下面的代码几乎是从官方flutter GitHub底部导航栏示例中复制粘贴而来的。问题是，在打开包含视图的小部件时，所有内容都是预先加载的。每次我按下底部导航栏元素时，都会重新加载所有视图。换句话说，当我按下第一个元素时，日志显示其他人（第二个和第三个）正在进行网络呼叫。当我按第二个时，日志显示第一个也在打网络电话。调试时，我发现每次从底部栏中选择一个小部件时，包含该小部件的小部件都会重新构建（有一个对方法build的调用）。

class MainScreen extends StatefulWidget{
    
    @override
      State<StatefulWidget> createState()=> MainScreenState();
    
}

class MainScreenState extends State<MainScreen> with TickerProviderStateMixin{
      GlobalKey<ScaffoldState> scaffoldState = new GlobalKey();
      int _currentSelection = 0;
      BottomNavigationBarType _navigationBarType = BottomNavigationBarType.fixed;
      Li

    st<NavigationIconView> _navigationIcons;

    @override
      void initState() {
       

    super.initState();
        _navigationIcons = <NavigationIconView>[
          new NavigationIconView(
        

    icon: const Icon(Icons.home),
        title: 'Главная',
        vsync: this

      ),
      new NavigationIconView(
        icon: const Icon(Icons.map),
        title: 'Квесты',
        vsync: this
      ),
      new NavigationIconView(
        icon: const Icon(Icons.dehaze),
        title: 'Профиль',
        vsync: this
      )
    ];

    for(NavigationIconView v in _navigationIcons)
      v.controller.addListener(_rebuild);

    _navigationIcons[_currentSelection].controller.value = 1.0;
      }

    @override
      void dispose() {
        for(NavigationIconView v in _navigationIcons)
          v.controller.dispose();
        super.dispose();
    }

     @override
      Widget build(BuildContext context) {
        final BottomNavigationBar botNavBar = new BottomNavigationBar(
          items: _navigationIcons
              .map((NavigationIconView navigationView) => navigationView.item)
              .toList(),
          currentIndex: _currentSelection,
          fixedColor: Colors.green,
          type: _navigationBarType,
          onTap: (int index) {
            setState(() {
              _navigationIcons[_currentSelection].controller.reverse();
              _currentSelection = index;
              _navigationIcons[_currentSelection].controller.forward();
              print('pressed : $_currentSelection');
            });
          },
        );
        return new Scaffold(
          key: scaffoldState,
          body: new  Center(
            key: new Key('Main view container'),
            child: new FutureBuilder<Widget>(
                future: _buildTransitionsStack(),
                builder: (BuildContext context, AsyncSnapshot<Widget> snapshot){
                  if(!snapshot.hasError) return snapshot.data;
                  else{
                    print('sh3t happened in main : ${snapshot.error}');
                  }
                }
            ),
          ),
      bottomNavigationBar: botNavBar,

    );
  }

    Future<Widget> _buildTransitionsStack() {
        final List<FadeTransition> transitions = <FadeTransition>[];
    
        return _showMain().then((mainWidget){
          transitions.add(_navigationIcons[0]
           

    .transition(_navigationBarType,mainWidget, context));
          print('size ${transitions.length}');
        }).then((_){
          transitions.add(_navigationIcons[1].transition(_navigationBarType,
              _showQuest(), context));
    
          transitions.add(_navigationIcons[2].transition(_navigationBarType,
              _showProfile(), context));
    
          transitions.sort((FadeTransition a, FadeTransition b) {
          final Animation<double> aAnimation = a.opacity;
          final Animation<double> bAnimation = b.opacity;
          final double aValue = aAnimation.value;
          final double bValue = bAnimation.value;
          return aValue.compareTo(bValue);
        });
    
          return new Stack(children: transitions);
        });
        }

  Future<Widget> _showMain(){
     return _getToken().then((token){
      return new FeedView(token);
    });
  }

  Widget _showQuest(){
//    return DetailableListScreen(ViewModelType.QUEST);

    return new QuestScreen();

  }

  Widget _showProfile(){
    return new Text('profile');
//    TODO
  }

  void _rebuild() {
    setState(() {

    });
  }

  Future<String> _getToken() async{
    return await SharedPreferences.getInstance()
        .then((SharedPreferences sp)=> sp.getString(TOKEN)
    );
  }
}}