Related
I am trying to figure out how can i watch a StateNotifierProvider and trigger some methods (defined in the class subclassing StateNotifier) on this provider after having done some async computations in another Provider watching the StateNotifierProvider.
Loking at the example below
i need to perform a reset from the RandomAdderNotifierobject provided by the randomAdderProvider if the doneProvider return true.
I try to reset from the doReset Provider. However the provider has nothing to provide.
The point is that both the doneProvider and the doreset provider are not rebuild on state changes of AdderProvider.
import 'dart:math';
import 'package:flutter/material.dart';
import 'package:flutter/rendering.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';
import 'package:equatable/equatable.dart';
void main() {
runApp(
const ProviderScope(child: MyApp()),
);
}
class MyApp extends StatelessWidget {
const MyApp({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return MaterialApp(home: Home());
}
}
final randomProvider = Provider<Random>((ref) {
return Random(1234);
});
//immutable state
class RandomAdder extends Equatable {
final int sum;
const RandomAdder(this.sum);
#override
List<Object> get props => [sum];
}
//State notifier extension
class RandomAdderNotifier extends StateNotifier<RandomAdder> {
RandomAdderNotifier(this.ref) : super(const RandomAdder(0));
final Ref ref;
void randomIncrement() {
state = RandomAdder(state.sum + ref.read(randomProvider).nextInt(5));
}
void reset() {
state = RandomAdder(0);
}
}
/// Providers are declared globally and specify how to create a state
final randomAdderProvider =
StateNotifierProvider<RandomAdderNotifier, RandomAdder>(
(ref) {
return RandomAdderNotifier(ref);
},
);
Future<bool> delayedRandomDecision(ref) async {
int delay = ref.read(randomProvider).nextInt(5);
await Future.delayed(Duration(seconds: delay));
print("You waited $delay seconds for a decision.");
return delay > 4;
}
final doneProvider = FutureProvider<bool>(
(ref) async {
ref.watch(randomAdderProvider);
bool decision = await delayedRandomDecision(ref);
print("the decision is $decision");
return decision;
},
);
final doreset = Provider((ref) {
if (ref.watch(doneProvider).value!) {
ref.read(randomAdderProvider.notifier).reset();
}
});
class Home extends ConsumerWidget {
#override
Widget build(BuildContext context, WidgetRef ref) {
return Scaffold(
appBar: AppBar(title: const Text('Counter example')),
body: Center(
// Consumer is a widget that allows you reading providers.
child: Consumer(builder: (context, ref, _) {
final count = ref.watch(randomAdderProvider);
return Text('$count');
}),
),
floatingActionButton: FloatingActionButton(
// The read method is a utility to read a provider without listening to it
onPressed: () =>
ref.read(randomAdderProvider.notifier).randomIncrement(),
child: const Icon(Icons.add),
),
);
}
}
I think ref.listen is more appropriate for usage within the doreset function than ref.watch.
Similarly to ref.watch, it is possible to use ref.listen to observe a provider.
The main difference between them is that, rather than rebuilding the widget/provider if the listened to provider changes, using ref.listen will instead call a custom function.
As per the Riverpod documentation
For ref.listen we need an additional argument - the callback function that we wish to execute on state changes - Source
The ref.listen method needs 2 positional arguments, the first one is the Provider and the second one is the callback function that we want to execute when the state changes.
The callback function when called will be passed 2 values, the value of the previous State and the value of the new State.
&
We will need to handle an AsyncValue - Source
As you can see, listening to a FutureProvider inside a widget returns an AsyncValue – which allows handling the error/loading states.
In Practice
doreset function
I chose to handle the AsyncValue by only handling the data case with state.whenData()
final doReset = Provider<void>(
(ref) {
final done = ref.listen<AsyncValue<bool>>(doneProvider, (previousState, state) {
state.whenData((value) {
if (value) {ref.read(randomAdderProvider.notifier).reset();}
});
});
},
);
Don't forget to watch either doReset/doneProvider in your Home Widget's build method. Without that neither will kick off (Don't have an explanation for this behaviour)
class Home extends ConsumerWidget {
#override
Widget build(BuildContext context, WidgetRef ref) {
ref.watch(doReset);
...
Lastly, your random function will never meet the condition for true that you have setup as delay>4, as the max possible delay is 4. Try instead using delay>3 or delay=4.
Also perhaps disable the button to prevent clicks while awaiting updates
and in a case where you are using ChangeNotifier You can pass ref in you provider and use the ref same as we can use in ConsumerWidget.
import 'package:flutter/material.dart';
import 'package:flutter_riverpod/flutter_riverpod.dart';
class YourProvider extends ChangeNotifier {
Ref ref;
YourProvider(this.ref) : super();
callOtherProviderFromThisProvider() {
ref.read(otherProvider).someMethodINeedToTrigger();
}
}
final yourProvider = ChangeNotifierProvider<YourProvider>(
(ref) => YourProvider(ref));
I'm building three pages of alarms (stored in different lists) that I have displayed via a Scaffold and TabViewer. Each alarm is stored as a row with a toggle switch to enable it. The rows for each page are stored as a List. Despite making sure to use set state when changing values and even trying to assign unique keys nothing I do seems to preserve the state of the switches when I changed tabs.
This is my first time coding in Flutter/Dart or designing an app for mobile in general. As such I'm still learning about some basic features of this language.
I've tried adding keys to everything using Uniquekey() to generate up keys didn't work so I've removed them.
I've made sure all variable changes are inside set state functions.
I've tried to store the variable inside the immutable super class of AlarmToggle which is both ill-advised and doesn't work anyways.
I haven't tired using PageStorageKey as I'm not sure how they'd be implemented in my code but I feel this is likely the only solution.
class Alarms {
List<Widget> allAlarms = []; // Store all alarms for the object
buildAlarm(
GlobalKey<ScaffoldState>
pageKey,
[int hour,
int minute,
List<bool> alarmDaysOfWeek]) {
TimeOfDay alarmTime = TimeOfDay(hour: hour, minute: minute);
AlarmRow _newAlarm = new AlarmRow(UniqueKey(), alarmTime, alarmDaysOfWeek);
allAlarms.add(_newAlarm);
}
void removeAlarm(GlobalKey<ScaffoldState> pageKey) {allAlarms.removeLast();}}
class AlarmRow extends StatefulWidget {
final TimeOfDay _alarmTime;
final List<bool> _alarmDaysofWeek;
final UniqueKey key;
AlarmRow(this.key, this._alarmTime, this._alarmDaysofWeek);
AlarmRowState createState() => new AlarmRowState();
}
class AlarmRowState extends State<AlarmRow> {
bool _alarmIsActive;
AlarmRowState(){_alarmIsActive = _alarmIsActive ?? false;}
void toggleChanged(bool state) {this.setState(() {_alarmIsActive = state;});}
#override
Widget build(BuildContext context) {
return new Container(
child: Row(
children: <Widget>[
new AlarmIcon(_alarmIsActive),
new Column(
children: <Widget>[
new AlarmTime(widget._alarmTime),
new AlarmWeekly(widget._alarmDaysofWeek),
],
),
new AlarmToggle(
_alarmIsActive,
() => toggleChanged(!_alarmIsActive),
),
],
),
);
} // Build
} // Class
No matter what I seem to try the _alarmIsActive variable in AlarmRow() gets reset to null each time the tab is changed. I'm trying to preserve its state when changing pages.
The solution is as jdv stated to use AutomaticKeepAliveClientMixin, it's not hard to use but I figured I'd include the instruction that I found after searching here in case anyone searches and finds this and doesn't know automatically how to implement it like myself.
class AlarmRowState extends State<AlarmRow> with AutomaticKeepAliveClientMixin {
#override bool get wantKeepAlive => true;
It's implemented in the state class with any modifiable variables you want to preserve. The 'with' after the 'extends' adds a Mixin which is a sort of class inheritance. Finally, it requires you to set the 'wantKeepAlive' to true and it compiles and state is no longer lost while the widget is not being rendered.
Why a stateful widget loses state while it's not rendered is something I'm still searching for. But at least I have a solution.
I hope this will help you #Ender ! check for this code, you can create a Global shared preference and use it, as shown below:-
class AlarmRow extends StatefulWidget {
#override
State<StatefulWidget> createState() => new _AlarmRowState();
}
class _AlarmRowState extends State<AlarmRow>{
bool alarmIsActive;
#override
void initState() {
alarmIsActive = Global.shared.alarmIsActive;
super.initState();
}
#override
Widget build(BuildContext context) {
....
.....
....
....
body: new Container(
child: Switch(
value: alarmIsActive,
onChanged: (bool isEnabled) {
setState(() {
alarmIsActive = isEnabled;
Global.shared.alarmIsActive = isEnabled;
isEnabled =!isEnabled;
});
},
.....
......
),
),
);
}
}
class Global{
static final shared =Global();
bool alarmIsActive = false;
}
Switch enabled and will maintain its state
I am writing a code in flutter in which I am using an SQFlite database. I want to insert image widget from the assets, and I am getting the name of the image from database.
#override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(title: Text("Single Line diagram"),backgroundColor: Colors.red.shade700,),
body: SingleChildScrollView(
scrollDirection: Axis.horizontal,
child: Align(
//alignment: Alignment.center,
child: SingleChildScrollView(
scrollDirection: Axis.vertical,
child: Row(
//crossAxisAlignment: CrossAxisAlignment.center,
children: imageList(),
),
),
)
),
);
}
The above code calls imageList() for the list of images to display.
List<Widget> imageList(){
List<Widget> singleLineImages = new List();
List unit;
for (int i = 0; i <= widget.unitsList.length-1; i++){
for (int j = 1; j <= int.parse(widget.unitsList[i].quantity); j++){
print("${widget.unitsList[i].bulletin}, ${widget.unitsList[i].mountType}, ${widget.unitsList[i].disconnect}");
getfileName(widget.unitsList[i].bulletin, widget.unitsList[i].mountType, widget.unitsList[i].disconnect);
//if(fileName != null) {
singleLineImages.add(
Image.asset("images/SD_Files_2100/$fileName.jpg", height: 400.0, width: 200.0,));
//}
}
}
return singleLineImages;
}
I am getting the filename from getFileName() method which is using the database.
getfileName(String bulletin, String mountType, String disconnect)async {
fileNameList = await db.getSDfileName(bulletin, disconnect, mountType);
fileName = fileNameList[0]['FileName'];
print("filename: $fileName");
}
Now, after calling the getFileName(), the program is not waiting for the fileName and proceeding further, which takes filename as null. The filename is obtained correctly after the Image.asset code. Is there any way, so that the program waits untill it gets the proper filename?
Start fetching the list in initState() and call setState when the list is fetched, to do this asynchronously. Below you can find a simplified example of this process. Also note the await statement before get Filename. This makes sure you return to that piece of code after is is done executing.
class ListPage extends StatefulWidget {
#override
_ListPageState createState() => _ListPageState();
}
class _ListPageState extends State<ListPage> {
// This should actually be a List<MyClass> instead of widgets.
List<Widget> _list;
#override
void initState() {
super.initState();
_fetchList();
}
Future _fetchList() async {
List<Widget> singleLineImages = new List();
List unit;
for (int i = 0; i <= widget.unitsList.length-1; i++){
for (int j = 1; j <= int.parse(widget.unitsList[i].quantity); j++){
print("${widget.unitsList[i].bulletin}, ${widget.unitsList[i].mountType}, ${widget.unitsList[i].disconnect}");
String fileName = await getfileName(widget.unitsList[i].bulletin, widget.unitsList[i].mountType, widget.unitsList[i].disconnect);
singleLineImages.add(
Image.asset("images/SD_Files_2100/$fileName.jpg", height: 400.0, width: 200.0,));
}
}
// call setState here to set the actual list of items and rebuild the widget.
setState(() {
_list = singleLineImages;
});
}
#override
Widget build(BuildContext context) {
// Build the list, or for example a CircularProcessIndicator if it is null.
}
}
Sidenote: you are making a lot of calls to the database, which is probably inefficient. Try to get the required data in a single db call. But that is another topic.
Keep Future<Widget> _list; field in your class.
Add _list = _fetchList(); in the initState() function.
Also note that _fetchList should return Future<Widget> in this case.
Use FutureBuilder in your build function.
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