Pass StreamBuilder between BottomNavigationBar tabs => Bad state: Stream has already been listened to

Question

I'm trying to pass Stream from a tab to another tab but when I comeback in the home.dart May I should close/destroy the Stream when tab changed? When the app runs the data are fetched correctly and everything is good.The problem appears only when I change tab. The data are stored to a firestore database.

I get this error:

Bad state: Stream has already been listened to

Here my Home.dart

class HomePage extends StatefulWidget {
  HomePage({Key key}) : super(key: key);
  Home createState() => Home();
}

class Home extends State<HomePage> {
  int _currentIndex;
  var _tabs = [];

  List<Company> currentCompaniesList = List();
  StreamController<List<Company>> _streamController;
  Stream<List<Company>> companiesStream;

  _getData() async {
    _companyService
        .getCompanies()
        .then((value) => _streamController.sink.add(value));
  }

  @override
  void initState() {
    super.initState();
    _currentIndex = 0;

    _streamController = StreamController<List<Company>>();

    _getData();
    companiesStream = _streamController.stream;
  }
}


@override
  Widget build(BuildContext context) {
    _tabs = [
      CompanyTab(stream: companiesStream),
      MapTab(),
      Center(child: Text('Profile')),
      Center(child: Text('Settings')),
    ];

    return Scaffold(
    ...
        actions: ...,
      body: _tabs[_currentIndex],
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        backgroundColor: BACKGROUND_COLOR,
        type: BottomNavigationBarType.fixed,
        items: [
          BottomNavigationBarItem(
            ...
          ),
          BottomNavigationBarItem(
            ...
          ),
          BottomNavigationBarItem(
            ...
          ),
          BottomNavigationBarItem(
            ...
          )
        ],
        onTap: (index) {
          setState(() {
            _currentIndex = index;
          });
        },
      ),
    );
  }
}

Here my CompanyTab.dart

class CompanyTab extends StatefulWidget {
  Stream stream;

  CompanyTab({Key key, this.stream}) : super(key: key);

  @override
  _CompanyTabState createState() => _CompanyTabState(stream);
}

class _CompanyTabState extends State<CompanyTab> {
  Stream stream;

  _CompanyTabState(this.stream);

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

  StreamBuilder companyList() {
    return StreamBuilder<List<Company>>(
        initialData: [],
        stream: stream,
        builder: (BuildContext context, AsyncSnapshot<List<Company>> snapshot) {
          if (snapshot.hasError) {
            return Text("Something went wrong");
          }

          if (snapshot.connectionState == ConnectionState.waiting ||
              snapshot.connectionState == ConnectionState.none ||
              snapshot.data == null) {
            return LoadingWidget();
          } else {
            return ListView.builder(
                padding: const EdgeInsets.all(10),
                itemCount: snapshot.data.length,
                itemBuilder: (BuildContext context, int index) {
                  Company company = snapshot.data.elementAt(index);
                  return Padding(
                    padding: const EdgeInsets.symmetric(
                        vertical: 1.0, horizontal: 4.0),
                    child: Card(
                      child: ListTile(
                        onTap: () {},
                        title: Text(company.name),
                        ...
                        ),
                      ),
                    ),
                  );
                });
          }
        });
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Container(
        child: companyList(),
      ),
    );
  }
}

Answer 1

It's about widgets lifecycle. I can suggest you two options.

1. Move _streamController and _getData() method to _CompanyTabState.

By default BottomNavigationBar destroys tab when you go away from one and init it again when you return back to it. If it's desired behaviour you need to move _streamController and _getData() method into _CompanyTabState. Don't forget to call _streamController.close() inside dispose() method of _CompanyTabState, it's important. _companyService can be injected into _CompanyTabState. It's a matter of it's life time. Should work like this:

...
class _CompanyTabState extends State<CompanyTab> {
  final _streamController = StreamController<List<Company>>();
  final CompanyService _companyService;

  _CompanyTabState(this._companyService);

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

  StreamBuilder companyList() {
    return StreamBuilder<List<Company>>(
        initialData: [],
        stream: _streamController.stream,
        builder: (BuildContext context, AsyncSnapshot<List<Company>> snapshot) {
          if (snapshot.hasError) {
            return Text("Something went wrong");
          }

          if (snapshot.connectionState == ConnectionState.waiting ||
              snapshot.connectionState == ConnectionState.none ||
              snapshot.data == null) {
            return LoadingWidget();
          } else {
            return ListView.builder(
                padding: const EdgeInsets.all(10),
                itemCount: snapshot.data.length,
                itemBuilder: (BuildContext context, int index) {
                  Company company = snapshot.data.elementAt(index);
                  return Padding(
                    padding: const EdgeInsets.symmetric(
                        vertical: 1.0, horizontal: 4.0),
                    child: Card(
                      child: ListTile(
                        onTap: () {},
                        title: Text(company.name),
                        ...
                        ),
                      ),
                    ),
                  );
                });
          }
        });
  }

  @override
  Widget build(BuildContext context) {
    return Center(
      child: Container(
        child: companyList(),
      ),
    );
  }

  @override
  void dispose() {
    super.dispose();
    _streamController.close();
  }

  void _getData() {
    _companyService
        .getCompanies()
        .then((value) => _streamController.sink.add(value));
  }
}

2. Use IndexedStack

You can save tab's state and widget data (like scroll offset, entered text etc.) when you go away from the tab. It's iOS UITabBarController-like behaviour. Use IndexedStack to achieve this:

...
    return Scaffold(
    ...
        actions: ...,
      body: IndexedStack(
        children: _tabs,
        index: _currentIndex,
      ),
      bottomNavigationBar: BottomNavigationBar(
        currentIndex: _currentIndex,
        backgroundColor: BACKGROUND_COLOR,
        type: BottomNavigationBarType.fixed,
        items: [
          BottomNavigationBarItem(
            ...
          ),
          BottomNavigationBarItem(
            ...
          ),
          BottomNavigationBarItem(
            ...
          ),
          BottomNavigationBarItem(
            ...
          )
        ],
        onTap: (index) {
          setState(() {
            _currentIndex = index;
          });
        },
      ),
    );

What option to use is up to you, you can use both if you want. But I would strongly recommend to move _streamController to _CompanyTabState as their lifecycles should be the same.