十七 状态保持 AutomaticKeepAliveClientMixin

StatefulWidget需要保持状态，切换页面后，Widget树不会dispose,保持原有的状态：

• 混入AutomaticKeepAliveClientMixin抽象类，
• 重写方法wantKeepAlive
• 在build方法中 调用父类的方法 super.build(context) ```dart

class ChatPage extends StatefulWidget { @override _ChatPageState createState() => _ChatPageState(); }

class _ChatPageState extends State with AutomaticKeepAliveClientMixin {

@override bool get wantKeepAlive => true; @override Widget build(BuildContext context) { super.build(context); return Scaffold( appBar: AppBar( title: Text(‘微信’), backgroundColor: WeChatThemeColor,

  ),
  );

} }

使用bottomNavigationBar的情况下，要保持pages的状态，需使用PageController
```dart
import 'package:flutter/material.dart';
import 'pages/chat_page.dart';
import 'pages/friends/friends_page.dart';
import 'pages/mine_page.dart';
import 'pages/discover/discover_page.dart';
class RootPage extends StatefulWidget {
  @override
  _RootPageState createState() => _RootPageState();
}
class _RootPageState extends State<RootPage> {
  int _currentIndex = 0;
  final PageController _controller = PageController(initialPage: 0);
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: PageView(
        controller: _controller,
        physics: NeverScrollableScrollPhysics(), //不滚动
        children: [
          ChatPage(),
          FriendsPage(),
          DiscoverPage(),
          MinePage()],
      ),
      bottomNavigationBar: BottomNavigationBar(
        onTap: (index) {
          setState(() {
            _currentIndex = index;
            _controller.animateToPage(index, duration: Duration(microseconds: 100), curve: Curves.easeIn);
          });
        },
        selectedFontSize: 12.0, //选中的字体大小
        currentIndex: _currentIndex,
        fixedColor: Colors.green,
        type: BottomNavigationBarType.fixed,
        items: [
          BottomNavigationBarItem(
            icon: Image.asset(
              'images/tabbar_chat.png',
              height: 20,
              width: 20,
            ),
            activeIcon: Image.asset(
              'images/tabbar_chat_hl.png',
              height: 20,
              width: 20,
            ),
            title: Text('微信'),
          ),
          BottomNavigationBarItem(
            icon: Image.asset(
              'images/tabbar_friends.png',
              height: 20,
              width: 20,
            ),
            activeIcon: Image.asset(
              'images/tabbar_friends_hl.png',
              height: 20,
              width: 20,
            ),
            title: Text('通讯录'),
          ),
          BottomNavigationBarItem(
            icon: Image.asset(
              'images/tabbar_discover.png',
              height: 20,
              width: 20,
            ),
            activeIcon: Image.asset(
              'images/tabbar_discover_hl.png',
              height: 20,
              width: 20,
            ),
            title: Text('发现'),
          ),
          BottomNavigationBarItem(
            icon: Image.asset(
              'images/tabbar_mine.png',
              height: 20,
              width: 20,
            ),
            activeIcon: Image.asset(
              'images/tabbar_mine_hl.png',
              height: 20,
              width: 20,
            ),
            title: Text('我的'),
          ),
        ],
      ),
    );
  }
}