Flutter - Modify AppBar from a page

Question

So I have a Flutter application with multiple pages, this is done via a PageView. Before this page view I create my AppBar so it is persistent at the top of the application and doesn't animate when scrolling between pages. I then want on one of the pages to create a bottom App bar, but for that I need to access the App bar element, however I have no idea how to do this.

This is the main class, the page I am trying to edit the app bar on is PlanPage.

final GoogleSignIn googleSignIn = GoogleSignIn();
final FirebaseAuth auth = FirebaseAuth.instance;

void main() {
  runApp(MyApp());
}

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

    Widget _handleCurrentScreen() {
        return StreamBuilder<FirebaseUser>(
            stream: auth.onAuthStateChanged,
            builder: (BuildContext context, snapshot) {
                print(snapshot);
                if (snapshot.connectionState == ConnectionState.waiting) {
                    return SplashPage();
                } else {
                    if (snapshot.hasData) {
                        return Home();
                    }
                    return LoginPage();
                }
            }
        );
    }
}

class Home extends StatefulWidget {
  @override
  State<StatefulWidget> createState() {
      return HomeState();
  }
}

class HomeState extends State<Home> {
    PageController _pageController;

    PreferredSizeWidget bottomBar;

    int _page = 0;


  @override
  Widget build(BuildContext context) {
      return Scaffold(
          appBar: AppBar(
              bottom: bottomBar,
          ),
          body: PageView(
              children: [
                  Container(
                      child: SafeArea(
                          child: RecipesPage()
                      ),
                  ),
                  Container(
                      child: SafeArea(
                          child: PlanPage()
                      ),
                  ),
                  Container(
                      child: SafeArea(
                          child: ShoppingListPage()
                      ),
                  ),
                  Container(
                      child: SafeArea(
                          child: ExplorePage()
                      ),
                  ),
              ],

              /// Specify the page controller
              controller: _pageController,
              onPageChanged: onPageChanged
          ),
          bottomNavigationBar: BottomNavigationBar(
              type: BottomNavigationBarType.fixed,
              items: [
                  BottomNavigationBarItem(
                      icon: Icon(Icons.book),
                      title: Text('Recipes')
                  ),
                  BottomNavigationBarItem(
                      icon: Icon(Icons.event),
                      title: Text('Plan')
                  ),
                  BottomNavigationBarItem(
                      icon: Icon(Icons.shopping_cart),
                      title: Text('Shopping List')
                  ),
                  BottomNavigationBarItem(
                      icon: Icon(Icons.public),
                      title: Text("Explore"),
                  ),
              ],
              onTap: navigationTapped,
              currentIndex: _page,
          ),

      );
  }

    void onPageChanged(int page){
        setState((){
            this._page = page;
        });
    }

    void setBottomAppBar(PreferredSizeWidget appBar) {
        this.bottomBar = appBar;
        print("setBottomAppBar: "+ appBar.toString());
    }

    /// Called when the user presses on of the
    /// [BottomNavigationBarItem] with corresponding
    /// page index
    void navigationTapped(int page){

        // Animating to the page.
        // You can use whatever duration and curve you like
        _pageController.animateToPage(
            page,
            duration: const Duration(milliseconds: 300),
            curve: Curves.ease
        );
    }

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

        _pageController = PageController();
    }

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

The PlanPage class looks like this

class PlanPage extends StatefulWidget {
    var homeState;

    PlanPage(this.homeState);

    @override
    State<StatefulWidget> createState() {
        return _PlanState(homeState);
    }

}

class _PlanState extends State<PlanPage> with AutomaticKeepAliveClientMixin<PlanPage>, SingleTickerProviderStateMixin {
    var homeState;
    TabController _tabController;

    _PlanState(this.homeState);

    @override
    bool get wantKeepAlive => true;

    @override
    Widget build(BuildContext context) {
        //homeState.setBottomAppBar(_buildTabBar());

        return Scaffold(
            appBar: AppBar(
                bottom: _buildTabBar(),
            ),
            body: TabBarView(
                controller: _tabController,
                children: Plan.now().days.map((day) {
                    return ListView.builder(
                        itemCount: MealType.values.length,
                        itemBuilder: (BuildContext context, int index){
                            var mealType = MealType.values[index];
                            return Column(
                                children: <Widget>[
                                    Text(
                                        mealType.toString().substring(mealType.toString().indexOf('.')+1),
                                        style: TextStyle(
                                            //decoration: TextDecoration.underline,
                                            fontSize: 30.0,
                                            fontWeight: FontWeight.bold
                                        ),
                                    ),
                                    Column(
                                        children: day.meals.where((meal) => meal.mealType == mealType).map((meal) {
                                            return RecipeCard(meal.recipe);
                                        }).toList(),
                                    )

                                ],
                            );
                        }
                    );
                }).toList(),
            )
        );
    }

    Widget _buildTabBar() {
        return TabBar(
            controller: _tabController,
            isScrollable: true,
            tabs: List.generate(Plan.now().days.length,(index) {
                return Tab(
                    child: Column(
                        children: <Widget>[
                            Text(DateFormat.E().format(Plan.now().days[index].day)),
                            Text(DateFormat('d/M').format(Plan.now().days[index].day)),
                        ],
                    ),
                );
            }, growable: true),
        );
    }

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

        _tabController = new TabController(
            length: Plan.now().days.length,
            vsync: this,
            initialIndex: 1
        );
    }
}

However the way it works now, makes it show 2 app bars.[

Answer 1

Usually it's a not a best practice to have two nested scrollable areas. Same for two nested Scaffolds.

That said, you can listen to page changes ( _pageController.addListener(listener) ) to update a page state property, and build a different AppBar.bottom (in the Home widget, so you can remove the Scaffold in PlanPage) depending on the page the user is viewing.

-EDIT-

In your Home widget you can add a listener to the _pageController like so:

void initState() {
    super.initState();
    _pageController = PageController()
        ..addListener(() {
            setState(() {});
        });
}

to have your widget rebuilt every time the user scrolls within your PageView. The setState call with an empty function might looks confusing, but it simply allows you to have the widget rebuilt when _pageController.page changes, which is not the default behavior. You could also have a page state property and update it in the setState call to reflect the _pageController.page property, but the result would be the same.

This way you can build a different AppBar.bottom depending on the _pageController.page:

// in your build function

final bottomAppBar = _pageController.page == 2 ? TabBar(...) : null;

final appBar = AppBar(
    bottom: bottomAppBar,
    ...
);

return Scaffold(
    appBar: appBar,
    ...
);