页面状态保持

状态保持需要做到以下两点：
1:with AutomaticKeepAliveMixIn
2:@override
　　bool get wantKeepAlive=>true;



完整示例：
main.dart:
import 'package:flutter/material.dart';
import 'pageview.dart';

void main()=>runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomePage(),
    );
  }
}

class HomePage extends StatefulWidget {
  @override
  _HomePageState createState() => _HomePageState();
}

class _HomePageState extends State<HomePage> with SingleTickerProviderStateMixin{
  TabController _tabController;
  @override
  void initState() {
    // TODO: implement initState
    _tabController=TabController(length: 3, vsync: this);
    super.initState();
  }
  @override
  void dispose() {
    // TODO: implement dispose
    _tabController.dispose();
    super.dispose();
  }
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('AutomaticKeepAlive'),
        bottom: TabBar(
          controller: _tabController,
          tabs: <Widget>[
            Tab(icon: Icon(Icons.send),text: 'send',),
            Tab(icon: Icon(Icons.event),text: 'event',),
            Tab(icon: Icon(Icons.map),text: 'map',),
          ],
        ),
      ),
      body: TabBarView(
        controller: _tabController,
        children: <Widget>[
          PageViewWidget(index:6),
          PageViewWidget(index:5),
          PageViewWidget(index:3),
        ],
      ),

    );
  }
}

pageview.dart:
import 'package:flutter/material.dart';
import 'dart:ui';

class PageViewWidget extends StatefulWidget {
  int index=0;
  PageViewWidget({Key key,@required this.index}):super(key:key);
  @override
  _PageViewWidgetState createState() => _PageViewWidgetState();
}

class _PageViewWidgetState extends State<PageViewWidget> with AutomaticKeepAliveClientMixin{
  int count=0;
  @override
  bool get wantKeepAlive=>true;

  @override
  Widget build(BuildContext context) {
    return Container(
      width: window.physicalSize.width,
      height: window.physicalSize.height,
      child: Stack(
        children: <Widget>[
          Image.network('http://www.ecobentech.com/images/${widget.index}.jpg',fit: BoxFit.cover,width: window.physicalSize.width,),
          Positioned(
            top: 200,
            left: 60,
            child: ActionChip(label: Text('$count'), onPressed:(){
              setState(() {
                count++;
              });
            }),
          )
        ],
      ),
    );
  }
}