Flutter为了节约内存不会保存widget状态，widget都是零时变量。当我们切换tab的时候就会重新加载创建。
使用AutomaticKeepAliveClientMixin保持状态

class _HomePageState extends State<HomePage> with AutomaticKeepAliveClientMixin{
  @override
  bool get wantKeepAlive => true;
  ...
  }

下拉刷新需要引入包

import 'package:flutter_easyrefresh/easy_refresh.dart';
//需要有key
GlobalKey<RefreshFooterState> _footerKey = new GlobalKey<RefreshFooterState>();
.....其余代码
	return EasyRefresh(
		     refreshFooter: ClassicsFooter(
		       key: _footerKey,
		       bgColor: Colors.white,
		       textColor: Colors.blueGrey,
		       moreInfoColor: Colors.blueGrey,
		       showMore: true,
		       noMoreText: 'no more',
		       moreInfo: '加载中',
		       loadReadyText: '上拉加载',
		     ),
		     child: ListView(
		       children: <Widget>[
		
		       ],
		     ),
		     loadMore: () async {
		       print("调后台接口异步加载更多");
		     },
   );