart'';
import ''package:flutter/material.dart'';
import ''home_page.dart'';
import ''category_page.dart'';
import ''cart_page.dart'';
import ''member_page.dart'';


class IndexPage extends StatefulWidget {
  _IndexPageState createState() => _IndexPageState();
}

class _IndexPageState extends State<IndexPage>{

  PageController _pageController;

  final List<BottomNavigationBarItem> bottomTabs = [
    BottomNavigationBarItem(
      icon:Icon(CupertinoIcons.home),
      title:Text(''首页'')
    ),
    BottomNavigationBarItem(
      icon:Icon(CupertinoIcons.search),
      title:Text(''分类'')
    ),
    BottomNavigationBarItem(
      icon:Icon(CupertinoIcons.shopping_cart),
      title:Text(''购物车'')
    ),
     BottomNavigationBarItem(
      icon:Icon(CupertinoIcons.profile_circled),
      title:Text(''会员中心'')
    ),
  ];

  final List<Widget> tabBodies = [
    HomePage(),
    CategoryPage(),
    CartPage(),
    MemberPage()
  ];

  int currentIndex= 0;
  var currentPage ;

  @override
  void initState() {
   currentPage=tabBodies[currentIndex];
   _pageController=new PageController()
      ..addListener(() {
        if (currentPage != _pageController.page.round()) {
          setState(() {
            currentPage = _pageController.page.round();
          });
        }
  });
  super.initState();
  }

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      backgroundColor: Color.fromRGBO(244, 245, 245, 1.0),
      bottomNavigationBar: BottomNavigationBar(
        type:BottomNavigationBarType.fixed,
        currentIndex: currentIndex,
        items:bottomTabs,
        onTap: (index){
          setState(() {
           currentIndex=index;
            currentPage =tabBodies[currentIndex]; 
          });

        },
      ),
      body: IndexedStack(
        index: currentIndex,
        children: tabBodies
      )
    );
  }
}

3、加入Mixin保持页面状态

在 home_page.dart 里加入 AutomaticKeepAliveClientMixin 混入，加入后需要重写 wantKeepAlive 方法。

主要代码如下：

class _HomePageState extends State<HomePage> with AutomaticKeepAliveClientMixin {

  @override
  bool get wantKeepAlive =>true;
}

为了检验结果，我们在 HomePageState 里增加一个 initState，在里边 print 一些内容，如果内容输出了，证明我们的页面重新加载了，如果没输出，证明我们的页面保持了状态。

@override
void initState() {
    super.initState();
    print(''我打印了哈哈哈哈哈'');
}

class ApplicationPage extends StatefulWidget {
　　//@override
　　//_ApplicationPageState createState() => new _ApplicationPageState();
　　 等同于上边注释掉的 createState();
    @override
    State<StatefulWidget> createState() {
      // TODO: implement createState
      return _ApplicationPageState();
    }

}

class _ApplicationPageState extends State<ApplicationPage> {

  int _currentPageIndex = 0;
  var _pageController = new PageController(initialPage: 0);

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
      appBar:
       new AppBar(
         title: new Text("我是AppBar"),
         centerTitle: true,
       ),
      body: new PageView.builder(
          onPageChanged:_pageChange,
          controller: _pageController,
          itemBuilder: (BuildContext context,int index){
            return index==1?new Text("我是第一页"):new Text("我是第二页");
          },
          itemCount: 2,
      ),
      bottomNavigationBar: new BottomNavigationBar(items: [
              BottomNavigationBarItem(
                  icon: new Icon(Icons.category), title: new Text("首页")),
              BottomNavigationBarItem(
                  icon: new Icon(Icons.message), title: new Text("我的")),
          ],
          currentIndex: _currentPageIndex,
          onTap: onTap,
      ),
    );
  }

  // bottomnaviagtionbar 和 pageview 的联动
  void onTap(int index) {
    // 过pageview的pagecontroller的animateToPage方法可以跳转
    _pageController.animateToPage(index,
        duration: const Duration(milliseconds: 300), curve: Curves.ease);
  }

  void _pageChange(int index) {
    setState(() {
      if (_currentPageIndex != index) {
        _currentPageIndex = index;
      }
    });
  }

}

import ''package:flutter/material.dart'';
import ''News.dart'';
import ''Video.dart'';
import ''Chat.dart'';
import ''MyId.dart'';

class AppTwo extends StatelessWidget {
  @override
  Widget build(BuildContext context) => MaterialApp(home: Home());
}

class Home extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => _HomeState();
}

class _HomeState extends State<Home> {
  int _currentIndex = 0;
  final List<Widget> _children = [
    NewsPage(),
    VideoPage(),
    ChatPage(),
    MyIdPage()
  ];

  @override
  Widget build(BuildContext context) {
    return new Scaffold(
//      appBar: AppBar(
//        title: Text("第二种方式"),
//        centerTitle: true,
//      ),
      body: _children[_currentIndex],
//      CupertinoTabBar 是IOS分格
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        onTap: onTabTapped,
        items: [
          BottomNavigationBarItem(
              title: new Text("Home"), icon: new Icon(Icons.home)),
          BottomNavigationBarItem(
              title: new Text("List"), icon: new Icon(Icons.list)),
          BottomNavigationBarItem(
              title: new Text("Message"), icon: new Icon(Icons.message)),
        ],
      ),
    );

  }

  void onTabTapped(int index) {
    setState(() {
      _currentIndex = index;
    });
  }
}

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

void main() => runApp(new AppTwo());