I have a function in my initState:
#override
void initState() {
_fetchListItems();
super.initState();
}
This function is very simple. It has two async await operations of sqflite, one of which waits for the other to complete in it:
_fetchListItems() async {
wait() async {
number = await db.getNumber(userId); }
await wait();
List rawFavouriteList = await db.getList(number);
setState((){
rawFavouriteList.forEach((item){
_favouriteList.add(Model.map(item));
}});
}
I have to wait for number to be fetched, only then I can fetch serialized List, which then I deserialize and populate then add to the List which is displayed in ListView.
The problem is, this is not working in initState. It's working fine when it's called through onPressed() of a button, but not in initState.
Points to Note:
1) There is no error being thrown
2) I have already tried more conservative alternatives by using await for even rawFavouriteListthrough a separate function like wait() before using setState() top populate the _favouriteList, even though it's working fine manually through buttons.
3) Everything works fine if I manually enter number value in the second database query and just remove the first query, i.e.
_fetchListItems() async {
List rawFavouriteList = await db.getList(42);
setState((){
rawFavouriteList.forEach((item){
_favouriteList.add(Model.map(item));
}});
}
Related
I have an async function which is called multiple times synchoronusly.
List response = await Future.wait([future, future])
Inside, it popups a form and waiting for it to be submitted or cancelled.
var val = await Navigator.push(
context,
MaterialPageRoute(builder : (context) => const TheForm())
);
The first served Future will popup the form first and waiting for the return. No problem with that. But I want the second Future to check first if the form is already popped up. If it is, it just waiting for it to conclude and receive the same returned value.
I'm aware that receiving same function return from two calls sounds crazy and impossible. I'm just looking for a way to hold the second Future call on and trigger to conclude it from somewhere else.
Kindly tell me what I was missing and I'll provide the required information.
I try to use ValueNotifier's. Unfortunately ValueNotifier.addListener() only accept a VoidCallback. As for now, this is my solution. Still looking for a better way to replace the loop.
Future future() async{
if(ison) await Future.doWhile(() async {
await Future.delayed(Duration(seconds: 1));
return ison;
});
else{
ison = true;
result = ... //Popup form
ison = false;
}
return await result;
}
It sounds like you want to coalesce multiple calls to an asynchronous operation. Make your asynchronous operation cache the Future it returns and make subsequent calls return that Future directly. For example:
Future<Result>? _pending;
Future<Result> foo() {
if (_pending != null) {
return _pending!;
}
Future<Result> doActualWork() async {
// Stuff goes here (such as showing a form).
}
return _pending = doActualWork();
}
Now, no matter how many times you do await foo();, doActualWork() will be executed at most once.
If you instead want to allow doActualWork() to be executed multiple times and just to coalesce concurrent calls, then make doActualWork set _pending = null; immediately before it returns.
im using post method to get api data/response.body in initState and i get all the response/data then i put it in my variables surveyData, but when i load the pages with widget looped by the response.body length it goes error and say response.body.length is null but when i save the text editor/ hot reload, all the data entered and it's not null anymore and the widget appear.
fyi: the http.post method is my own function not from import 'package:http/http.dart' as http; so don't mind it
Variables that contain the response
dynamic surveyData;
initState Code
#override
void initState() {
super.initState();
// GET PAGES
surveyPages = widget.surveyPages;
// GET FORM DETAIL
http.post(
'survey-pre-mitsu/form-detail',
body: {
"survey_form_header_id": 1,
},
).then(
(res) {
surveyData = res['data'];
},
);
}
Widget that looped by surveyData.length
for (var i = 0; i < surveyData.length; i++)
AdditionalForm(questionLabel: surveyData[i]['label'],
questionType: surveyData[i]['type'],),
This is what the error
And this is what it looks like when i do hot reload
First, I suggest you to use future builder to resolve this problem.
First, I suggest performing post(even though its your own code) as a asynchronous operation, hence not inside initState(). Preferably write the logic in a separate class/file with packages like a provider package. Try this first and then post the error after that.
You need to add Some delay , In this Delay you can show a loading icon . for delay you can use :
Future.delayed(const Duration(milliseconds: 500), () {
// Here you can write your code
setState(() {
// Here you can write your code for open new view
});
});
This will solve your problem
Hi I am using this code here to retrieve data from Firestore. The data is in a Map and here is the picture of my Firestore.
And here is my code when I retrieve from Firestore
Future <void> getRoute() async{
Firestore.instance.collection('routes').snapshots().listen((RouteData) {
if(RouteData.documents.isNotEmpty){
for (int i = 0; i < RouteData.documents.length; i++){
_getPolyline(RouteData.documents[i].data, RouteData.documents[i].documentID);
}
}
});
setState(() {
});
}
void _getPolyline(info, infoID) async{
print(info);}
When Im printing the info it will display the data many times.
here is the result. As you can see it starts off in test then test1 then test again and repeated for some time. Is there something wrong with my code? Thank you in advance.
Calling setState will trigger a rebuild of your widget. What i think is happening is that when getRoute() u are also triggering your build function which will trigger getRoute() to run again.
solution:
make sure your function is not being triggered during a build. you can do this by triggering your function only once in initState
Either that or because u are using snapshots().listen. this will listen to any changes that happen on that collection.
solution:
use .get() instead of .listen()
To read a collection or document once, call the Query.get or DocumentReference.get methods. In the below example a FutureBuilder is used to help manage the state of the request:
https://firebase.flutter.dev/docs/firestore/usage/#one-time-read
I'm developing a Flutter app which has some tabs inside, each of them depend on the database that is loaded on the first run. State is stored in a ScopedModel.
On every tab I have this code:
#override
void initState() {
super.initState();
loadData();
}
void loadData() async {
await MyModel.of(context).scopedLoadData();
_onCall = MyModel.of(context).onCall;
setState(() {
});
}
And this is the code snippet that matters for the ScopedModel:
Future<Null> scopedLoadData() async {
if (_isLoading) return;
_isLoading = true;
(...)
_isLoading = false;
}
If the user waits on the first tab for a few seconds everything is fine, since Database is loaded. However, if the user switches tabs right after app launch, the method scopedLoadData is still being executed so that I get a runtime error ("Unhandled Exception: NoSuchMethodError: The method 'ancestorWidgetOfExactType' was called on null.").
This exception happens because the scopedLoadData has not yet been completed. So I'm looking for a way to wait for a method that is still being executed.
Thanks!
Not sure without seeing your build method but I would start your build method with a guard clause.
if (_oncall == null) return Container(); // or Text("loading") or similar
use should be using a FutureBuilder on each tab to make sure the data is loaded before you try to build the widget... more code would be helpful
I solved the exception by getting rid of every:
setState(() { });
and implementing ScopedModelDescendant on every relevant tab on top of using notifyListeners() at the end of the database loading method.
This pulls the responsibility from the tabs for updating views and gives it to the scopedLoadData().
I want to know the point behind calling setState without setting a new value to the variables.
readLocal() async {
prefs = await SharedPreferences.getInstance();
id = prefs.getString('id') ?? '';
if (id.hashCode <= peerId.hashCode) {
groupChatId = '$id-$peerId';
} else {
groupChatId = '$peerId-$id';
}
setState(() {});
}
I would say it's just a convention. The above can be re-written as
readLocal() async {
prefs = await SharedPreferences.getInstance();
setState(() {
id = prefs.getString('id') ?? '';
if (id.hashCode <= peerId.hashCode) {
groupChatId = '$id-$peerId';
} else {
groupChatId = '$peerId-$id';
}
});
}
Both will do the same thing. Calling setState(() {}) after mutating the state variable looks neat and reabable.
As per the implementation section of setState, it will below things in order.
Assertions. If any assert fails, throws exception and stops there.
Execute the callback function (final dynamic result = fn() as dynamic;)
Ask framework to rebuild(_element.markNeedsBuild();)
The documentation says [ https://docs.flutter.io/flutter/widgets/State/setState.html ]:
Calling setState notifies the framework that the internal state of this object has changed in a way that might impact the user interface in this subtree, which causes the framework to schedule a build for this State object.
The empty bracket { } is the empty callback (because you apparently don't need one):
The provided callback is immediately called synchronously. [...]
In short:
setState(() {});
is a way to tell the framework to build the state object anew, without using the possibility to pass a callback which would be called right after the build
Adding to the other answers, there is one difference.
When setState() is called on an unmounted (mounted == false) widget, it will fail. This means that whatever is wrapped inside the setState callback won't be called, whereas if you would run it outside setState it would be executed.