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I am running into an issue with flutter when I try to read data from local storage when the app loads.
I have an inherited widget that holds authentication information for the current user. When the app loads I want to look into local storage for session tokens. If the session tokens exist I would like to update the inherited widget with this information.
My screens are dynamic. If it knows the user is authenticated it takes them to the requested screen, otherwise it takes them to the register screen.
The issue I am running into is that I cannot update the inherited widget's state from an initState() method from a widget that depends on the inherited widget (My router widget)
How can I read from local storage when the app loads and update the inherited widget?
Error when running app:
flutter: ══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
flutter: The following assertion was thrown building _InheritedAuthContainer:
flutter: inheritFromWidgetOfExactType(_InheritedAuthContainer) or inheritFromElement() was called before
flutter: RootState.initState() completed.
flutter: When an inherited widget changes, for example if the value of Theme.of() changes, its dependent
flutter: widgets are rebuilt. If the dependent widget's reference to the inherited widget is in a constructor
flutter: or an initState() method, then the rebuilt dependent widget will not reflect the changes in the
flutter: inherited widget.
flutter: Typically references to inherited widgets should occur in widget build() methods. Alternatively,
flutter: initialization based on inherited widgets can be placed in the didChangeDependencies method, which
flutter: is called after initState and whenever the dependencies change thereafter.
Router Widget (Root)
class Root extends StatefulWidget {
#override
State createState() => RootState();
}
class RootState extends State<Root> {
static Map<String, Widget> routeTable = {Constants.HOME: Home()};
bool loaded = false;
bool authenticated = false;
#override
void initState() {
super.initState();
if (!loaded) {
AuthContainerState data = AuthContainer.of(context);
data.isAuthenticated().then((authenticated) {
setState(() {
authenticated = authenticated;
loaded = true;
});
});
}
}
#override
Widget build(BuildContext context) {
return MaterialApp(
initialRoute: '/',
onGenerateRoute: (routeSettings) {
WidgetBuilder screen;
if (loaded) {
if (authenticated) {
screen = (context) => SafeArea(
child: Material(
type: MaterialType.transparency,
child: routeTable[routeSettings.name]));
} else {
screen = (conext) => SafeArea(
child: Material(
type: MaterialType.transparency, child: Register()));
}
} else {
screen = (context) => new Container();
}
return new MaterialPageRoute(
builder: screen,
settings: routeSettings,
);
});
}
}
Inherited Widget method that checks for auth and updates itself which triggers a rerender of my router widget
Future<bool> isAuthenticated() async {
if (user == null) {
final storage = new FlutterSecureStorage();
List results = await Future.wait([
storage.read(key: 'idToken'),
storage.read(key: 'accessToken'),
storage.read(key: 'refreshToken'),
storage.read(key: 'firstName'),
storage.read(key: 'lastName'),
storage.read(key: 'email')
]);
if (results != null && results[0] != null && results[1] != null && results[2] != null) {
//triggers a set state on this widget
updateUserInfo(
identityToken: results[0],
accessToken: results[1],
refreshToken: results[2],
firstName: results[3],
lastName: results[4],
email: results[5]
);
}
}
return user != null && (JWT.isActive(user.identityToken) || JWT.isActive(user.refreshToken));
}
Main
void main() => runApp(
EnvironmentContainer(
baseUrl: DEV_API_BASE_URL,
child: AuthContainer(
child: Root()
)
)
);
What is a correct way of checking local storage on app load and updating the inherited widget that holds this information?
Actually you cannot access InheritedWidget from an initState method. Instead try accessing it from didChangeDependencies.
Example:
#override
void didChangeDependencies() {
super.didChangeDependencies();
if (!loaded) {
AuthContainerState data = AuthContainer.of(context);
data.isAuthenticated().then((authenticated) {
setState(() {
authenticated = authenticated;
loaded = true;
});
});
}
}
Another way would be to schedule the data fetch in initState with SchedulerBinding. You can find the docs here
SchedulerBinding.instance.addPostFrameCallback((_) {
// your login goes here
});
Note: remember the didChangeDependencies will be called whenever the state or dependencies of any parent InheritedWidget changes. Please look at the docs here.
Hope this helps!
While the answer by #hemanth-raj is correct, I would actually advocate a slightly different way of doing this. Instead of constructing the AuthContainer with no data, you could actually do the user session loading before you construct your widgets and pass the data in directly. This example uses the scoped_model plugin to abstract away the inherited widget boilerplate (which I highly recommend over writing inherited widgets manually!) but is otherwise pretty similar to what you've done.
Future startUp() async {
UserModel userModel = await loadUser();
runApp(
ScopedModel<UserModel>(
model: userModel,
child: ....
),
);
}
void main() {
startup();
}
This is more or less what I do in my app and I haven't had any problems with it (although you'd probably want to put in some error handling if there's any chance of loadUser failing)!
This should made your userState code much cleaner =).
And an FYI, what I've done in my UserModel is have a bool get loggedIn => ... that knows which information needs to be in there to tell whether the user is logged in or not. That way I don't need to track it separately but I still get a nice simple way to tell from outside the model.
Do like this as example given in this :
void addPostFrameCallback(FrameCallback callback) {
// Login logic code
}
Schedule a callback for the end of this frame.
Does not request a new frame.
This callback is run during a frame, just after the persistent frame callbacks (which is when the main rendering pipeline has been flushed). If a frame is in progress and post-frame callbacks haven't been executed yet, then the registered callback is still executed during the frame. Otherwise, the registered callback is executed during the next frame.
The callbacks are executed in the order in which they have been added.
Post-frame callbacks cannot be unregistered. They are called exactly once.
StreamBuilder is rebuild whenever it get new event. This cause problem with for example navigation (Navigator.push) because if new event is receive while navigate then this trigger rebuild. Because try to navigate while widget tree still being built, this will throw error.
It is not possible to prevent rebuild to avoid this issue as required.
Suggested workaround is basically take stream from cache.
Also:
here and
here
But this mean cannot have StreamBuilder build list which constantly update if also want to provide navigation from cards on list. For example in card onPressed(). See here.
So to refresh data must use pull to refresh…
Anyone have better solution?
Or is Flutter team work on solve this limitation for example by allow prevent rebuild if card is tap by user?
UPDATE:
TL;DR Is pull to refresh only way to update data since stream in StreamBuilder must be cached to prevent it rebuilding every time new event is received?
UPDATE 2:
I have try implement cache data but my code not work:
Stream<QuerySnapshot> infoSnapshot;
fetchSnapshot() {
Stream<QuerySnapshot> infoSnapshot = Firestore.instance.collection(‘info’).where(‘available’, isEqualTo: true).snapshots();
return infoSnapshot;
}
#override
void initState() {
super.initState();
fetchSnapshot();
}
...
child: StreamBuilder(
stream: infoSnapshot,
builder: (context, snapshot) {
if(snapshot.hasData) {
return ListView.builder(
itemBuilder: (context, index) =>
build(context, snapshot.data.documents[index]),
itemCount: snapshot.data.documents.length,
);
} else {
return _emptyStateWidget();
}
UPDATE 3:
I have try use StreamController but cannot implement correct:
Stream<QuerySnapshot> infoStream;
StreamController<QuerySnapshot> infoStreamController = StreamController<QuerySnapshot>();
#override
void initState() {
super.initState();
infoStream = Firestore.instance.collection(‘info’).where(‘available’, isEqualTo: true).snapshots();
infoStreamController.addStream(infoStream);
}
…
child: StreamBuilder(
stream: infoStreamController.stream,
builder: (context, snapshot) {
UPDATE 4:
Suggestion to use _localStreamController give error:
StreamController<QuerySnapshot> _localStreamController = StreamController<QuerySnapshot>();
#override
void initState() {
super.initState();
Firestore.instance.collection(‘info’).snapshots().listen((QuerySnapshot querySnapshot) {
// if(userAdded == null) {
_localStreamController.add(querySnapshot);
// }
});
...
child: StreamBuilder(
stream: _localStreamController.stream,
builder: (context, snapshot) {
The getter 'stream' was called on null.
The method 'add' was called on
null.
It seems like the actual problem based on your comments above is that it crashes after you navigate away from the view using the stream. You have to either:
Cancel your stream controller when you navigate away so that it's not listening for any more events.
Or just don't emit any new values through the stream after navigation. Add a pause on it until you come back to the view
Update: Adding code with pseudo example
class Widget {
// Your local stream
Stream<String> _localStream;
// Value to indicate if you have navigated away
bool hasNavigated = false;
...
void init() {
// subscribe to the firebase stream
firebaseStream...listen((value){
// If this value is still false then emit the same value to the localStream
if(!hasNavigated) {
_localStream.add(value);
}
});
}
Widget build() {
return StreamBuilder(
// subscribe to the local stream NOT the firebase stream
stream: _localStream,
// handle the same way as you were before
builder: (context, snapshot) {
return YourWidgets();
}
);
}
}
Try breaking everything into widgets
Running the query should cache it even if you fully close your app(I believe only cache it on fully closed for up to 30 minutes but if you remain without internet connection, you still have access to past previous cached queries from Firestore)
Try something like this:
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text('Please work')),
body: _buildStream(context),
);
}
Widget _buildStream(BuildContext context) {
return StreamBuilder(
stream: yourFireStoreStream,
builder: (context, snapshot) {
if (!snapshot.hasData) return LinearProgressIndicator();
return _buildAnotherwidget(context, snapshot.data.documents);
},
);
}
Widget _buildAnotherwidget(Buildcontext context, List<DocumentSnapshot> snaps){
return ListView.Builder(
itemCount: snaps.length,
itemBuilder:(context, index) {
..dostuff here...display your cards etc..or build another widget to display cards
}
);
}
focus on the breaking into more widgets. The highest part should have the streambuilder along with the stream. then go deep down into more widgets.
The streambuilder automatically will listen and subscribe to the given stream.
When streambuilder updates, it will update the lower widgets.
Now this way, when you tap on a card in a lower widget to navigate, it should not affect the highest widget because it will effect only the UI.
we placed the streambuilder in its own top level widget...
I hope i made some sense :(
I wrote the code out without testing but im sure you can get it to work
I was wondering when I should use the future builder. For example, if I want to make an http request and show the results in a list view, as soon as you open the view, should I have to use the future builder or just build a ListViewBuilder like:
new ListView.builder(
itemCount: _features.length,
itemBuilder: (BuildContext context, int position) {
...stuff here...
}
Moreover, if I don't want to build a list view but some more complex stuff like circular charts, should I have to use the future builder?
Hope it's clear enough!
FutureBuilder removes boilerplate code.
Let's say you want to fetch some data from the backend on page launch and show a loader until data comes.
Tasks for ListBuilder:
Have two state variables, dataFromBackend and isLoadingFlag
On launch, set isLoadingFlag = true, and based on this, show loader.
Once data arrives, set data with what you get from backend and set isLoadingFlag = false (inside setState obviously)
We need to have a if-else in widget creation. If isLoadingFlag is true, show the loader else show the data. On failure, show error message.
Tasks for FutureBuilder:
Give the async task in future of Future Builder
Based on connectionState, show message (loading, active(streams), done)
Based on data(snapshot.hasError), show view
Pros of FutureBuilder
Does not use the two state variables and setState
Reactive programming (FutureBuilder will take care of updating the view on data arrival)
Example:
FutureBuilder<String>(
future: _fetchNetworkCall, // async work
builder: (BuildContext context, AsyncSnapshot<String> snapshot) {
switch (snapshot.connectionState) {
case ConnectionState.waiting: return Text('Loading....');
default:
if (snapshot.hasError)
return Text('Error: ${snapshot.error}');
else
return Text('Result: ${snapshot.data}');
}
},
)
Performance impact:
I just looked into the FutureBuilder code to understand the performance impact of using this.
FutureBuilder is just a StatefulWidget whose state variable is _snapshot
Initial state is _snapshot = AsyncSnapshot<T>.withData(ConnectionState.none, widget.initialData);
It is subscribing to future which we send via the constructor and update the state based on that.
Example:
widget.future.then<void>((T data) {
if (_activeCallbackIdentity == callbackIdentity) {
setState(() {
_snapshot = AsyncSnapshot<T>.withData(ConnectionState.done, data);
});
}
}, onError: (Object error) {
if (_activeCallbackIdentity == callbackIdentity) {
setState(() {
_snapshot = AsyncSnapshot<T>.withError(ConnectionState.done, error);
});
}
});
So the FutureBuilder is a wrapper/boilerplate of what we do typically, hence there should not be any performance impact.
FutureBuilder Example
When you want to rander widget after async call then use FutureBuilder()
class _DemoState extends State<Demo> {
#override
Widget build(BuildContext context) {
return FutureBuilder<String>(
future: downloadData(), // function where you call your api
builder: (BuildContext context, AsyncSnapshot<String> snapshot) { // AsyncSnapshot<Your object type>
if( snapshot.connectionState == ConnectionState.waiting){
return Center(child: Text('Please wait its loading...'));
}else{
if (snapshot.hasError)
return Center(child: Text('Error: ${snapshot.error}'));
else
return Center(child: new Text('${snapshot.data}')); // snapshot.data :- get your object which is pass from your downloadData() function
}
},
);
}
Future<String> downloadData()async{
// var response = await http.get('https://getProjectList');
return Future.value("Data download successfully"); // return your response
}
}
In future builder, it calls the future function to wait for the result, and as soon as it produces the result it calls the builder function where we build the widget.
AsyncSnapshot has 3 state:
connectionState.none = In this state future is null
connectionState.waiting = [future] is not null, but has not yet completed
connectionState.done = [future] is not null, and has completed. If the future completed successfully, the [AsyncSnapshot.data] will be set to the value to which the future completed. If it completed with an error, [AsyncSnapshot.hasError] will be true
FutureBuilder is a Widget that will help you to execute some asynchronous function and based on that function’s result your UI will update.
I listed some use cases, why you will use FutureBuilder?
If you want to render widget after async task then use it.
We can handle loading process by simply using ConnectionState.waiting
Don't need any custom error controller. Can handle error simply dataSnapshot.error != null
As we can handle async task within the builder we do not need any setState(() { _isLoading = false; });
When we use the FutureBuilder widget we need to check for future state i.e future is resolved or not and so on. There are various State as follows:
ConnectionState.none: It means that the future is null and initialData is used as defaultValue.
ConnectionState.active: It means the future is not null but it is not resolved yet.
ConnectionState.waiting: It means the future is being resolved, and we will get the result soon enough.
ConnectionState.done: It means that the future has been resolved.
A simple implementation
Here OrdersProvider is a provider class and fetchAndSetOrders() is the method of that provider class.
body: FutureBuilder(
future: Provider.of<OrdersProvider>(context, listen: false)
.fetchAndSetOrders(),
builder: (context, dataSnapshot) {
if (dataSnapshot.connectionState == ConnectionState.waiting) {
return Center(
child: CircularProgressIndicator(),
);
} else {
if (dataSnapshot.error != null) {
return Center(
child: Text('An error occured'),
);
} else {
return Consumer<OrdersProvider>(
builder: (context, orderData, child) => ListView.builder(
itemCount: orderData.orders.length,
itemBuilder: (context, i) => OrderItem(orderData.orders[i]),
),
);
}
}
},
),
class MyPage extends StatelessWidget {
#override
Widget build(BuildContext context) {
return DefaultTabController(
length: 2,
child: new Scaffold(
appBar: TabBar(
tabs: [
Tab(child: Text("MY INFORMATION",style: TextStyle(color: Colors.black54),)),
Tab(child: Text("WEB CALENDER",style: TextStyle(color: Colors.black54),)),
],
),
body:PersonalInformationBlocProvider(
movieBloc: PersonalInformationBloc(),
child: TabBarView(
children: [
MyInformation(),
new SmallCalendarExample(),
],
),
),
),
);
}
}
class MyInformation extends StatelessWidget{
// TODO: implement build
var deviceSize;
//Column1
Widget profileColumn(PersonalInformation snapshot) => Container(
height: deviceSize.height * 0.24,
child: Column(
mainAxisAlignment: MainAxisAlignment.center,
children: <Widget>[
Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: <Widget>[
Container(
decoration: BoxDecoration(
borderRadius:
new BorderRadius.all(new Radius.circular(50.0)),
border: new Border.all(
color: Colors.black,
width: 4.0,
),
),
child: CircleAvatar(
backgroundImage: NetworkImage(
"http://www.binaythapa.com.np/img/me.jpg"),
foregroundColor: Colors.white,
backgroundColor: Colors.white,
radius: 40.0,
),
),
ProfileTile(
title: snapshot.firstName,
subtitle: "Developer",
),
SizedBox(
height: 10.0,
),
],
)
],
),
);
Widget bodyData(PersonalInformation snapshot) {
return SingleChildScrollView(
child: Column(
children: <Widget>[
profileColumn(snapshot)
],
),
);
}
#override
Widget build(BuildContext context) {
final personalInformationBloc = PersonalInformationBlocProvider.of(context);
deviceSize = MediaQuery.of(context).size;
return StreamBuilder(
stream: personalInformationBloc.results,
builder: (context,snapshot){
if (!snapshot.hasData)
return Center(
child: CircularProgressIndicator(),
);
return bodyData(snapshot.data);
}
);
}
}
I am using Bloc Pattern for retrieving data from Rest API (just called the whole object from JSON and parsed user name only). The Page consists of two tabs MyInformation and SmallCalendar. When the app runs the data are fetched correctly and everything is good. When I go to tab two and return to tab one then the whole screens in tab one goes to red showing error:
Bad state: Stream has already been listened to.
You should use the following.
StreamController<...> _controller = StreamController<...>.broadcast();
The most common form of Stream can be listened only once at a time. If you try to add multiple listeners, it will throw
Bad state: Stream has already been listened to
To prevent this error, expose a broadcast Stream. You can convert your stream to a broadcast using myStream.asBroadcastStream
This needs to be done inside your class that expose Stream. Not as parameter of StreamBuilder. Since asBroadcastStream internally listen to the original stream to generate the broadcast one, this imply you can't call this method twice on the same stream.
You could use broadcast, which allows to listen stream more than once, but it also prevents from listening past events:
Broadcast streams do not buffer events when there is no listener.
A better option is to use BehaviorSubject from rxdart package class as StreamController. BehaviorSubject is:
A special StreamController that captures the latest item that has been added to the controller, and emits that as the first item to any new listener.
The usage is as simple as:
StreamController<...> _controller = BehaviorSubject();
In my case, I was getting this error because the same line of code myStream.listen() was being called twice in the same widget on the same stream. Apparently this is not allowed!
UPDATE:
If you intend to subscribe to the same stream more than once, you should use a behavior subject instead:
// 1- Create a behavior subject
final _myController = BehaviorSubject<String>();
// 2- To emit/broadcast new events, we will use Sink of the behavior subject.
Sink<String> get mySteamInputSink => _myController.sink;
// 3- To listen/subscribe to those emitted events, we will use Stream (observable) of the behavior subject.
Stream<String> get myStream => _myController.stream;
// 4- Firstly, Listen/subscribe to stream events.
myStream.listen((latestEvent) {
// use latestEvent data here.
});
// 5- Emit new events by adding them to the BehaviorSubject's Sink.
myStreamInputSink.add('new event');
That's it!
However, there is one final important step.
6- We must unsubscribe from all stream listeners before a widget is destroyed.
Why? (You might ask)
Because if a widget subscribes to a stream, and when this widget is destroyed, the destroyed widget stream subscription will remain in app memory causing memory leaks and unpredictable behavior.:
_flush() {
_myController.close();
_myController = StreamController<String>();
}
###############################
###############################
Old Answer:
What fixed it for me is to both create a my stream controller as a broadcast stream controller:
var myStreamController = StreamController<bool>.broadcast();
AND
use stream as a broadcast stream:
myStreamController.stream.asBroadcastStream().listen(onData);
The problem was due to not disposing the controllers in bloc.
void dispose() {
monthChangedController.close();
dayPressedController.close();
resultController.close();
}
Just to sum up:
The main difference is broadcast() creates a Stream listenable for multiple sources but it needs to be listened for at least one source to start emitting items.
A Stream should be inert until a subscriber starts listening on it (using the [onListen] callback to start producing events).
asBroadcastStream turns an existing Stream into a multi listenable one but it doesn't need to be listened to start emitting since it calls onListen() under the hood.
I have had the same issue when I used a result of Observable.combineLatest2 for StreamBuilder into Drawer:
flutter: Bad state: Stream has already been listened to.
As for me, the best solution has added the result of this combine to new BehaviorSubject and listen new one.
Don't forget to listen old one !!!
class VisitsBloc extends Object {
Map<Visit, Location> visitAndLocation;
VisitsBloc() {
visitAndLocations.listen((data) {
visitAndLocation = data;
});
}
final _newOne = new BehaviorSubject<Map<Visit, Location>>();
Stream<Map<Visit, Location>> get visitAndLocations => Observable.combineLatest2(_visits.stream, _locations.stream, (List<vis.Visit> visits, Map<int, Location> locations) {
Map<vis.Visit, Location> result = {};
visits.forEach((visit) {
if (locations.containsKey(visit.skuLocationId)) {
result[visit] = locations[visit.skuLocationId];
}
});
if (result.isNotEmpty) {
_newOne.add(result);
}
});
}
I didn't use .broadcast because it slowed my UI.
I think not all of the answers take into account the situation where you do not want or simply can't use broadcast stream.
More often than not, you have to rely on receiving past events because the listener might be created later than the stream it listens to and it's important to receive such information.
In Flutter what will often happen is that widget listening to the stream ("listener") gets destroyed and built again. If you attempt to attach listener to the same stream as before, you will get this error.
To overcome this, you will have to manage your streams manually. I created this gist demonstrating how that can be done. You can also run this code on this dartpad to see how it behaves and play with it. I have used simple String ids to refer to specific StreamController instances but there might be better solutions too (perhaps symbols).
The code from the gist is:
/* NOTE: This approach demonstrates how to recreate streams when
your listeners are being recreated.
It is useful when you cannot or do not want to use broadcast
streams. Downside to broadcast streams is that it is not
guaranteed that your listener will receive values emitted
by the stream before it was registered.
*/
import 'dart:async';
import 'dart:math';
// [StreamService] manages state of your streams. Each listener
// must have id which is used in [_streamControllers] map to
// look up relevant stream controller.
class StreamService {
final Map<String, StreamController<int>?> _streamControllers = {};
Stream<int> getNamedStream(String id) {
final controller = _getController(id);
return controller.stream;
}
// Will get existing stream controller by [id] or create a new
// one if it does not exist
StreamController<int> _getController(String id) {
final controller = _streamControllers[id] ?? _createController();
_streamControllers[id] = controller;
return controller;
}
void push(String id) {
final controller = _getController(id);
final rand = Random();
final value = rand.nextInt(1000);
controller.add(value);
}
// This method can be called by listener so
// memory leaks are avoided. This is a cleanup
// method that will make sure the stream controller
// is removed safely
void disposeController(String id) {
final controller = _streamControllers[id];
if (controller == null) {
throw Exception('Controller $id is not registered.');
}
controller.close();
_streamControllers.remove(id);
print('Removed controller $id');
}
// This method should be called when you want to remove
// all controllers. It should be called before the instance
// of this class is garbage collected / removed from memory.
void dispose() {
_streamControllers.forEach((id, controller) {
controller?.close();
print('Removed controller $id during dispose phase');
});
_streamControllers.clear();
}
StreamController<int> _createController() {
return StreamController<int>();
}
}
class ManagedListener {
ManagedListener({
required this.id,
required StreamService streamService,
}) {
_streamService = streamService;
}
final String id;
late StreamService _streamService;
StreamSubscription<int>? _subscription;
void register() {
_subscription = _streamService.getNamedStream(id).listen(_handleStreamChange);
}
void dispose() {
_subscription?.cancel();
_streamService.disposeController(id);
}
void _handleStreamChange(int n) {
print('[$id]: streamed $n');
}
}
void main(List<String> arguments) async {
final streamService = StreamService();
final listener1Id = 'id_1';
final listener2Id = 'id_2';
final listener1 = ManagedListener(id: listener1Id, streamService: streamService);
listener1.register();
streamService.push(listener1Id);
streamService.push(listener1Id);
streamService.push(listener1Id);
await Future.delayed(const Duration(seconds: 1));
final listener2 = ManagedListener(id: listener2Id, streamService: streamService);
listener2.register();
streamService.push(listener2Id);
streamService.push(listener2Id);
await Future.delayed(const Duration(seconds: 1));
listener1.dispose();
listener2.dispose();
streamService.dispose();
}
For those of you running into this while doing Future.asStream(), you'll need Future.asStream().shareReplay(maxSize: 1) to make it a broadcast/hot stream.
For me defining my stream as a global variable worked
Stream infostream (was inside the ...State in a stateful widget i defined it outside the widget and it worked
(not sure if the best solution but give it a try)
Call .broadcast() on your stream controller
example:
StreamController<T> sampleController =
StreamController<T>.broadcast();
StreamSplitter.split() from the async can be used for this use case
import 'package:async/async.dart';
...
main() {
var process = Process.start(...);
var stdout = StreamSplitter<List<int>>(process.stdout);
readStdoutFoo(stdout.split());
readStdoutBar(stdout.split());
}
readStdoutFoo(Stream<List<int>> stdout) {
stdout.transform(utf8.decoder)...
}
readStdoutBar(Stream<List<int>> stdout) {
stdout.transform(utf8.decoder)...
}
In my case I was Using the Package Connectivity while on flutter web.
Commenting all Connectivity calls solved the issue.
I'm now just using Connectivity while only on Android/iOS.
So maybe check your Packages im you are using some packages that have some issues on Web in case you are developing for web.
Hopefully I could help someone with this Information.
This is a problem for the provider, I solved it by change provider initialization
Eg
locator.registerSingleton<LoginProvider>(LoginProvider());
TO
locator.registerFactory(() => TaskProvider());
Where locator is
GetIt locator = GetIt.instance;
This could help any other person. In my case i was using two StreamBuilder one in each tab. So when i swipe to above the tab and back. The other stream was already listened so i get the error.
What i did was to remove the StreamBuilder from the tabs and put it on top. I setState each time there is a change. I return an empty Text('') to avoid showing anything. I hope this methods
For other case scenarios. Watch out if you are somehow using a stream inside a stateless class. This is one of the reasons you get the above error.
Convert the stateless class to stateful and call init and dispose method on the streamController:
#override
void initState() {
super.initState();
YourStreamController.init();
}
#override
void dispose() {
YourStreamController.dispose();
super.dispose();
}
make sure you dispose controllers!
#override
void dispose() {
scrollController.dispose();
super.dispose();
}
I was getting this error when navigating away and then back to the view listening to the stream because I was pushing a new instance of the same view into the Navigator stack, which effectively ended up creating a new listener even though it was the same place in code.
Specifically and in more detail, I had a ListItemsView widget which uses StreamBuilder to show all the items in a stream. User taps on the "Add Item" button which pushes the AddItemView in the Navigator stack, and after submitting the form, the user is brought back to the ListItemsView, where the "Bad state: Stream has already been listened to." error happens.
For me the fix was to replace Navigator.pushNamed(context, ListItemsView.routeName) with Navigator.pop(context). This effectively prevents the instantiation of a new ListItemsView (as the second subscriber to the same stream), and just takes the user back to the previous ListItemsView instance.
I experienced this because, i was using a stream builder to create a list for tabs of tabview and anytime i switch tabs and come back to the previous i get this error. "wrapping the stream builder with a builder widget" did the magic for me.
i have experienced this, always closing the streamcontroller worked for me.
Edit: I've edited the code below to feature the method that fetches the data along with the widgets that build the train estimates (replacing any API information along the way with "API_URL" and "API_STOP_ID"). I hope this even better helps us figure out the problem! I really appreciate any information anyone can give -- I've been working very hard on this project! Thank you all again!
Original post:
I have a ListView of ListTiles that each have a trailing widget which builds train arrival estimates in a new Text widget. These trailing widgets are updated every five seconds (proven by print statements). As a filler for when the app is fetching data from the train's API, it displays a "no data" Text widget which is built by _buildEstimatesNull().
However, the problem is that "no data" is still being shown even when the app has finished fetching data and _isLoading = false (proven by print statements). Still, even if that was solved, the train estimates would become quickly outdated, as the trailing widgets are updating every five seconds on their own but this would not be reflected in the actual app as the widgets were built on page load. Thus, I need a way to rebuild those trailing widgets whenever they fetch new information.
Is there a way to have Flutter automatically rebuild the ListTile's trailing widget every five seconds as well (or whenever _buildEstimatesS1 is updated / the internals of the trailing widget is updated)?
class ShuttleApp extends StatefulWidget {
#override
State<StatefulWidget> createState() {
return new ShuttleState();
}
}
class ShuttleState extends State<ShuttleApp> {
#override
Widget build(BuildContext context) {
return new MaterialApp(
home: new HomeScreen(),
);
}
}
class HomeScreen extends StatefulWidget {
#override
State<StatefulWidget> createState() {
return new HomeState();
}
}
class HomeState extends State<HomeScreen> {
var _isLoading = true;
void initState() {
super.initState();
_fetchData();
const fiveSec = const Duration(seconds: 5);
new Timer.periodic(fiveSec, (Timer t) {
_fetchData();
});
}
var arrivalsList = new List<ArrivalEstimates>();
_fetchData() async {
arrivalsList.clear();
stopsList.clear();
final url = "API_URL";
print("Fetching: " + url);
final response = await http.get(url);
final busesJson = json.decode(response.body);
if (busesJson["service_id"] == null) {
globals.serviceActive = false;
} else {
busesJson["ResultSet"]["Result"].forEach((busJson) {
if (busJson["arrival_estimates"] != null) {
busJson["arrival_estimates"].forEach((arrivalJson) {
globals.serviceActive = true;
final arrivalEstimate = new ArrivalEstimates(
arrivalJson["route_id"], arrivalJson["arrival_at"], arrivalJson["stop_id"]
);
arrivalsList.add(arrivalEstimate);
});
}
});
}
setState(() {
_isLoading = false;
});
}
Widget _buildEstimateNull() {
return new Container(
child: new Center(
child: new Text("..."),
),
);
}
Widget _buildEstimateS1() {
if (globals.serviceActive == false) {
print('serviceNotActive');
_buildEstimateNull();
} else {
final String translocStopId = "API_STOP_ID";
final estimateMatches = new List<String>();
arrivalsList.forEach((arrival) {
if (arrival.stopId == translocStopId) {
estimateMatches.add(arrival.arrivalAt);
}
});
estimateMatches.sort();
if (estimateMatches.length == 0) {
print("zero");
return _buildEstimateNull();
} else {
return new Container(
child: new Center(
child: new Text(estimateMatches[0]),
),
);
}
}
}
#override
Widget build(BuildContext context) {
return new Scaffold(
backgroundColor: const Color(0xFF171717),
appBar: new AppBar(),
body: new DefaultTextStyle(
style: new TextStyle(color: const Color(0xFFaaaaaa),),
child: new ListView(
children: <Widget>[
new ListTile(
title: new Text('S1: Forest Hills',
style: new TextStyle(fontWeight: FontWeight.w500, fontSize: 20.0)),
subtitle: new Text('Orange Line'),
contentPadding: new EdgeInsets.symmetric(vertical: 16.0, horizontal: 16.0),
trailing: _isLoading ? _buildEstimateNull() : _buildEstimateS1(),
),
],
),
)
);
}
class ArrivalEstimates {
final String routeId;
final String arrivalAt;
final String stopId;
ArrivalEstimates(this.routeId, this.arrivalAt, this.stopId);
}
Thank you so much in advance for any help you can give! I really super appreciate it! :)
There are a few ways you could tackle this. It is slightly difficult however to tell what's going on without seeing a bit more of your code - specifically how you're getting the data and what you're doing with it. But I think I can give you a sufficient answer anyways.
The simple way of doing this is to either:
Have a StatefulWidget which keeps track of the build estimates for all of the items in the list. It should request data from your API, get the results, and then call setState(() => this.listData = data);. The call to setState is what tells the widget that it needs to rebuild.
Have a StatefulWidget for each item in the list. They would all each perform an API request every 5 seconds, get the results, and then each would call setState(() => this.itemData = data);. This means multiple calls to the API etc.
The advantage of #1 is that you can batch API calls, whereas the advantage to #2 is that your build would change less overall (although the way flutter works, this would be pretty minimal)... so I would probably go with #1 if possible.
However, there is a better way of doing this!
The better way of doing this is to have some sort of API Manager (or whatever you want to call it) which handles the communication with your API. It probably would live higher up in your widget tree and would be started/stopped with whatever logic you want. Depending on how far up the widget tree is, you could either pass it into each child or more likely hold it in an InheritedWidget which could then be used to retrieve it from each list element or from the overall list.
The API manager would provide various streams - either with a bunch of named fields/methods or with a getStream(id) sort of structure depending on your API.
Then, within your various list elements, you would use StreamBuilder widgets to build each of the elements based on the data - by using a StreamBuilder you get a ConnectionState object that lets you know whether the stream has received any data yet so you can choose to show an isLoading type widget instead of the one that shows data.
By using this more advanced method, you get:
Maintainability
If your API changes, you only have to change the API manager
You can write better testing as the API interactions and the UI interactions are separated
Extensibility
If you, later on, use push notifications for updates rather than pinging a server every 5 seconds, that can be incorporated into the API manager so that it can simply update the stream without touching the UI
EDIT: as per OP's comments, they have already implemented more or less the first suggestion. However, there are a few problems with the code. I'll list them below and I've posted the code with a couple of changes.
The arrivalsList should be replaced each time a new build is done rather than simply being changed. This is because dart compares the lists and if it finds the same list, it doesn't necessarily compare all of the elements. Also, while changing it in the middle of a function isn't necessarily going to cause problems, it's generally better to use a local variable and then change the value at the end. Note that the member is actually set within setState.
If serviceActive == false, the return was missed from return _buildEstimateNull();.
Here's the code:
class HomeState extends State<HomeScreen> {
var _isLoading = true;
void initState() {
super.initState();
_fetchData();
const fiveSec = const Duration(seconds: 5);
new Timer.periodic(fiveSec, (Timer t) {
_fetchData();
});
}
var arrivalsList = new List<ArrivalEstimates>();
_fetchData() async {
var arrivalsList = new List<ArrivalEstimates>(); // *********** #1
stopsList.clear();
final url = "API_URL";
print("Fetching: " + url);
final response = await http.get(url);
final busesJson = json.decode(response.body);
if (busesJson["service_id"] == null) {
print("no service id");
globals.serviceActive = false;
} else {
busesJson["ResultSet"]["Result"].forEach((busJson) {
if (busJson["arrival_estimates"] != null) {
busJson["arrival_estimates"].forEach((arrivalJson) {
globals.serviceActive = true;
final arrivalEstimate = new ArrivalEstimates(
arrivalJson["route_id"], arrivalJson["arrival_at"], arrivalJson["stop_id"]
);
arrivalsList.add(arrivalEstimate);
});
}
});
}
setState(() {
_isLoading = false;
this.arrivalsList = arrivalsList; // *********** #1
});
}
Widget _buildEstimateNull() {
return new Container(
child: new Center(
child: new Text("..."),
),
);
}
Widget _buildEstimateS1() {
if (globals.serviceActive == false) {
print('serviceNotActive');
return _buildEstimateNull(); // ************ #2
} else {
final String translocStopId = "API_STOP_ID";
final estimateMatches = new List<String>();
print("arrivalsList length: ${arrivalsList.length}");
arrivalsList.forEach((arrival) {
if (arrival.stopId == translocStopId) {
print("Estimate match found: ${arrival.stopId}");
estimateMatches.add(arrival.arrivalAt);
}
});
estimateMatches.sort();
if (estimateMatches.length == 0) {
print("zero");
return _buildEstimateNull();
} else {
return new Container(
child: new Center(
child: new Text(estimateMatches[0]),
),
);
}
}
}
#override
Widget build(BuildContext context) {
return new Scaffold(
backgroundColor: const Color(0xFF171717),
appBar: new AppBar(),
body: new DefaultTextStyle(
style: new TextStyle(color: const Color(0xFFaaaaaa),),
child: new ListView(
children: <Widget>[
new ListTile(
title: new Text('S1: Forest Hills',
style: new TextStyle(fontWeight: FontWeight.w500, fontSize: 20.0)),
subtitle: new Text('Orange Line'),
contentPadding: new EdgeInsets.symmetric(vertical: 16.0, horizontal: 16.0),
trailing: _isLoading ? _buildEstimateNull() : _buildEstimateS1(),
),
],
),
)
);
}
Instead of clearing and re-using the arrivalsList, create a new list every time the data is fetched. Otherwise Flutter is unable to detect if the list has changed.
Also, the code would clearer if you called setState whenever you change the list.
_fetchData() async {
final url = "API_URL";
print("Fetching: " + url);
final response = await http.get(url);
final busesJson = json.decode(response.body);
if (busesJson["service_id"] == null) {
globals.serviceActive = false;
setState(() {
_isLoading = false;
});
} else {
final newArrivalsList = new List<ArrivalEstimates>();
busesJson["ResultSet"]["Result"].forEach((busJson) {
if (busJson["arrival_estimates"] != null) {
busJson["arrival_estimates"].forEach((arrivalJson) {
globals.serviceActive = true;
final arrivalEstimate = new ArrivalEstimates(
arrivalJson["route_id"], arrivalJson["arrival_at"], arrivalJson["stop_id"]
);
newArrivalsList.add(arrivalEstimate);
});
}
});
setState(() {
arrivalsList = newArrivalsList;
_isLoading = false;
});
}
}
A few side notes:
I'm not sure if you actually want to clear the list before you fetch the data. If the state was updated properly, that would cause a flicker every 5 seconds.
I'm not sure if you simplified the code, but calling the _fetchData method every five seconds may become a problem if the network is slow.
If you are certain that you want a child widget to rebuild every time you call setState() and it is stubbornly refusing, you can give it a UniqueKey(). This will ensure that when setState() triggers a rebuild the child widget keys will not match, the old widget will be popped and disposed of, and, the new widget will replace it in the widget tree.
Note that this is using keys in sort of the opposite way for which they were intended (to reduce rebuilding) but if something beyond your control is hindering necessary rebuilds then this is a simple, built-in way to achieve the desired goal.
Here is a very helpful Medium article on keys from one the Flutter team members, Emily Fortuna:
https://medium.com/flutter/keys-what-are-they-good-for-13cb51742e7d
I am not sure if this is what your looking for but and im probably late on this but i believe you can use a change notifier efficiently to achieve this. Basically a change notifier is hooked to your backed logic() for instance an api data fetch. A widget is then registered with a change notifier of the same type as the change notifier provider. In event of data change, the widgets registered with the change notifier will be rebuild.
For instance
// extend the change notifier class
class DataClass extends ChangeNotifier {
....
getData(){
Response res = get('https://data/endpoint')
notifyListeners()
}
void onChange() {
notifyListeners();
}
....
}
Every time there is change in data you call the notifyListeners() that will trigger rebuild of consuming widgets.
Register you widget with a changenotifier
class View extends StatefulWidget {
Widget create(BuildContext context) {
return ChangeNotifierProvider<ModelClass>(
builder: (context) => DataClass(auth: auth),
child: Consumer<ModelClass>(
builder: (context, model, _) => View(model: model),
),
);
}
}
You can also user a Consumer for the same. Get more on this from the Documentation