CODEWITHSUNDEEP
dartflutter
ArtBelch
ArtBelch

Reputation: 1399

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

I'm currently working on building a Flutter app that will preserve states when navigating from one screen, to another, and back again when utilizing BottomNavigationBar. Just like it works in the Spotify mobile application; if you have navigated down to a certain level in the navigation hierarchy on one of the main screens, changing screen via the bottom navigation bar, and later changing back to the old screen, will preserve where the user were in that hierarchy, including preservation of the state.

I have run my head against the wall, trying various different things without success.

I want to know how I can prevent the pages in pageChooser(), when toggled once the user taps the BottomNavigationBar item, from rebuilding themselves, and instead preserve the state they already found themselves in (the pages are all stateful Widgets).

import 'package:flutter/material.dart';
import './page_plan.dart';
import './page_profile.dart';
import './page_startup_namer.dart';

void main() => runApp(new Recipher());

class Recipher extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return new Pages();
  }
}

class Pages extends StatefulWidget {
  @override
  createState() => new PagesState();
}

class PagesState extends State<Pages> {
  int pageIndex = 0;
  

  pageChooser() {
    switch (this.pageIndex) {
      case 0:
        return new ProfilePage();
        break;
      
      case 1:
        return new PlanPage();
        break;

      case 2:
        return new StartUpNamerPage(); 
        break;  
      
      default:
        return new Container(
          child: new Center(
            child: new Text(
              'No page found by page chooser.',
              style: new TextStyle(fontSize: 30.0)
              )
            ),
          );     
    }
  }

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
      home: new Scaffold(
        body: pageChooser(),
        bottomNavigationBar: new BottomNavigationBar(
          currentIndex: pageIndex,
          onTap: (int tappedIndex) { //Toggle pageChooser and rebuild state with the index that was tapped in bottom navbar
            setState(
              (){ this.pageIndex = tappedIndex; }
              ); 
            },
          items: <BottomNavigationBarItem>[
            new BottomNavigationBarItem(
              title: new Text('Profile'),
              icon: new Icon(Icons.account_box)
              ),
              new BottomNavigationBarItem(
                title: new Text('Plan'),
                icon: new Icon(Icons.calendar_today)
              ),
                new BottomNavigationBarItem(
                title: new Text('Startup'),
                icon: new Icon(Icons.alarm_on)
              )
            ],
          )
      )
    );
  }
}

Upvotes: 139

Views: 92943

Answers (12)

Tayan
Tayan

Reputation: 2307

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

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.

Upvotes: 125

ED-209
ED-209

Reputation: 4746

All the examples here are using BottomNavigationBar which has now (over 6 years since the question was asked - which admittedly refers to specifically to BottomNavigationBar) been replaced for Material 3 by NavigationBar, and none of them seem to use a PageTransitionSwitcher, which to me is a default requirement for any app I build.

PageStorage is what I use to fulfil the OP's needs. Even the official Flutter documentation example of this uses an older BottomNavigationBar so I decided to share my implementation with the newer NavigationBar and a lovely FadeThroughTransition in this blog post.

Upvotes: 0

ibrahim Eltayfe
ibrahim Eltayfe

Reputation: 289

You can use a Lazy IndexedStack.
Normally, an IndexedStack loads all its child pages at once, which is not very good practice.

You can load pages only when needed by maintaining a list of loaded pages indexes and conditionally load pages as the user navigates between tabs.

class Home extends StatefulWidget {
  const Home({super.key});

  @override
  State<Home> createState() => _HomeState();
}

class _HomeState extends State<Home> {
  int _currentTab = 0;
  final List<int> loadedPages = [0];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      bottomNavigationBar: BottomNavBar(
        currentTab: _currentTab,
        onTap: (index){
          if (!loadedPages.contains(index)) {
            loadedPages.add(index);
          }

          setState(() {
            _currentTab = index;
          });
        },
      ),
      body:  IndexedStack(
        index: _currentTab,
        children: [
          loadedPages.contains(0)
            ? const FirstTab()
            : const SizedBox.shrink(),

          loadedPages.contains(1)
            ? const SecondTab()
            : const SizedBox.shrink(),

          loadedPages.contains(2)
            ? const ThirdTab()
            : const SizedBox.shrink(),
          )
        ],
      ),
    );
  }
}

Upvotes: 1

Charles Crete
Charles Crete

Reputation: 964

Use AutomaticKeepAliveClientMixin to force your tab content to not be disposed.

class PersistantTab extends StatefulWidget {
  @override
  _PersistantTabState createState() => _PersistantTabState();
}

class _PersistantTabState extends State<PersistantTab> with AutomaticKeepAliveClientMixin {
  @override
  Widget build(BuildContext context) {
    return Container();
  }

  // Setting to true will force the tab to never be disposed. This could be dangerous.
  @override
  bool get wantKeepAlive => true;
}

To make sure your tab does get disposed when it doesn't require to be persisted, make wantKeepAlive return a class variable. You must call updateKeepAlive() to update the keep alive status.

Example with dynamic keep alive:

// class PersistantTab extends StatefulWidget ...

class _PersistantTabState extends State<PersistantTab>
    with AutomaticKeepAliveClientMixin {
  bool keepAlive = false;

  @override
  void initState() {
    doAsyncStuff();
  }

  Future doAsyncStuff() async {
    keepAlive = true;
    updateKeepAlive();
    // Keeping alive...

    await Future.delayed(Duration(seconds: 10));

    keepAlive = false;
    updateKeepAlive();
    // Can be disposed whenever now.
  }

  @override
  bool get wantKeepAlive => keepAlive;

  @override
  Widget build(BuildContext context) {
    super.build(context);
    return Container();
  }
}

Upvotes: 48

DuocNP
DuocNP

Reputation: 64

Summary of Solutions: Two Approaches Offered

Solution 1: AutomaticKeepAliveClientMixin

class SubPage extends StatefulWidget {
  @override
  State<SubPage> createState() => _SubPageState();
}

class _SubPageState extends State<SubPage> with AutomaticKeepAliveClientMixin {
  @override
  Widget build(BuildContext context) {
    super.build(context); // Ensure that the mixin is initialized
    return Container();
  }

  @override
  bool get wantKeepAlive => true;
}
/*-------------------------------*/
class MainPage extends StatefulWidget {
  const MainPage({Key? key}) : super(key: key);

  @override
  State<MainPage> createState() => _MainPageState();
}

class _MainPageState extends State<MainPage> {
  int currentBottom = 0;

  static const List<Widget> _page = [
    SubPage(),
    SubPage(),
  ];

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        bottomNavigationBar: BottomNavigationBar(
          currentIndex: currentBottom,
          onTap: (index) => setState(() {
            currentBottom = index;
          }),
          items: const [
            BottomNavigationBarItem(icon: Icon(Icons.home)),
            BottomNavigationBarItem(icon: Icon(Icons.settings))
          ],
        ),
        body: _page.elementAt(currentBottom));
  }
}

Solution 2: IndexedStack

class MainPage extends StatefulWidget {
  @override
  _MainPageState createState() => _MainPageState();
}

class _MainPageState extends State<MainPage> {
  int _currentIndex = 0;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: IndexedStack(
        index: _currentIndex,
        children: <Widget>[
          Page1(),
          Page2(),
          Page3(),
        ],
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        onTap: (int index) {
          setState(() {
            _currentIndex = index;
          });
        },
        items: [
          BottomNavigationBarItem(
            icon: Icon(Icons.home),
            label: 'Page 1',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.search),
            label: 'Page 2',
          ),
          BottomNavigationBarItem(
            icon: Icon(Icons.settings),
            label: 'Page 3',
          ),
        ],
      ),
    );
  }
}
/*--------------------------------*/
class Page1 extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Container(
      // Page 1 content
    );
  }
}
// Repeat the above for Page2 and Page3

Upvotes: 2

maRci002
maRci002

Reputation: 381

This solution is based on CupertinoTabScaffold's implementation which won't load screens unnecessary.

import 'package:flutter/material.dart';

enum MainPage { home, profile }

class BottomNavScreen extends StatefulWidget {
  const BottomNavScreen({super.key});

  @override
  State<BottomNavScreen> createState() => _BottomNavScreenState();
}

class _BottomNavScreenState extends State<BottomNavScreen> {
  var currentPage = MainPage.home;

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: PageSwitchingView(
        currentPageIndex: MainPage.values.indexOf(currentPage),
        pageCount: MainPage.values.length,
        pageBuilder: _pageBuilder,
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: MainPage.values.indexOf(currentPage),
        onTap: (index) => setState(() => currentPage = MainPage.values[index]),
        items: const [
          BottomNavigationBarItem(
            label: 'Home',
            icon: Icon(Icons.home),
          ),
          BottomNavigationBarItem(
            label: 'Profile',
            icon: Icon(Icons.account_circle),
          ),
        ],
      ),
    );
  }

  Widget _pageBuilder(BuildContext context, int index) {
    final page = MainPage.values[index];

    switch (page) {
      case MainPage.home:
        return ...
      case MainPage.profile:
        return ...
    }
  }
}

/// A widget laying out multiple pages with only one active page being built
/// at a time and on stage. Off stage pages' animations are stopped.
class PageSwitchingView extends StatefulWidget {
  const PageSwitchingView({
    super.key,
    required this.currentPageIndex,
    required this.pageCount,
    required this.pageBuilder,
  });

  final int currentPageIndex;
  final int pageCount;
  final IndexedWidgetBuilder pageBuilder;

  @override
  State<PageSwitchingView> createState() => _PageSwitchingViewState();
}

class _PageSwitchingViewState extends State<PageSwitchingView> {
  final List<bool> shouldBuildPage = <bool>[];

  @override
  void initState() {
    super.initState();
    shouldBuildPage.addAll(List<bool>.filled(widget.pageCount, false));
  }

  @override
  void didUpdateWidget(PageSwitchingView oldWidget) {
    super.didUpdateWidget(oldWidget);

    // Only partially invalidate the pages cache to avoid breaking the current
    // behavior. We assume that the only possible change is either:
    // - new pages are appended to the page list, or
    // - some trailing pages are removed.
    // If the above assumption is not true, some pages may lose their state.
    final lengthDiff = widget.pageCount - shouldBuildPage.length;
    if (lengthDiff > 0) {
      shouldBuildPage.addAll(List<bool>.filled(lengthDiff, false));
    } else if (lengthDiff < 0) {
      shouldBuildPage.removeRange(widget.pageCount, shouldBuildPage.length);
    }
  }

  @override
  Widget build(BuildContext context) {
    return Stack(
      fit: StackFit.expand,
      children: List<Widget>.generate(widget.pageCount, (int index) {
        final active = index == widget.currentPageIndex;
        shouldBuildPage[index] = active || shouldBuildPage[index];

        return HeroMode(
          enabled: active,
          child: Offstage(
            offstage: !active,
            child: TickerMode(
              enabled: active,
              child: Builder(
                builder: (BuildContext context) {
                  return shouldBuildPage[index] ? widget.pageBuilder(context, index) : Container();
                },
              ),
            ),
          ),
        );
      }),
    );
  }
}

Upvotes: 1

Himmat rai
Himmat rai

Reputation: 87

proper way of preserving tabs state in bottom nav bar is by wrapping the whole tree with PageStorage() widget which takes a PageStorageBucket bucket as a required named parameter and for those tabs to which you want to preserve its state pas those respected widgets with PageStorageKey(<str_key>) then you are done !! you can see more details in this ans which i've answered few weeks back on one question : https://stackoverflow.com/a/68620032/11974847

there's other alternatives like IndexedWidget() but you should beware while using it , i've explained y we should be catious while using IndexedWidget() in the given link answer

good luck mate ..

Upvotes: 0

Swapnil Kadam
Swapnil Kadam

Reputation: 4293

Do not use IndexStack Widget, because it will instantiate all the tabs together, and suppose if all the tabs are making a network request then the callbacks will be messed up the last API calling tab will probably have the control of the callback.

Use AutomaticKeepAliveClientMixin for your stateful widget it is the simplest way to achieve it without instantiating all the tabs together.

My code had interfaces that were providing the respective responses to the calling tab I implemented it the following way.

Create your stateful widget

class FollowUpsScreen extends StatefulWidget {
  FollowUpsScreen();
        
  @override
  State<StatefulWidget> createState() {
    return FollowUpsScreenState();
  }
}
        
class FollowUpsScreenState extends State<FollowUpsScreen>
     with AutomaticKeepAliveClientMixin<FollowUpsScreen>
            implements OperationalControls {
    
  @override
  Widget build(BuildContext context) {
  //do not miss this line
    super.build(context);
    return .....;
  }

  @override
  bool get wantKeepAlive => true;
}

Upvotes: 3

Techalgoware
Techalgoware

Reputation: 620

Use “IndexedStack Widget” with “Bottom Navigation Bar Widget” to keep state of Screens/pages/Widget

Provide list of Widget to IndexedStack and index of widget you want to show because IndexedStack show single widget from list at one time.

final List<Widget> _children = [
    FirstClass(),
    SecondClass()
  ];

Scaffold(
  body: IndexedStack(
    index: _selectedPage,
    children: _children,
  ),
  bottomNavigationBar: BottomNavigationBar(
    ........
    ........
  ), 
);

Upvotes: 10

Newaj
Newaj

Reputation: 4450

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,
          ),
        );
      }

Upvotes: 130

Anirudh Agarwal
Anirudh Agarwal

Reputation: 2164

The most convenient way I have found to do so is using PageStorage widget along with PageStorageBucket, which acts as a key value persistent layer.

Go through this article for a beautiful explanation -> https://steemit.com/utopian-io/@tensor/persisting-user-interface-state-and-building-bottom-navigation-bars-in-dart-s-flutter-framework

Upvotes: 5

Hemanth Raj
Hemanth Raj

Reputation: 34769

Instead of returning new instance every time you run pageChooser, have one instance created and return the same.

Example:

class Pages extends StatefulWidget {
  @override
  createState() => new PagesState();
}

class PagesState extends State<Pages> {
  int pageIndex = 0;

  // Create all the pages once and return same instance when required
  final ProfilePage _profilePage = new ProfilePage(); 
  final PlanPage _planPage = new PlanPage();
  final StartUpNamerPage _startUpNamerPage = new StartUpNamerPage();


  Widget pageChooser() {
    switch (this.pageIndex) {
      case 0:
        return _profilePage;
        break;

      case 1:
        return _planPage;
        break;

      case 2:
        return _startUpNamerPage;
        break;

      default:
        return new Container(
          child: new Center(
              child: new Text(
                  'No page found by page chooser.',
                  style: new TextStyle(fontSize: 30.0)
              )
          ),
        );
    }
  }

  @override
  Widget build(BuildContext context) {
    return new MaterialApp(
        home: new Scaffold(
            body: pageChooser(),
            bottomNavigationBar: new BottomNavigationBar(
              currentIndex: pageIndex,
              onTap: (int tappedIndex) { //Toggle pageChooser and rebuild state with the index that was tapped in bottom navbar
                setState(
                        (){ this.pageIndex = tappedIndex; }
                );
              },
              items: <BottomNavigationBarItem>[
                new BottomNavigationBarItem(
                    title: new Text('Profile'),
                    icon: new Icon(Icons.account_box)
                ),
                new BottomNavigationBarItem(
                    title: new Text('Plan'),
                    icon: new Icon(Icons.calendar_today)
                ),
                new BottomNavigationBarItem(
                    title: new Text('Startup'),
                    icon: new Icon(Icons.alarm_on)
                )
              ],
            )
        )
    );
  }
}

Or you can make use of widgets like PageView or Stack to achieve the same.

Hope that helps!

Upvotes: 15

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