How to preserve widget states in flutter, when navigating using BottomNavigationBar?

For keeping state in BottomNavigationBar, you can use IndexedStack

    @override
      Widget build(BuildContext context) {
        return Scaffold(
          bottomNavigationBar: BottomNavigationBar(
            onTap: (index) {
              setState(() {
                current_tab = index;
              });
            },
            currentIndex: current_tab,
            items: [
              BottomNavigationBarItem(
                ...
              ),
              BottomNavigationBarItem(
                ...
              ),
            ],
          ),
          body: IndexedStack(
            children: <Widget>[
              PageOne(),
              PageTwo(),
            ],
            index: current_tab,
          ),
        );
      }

Late to the party, but I've got a simple solution. Use the PageView widget with the AutomaticKeepAliveClinetMixin.

The beauty of it that it doesn't load any tab until you click on it.

The page that includes the BottomNavigationBar:

var _selectedPageIndex;
List<Widget> _pages;
PageController _pageController;

@override
void initState() {
  super.initState();

  _selectedPageIndex = 0;
  _pages = [
    //The individual tabs.
  ];

  _pageController = PageController(initialPage: _selectedPageIndex);
}

@override
void dispose() {
  _pageController.dispose();

  super.dispose();
}

@override
Widget build(BuildContext context) {
  ...
    body: PageView(
      controller: _pageController,
      physics: NeverScrollableScrollPhysics(),
      children: _pages,
    ),
   bottomNavigationBar: BottomNavigationBar(
      ...
      currentIndex: _selectedPageIndex,
      onTap: (selectedPageIndex) {
        setState(() {
          _selectedPageIndex = selectedPageIndex;
          _pageController.jumpToPage(selectedPageIndex);
        });
      },
  ...
}

The individual tab:

class _HomeState extends State<Home> with AutomaticKeepAliveClientMixin<Home> {
  @override
  bool get wantKeepAlive => true;

  @override
  Widget build(BuildContext context) {
    //Notice the super-call here.
    super.build(context);
    ...
  }
}

I've made a video about it here.