I'm starting to use riverpod and I'm trying to migrate my existing code which was using provider.
With provider, the Providers were scoped in the widget tree. Only the children of the Provider widget could access its model.
It says in the riverpod's doc:
Allows easily accessing that state in multiple locations. Providers are a complete replacement for patterns like Singletons, Service Locators, Dependency Injection or InheritedWidgets.
* "Providers" here refers to the Provider class of the package riverpod.
And the provider package was a simplification/wrapper/API around InheritedWidgets so I guess what was possible with provider is also possible with riverpod.
But I cannot manage to find out how to do it.
Here is a small example of what I am trying to migrate.
class Counter extends ValueNotifier<int> {
Counter(): super(0);
}
class MyWidget extends StatelessWidget {
const MyWidget();
#override
Widget build(BuildContext context) {
return ChangeNotifierProvider<Counter>(
create: (_) => Counter();
child: Builder(
builder: (context) {
return Row(
children: [
IconButton(
onPressed: () {
context.read<Counter>().value++;
},
icon: Icon(Icons.add),
),
Text('Count: ${context.watch<Counter>().value}'),
],
);
},
),
);
}
}
Wherever there is a MyWidget widget, the sub-widget has access to a unique/scoped model Counter.
Scoping providers is done through ProviderScope
You can do:
final provider = ChangeNotifierProvider<Counter>((ref) => throw UnimplementedError());
// in some widget:
return ProviderScope(
overrides: [
provider.overrideWithProvider(
ChangeNotifierProvider((ref) => Counter());
),
],
)
Although if you can avoid scoping, do so. Scoping providers is generally not recommended and should be avoided as that's fairly advanced.
If you're using a scoped provider so that the state is destroyed when the user leaves the page, a much simpler solution is to use autoDispose:
final provider = ChangeNotifierProvider.autoDispose<Counter>((ref) => Counter());
// No longer needed to scope the provider
Inside ProviderScope is an InheritedWidget, so you can you multiple ProviderScope just like Provider. Please checkout the code of Remi in this ticket link.
Or you can use .family and .autoDispose for your purpose.
I'm using Obs/Obx in 2 routes, one works fine but one doesn't. There are 2 obs vars in the controller.
import 'package:get/get.dart';
class CountController extends GetxController {
final vcount = 0.obs;
final ncount = 0.obs;
static CountController get to => Get.find<CountController>();
#override
void onInit() {
super.onInit();
}
setVcount(int vehnum) {
vcount(vehnum);
}
setNcount(int notenum) {
ncount(notenum);
}
}
The one I use on a ListTile in the app drawer (vcount) works as expected. The other one is on an appbar action, the only action on this appbar. I want to disable the TextButton if ncount is 0.
appBar: AppBar(
actions: <Widget> [
Obx(() =>
TextButton(
child: _appBarChild(),
onPressed: () => nCountController.ncount.value == 0 ? null : _switchView(),
// onPressed: () => _switchView(),
),
),
],
)
The ncount value is set from a child of the route.
nCountController.setNcount(_queryResult.length);
Everything is the same, import controller.dart, "final (nC)countController = CountController.to;", setting the obs vars etc, the only difference is the type of widget, but when I run it I get the error:
======== Exception caught by widgets library =======================================================
The following message was thrown building Obx(has builder, dirty, state: _ObxState#11cd1):
[Get] the improper use of a GetX has been detected.
You should only use GetX or Obx for the specific widget that will be updated.
If you are seeing this error, you probably did not insert any observable variables into GetX/Obx
or insert them outside the scope that GetX considers suitable for an update
(example: GetX => HeavyWidget => variableObservable).
If you need to update a parent widget and a child widget, wrap each one in an Obx/GetX.
Am I missing something or doesn't Obx work in an appbar?
You are actually doing nothing with the view by using onPressed: () => nCountController.ncount.value == 0 ? null : _switchView()
You should use onPressed: nCountController.ncount.value == 0 ? null : ()=> _switchView()
Explanation: ()=> null & null aren't same. You have to pass onPressed: null in order to disable the button and not onPressed: ()=> null
The Background: I am intending to use Flutter's animation package to run a container transform effect between a material card and another page (a details page). To set this up, the cards are set into a list within a "search" page to which then the container is opened, the DetailsScreen is passed a data object (an object within objectList according to the card's index). See below code:
SEARCHSCREEN
...
child: ListView.builder(
controller: _listController,
itemCount: objectList.length,
itemBuilder: (BuildContext context, int index) {
return OpenContainer(
transitionDuration: Duration(milliseconds: 750),
closedBuilder: (_, openContainer) {
return AnimatedCard(
direction: AnimatedCardDirection.left,
initDelay: Duration(milliseconds: 0),
duration: Duration(milliseconds: 500),
curve: Curves.easeOutBack,
child: ObjectCard(objectList[index],
savedObjectList, openContainer),
);
},
openBuilder: (_, closeContainer) {
return DetailScreen(objectList[index], closeContainer);
},
);
},
),
DETAILSCREEN
class DetailScreen extends StatefulWidget {
const DetailScreen(this.object, this.closeContainer);
final object;
final closeContainer;
#override
_DetailScreenState createState() => _DetailScreenState();
}
class _DetailScreenState extends State<DetailScreen> {
#override
Widget build(BuildContext context) {
return Scaffold(
...
The Problem: Whenever a card is clicked upon, the following error is returned:
The widget which was currently being built when the offending call was made was:
DetailScreen-[LabeledGlobalKey<_SearchScreenState>#b16ed]
When the exception was thrown, this was the stack:
...
═════ Exception caught by animation library ═════════════════════════════════
setState() or markNeedsBuild() called during build.
════════════════════════════════════════════════════════════════════════════════
Another exception was thrown: setState() or markNeedsBuild() called during build.
Another exception was thrown: Duplicate GlobalKey detected in widget tree.
════════ Exception caught by widgets library ═══════════════════════════════════
Duplicate GlobalKey detected in widget tree.
Having tried many solutions of adding and passing keys first to the constructors of each object, then to all of them, nothing has worked. The only thing that makes the animation work is if I remove the [index] from "return DetailScreen(objectList[index], closeContainer);". The animation will fire, but obviously is not desirable as now no object has been passed and hence nothing on the DetailScreen will work. What am I missing here? I was under the impression that on each openContainer click, a DetailsScreen was created, populated with the passed object's data and the destroyed when I used the closeContainer callback. Under this assumption, this should work because only one screen will exist at a time and no duplication of "Global Keys" will occur. But this now does not seem to be the case. Is there something up that I need to include higher up the widget chain, such as within my "SearchScreen" itself, which is hosting this code? Any help or insight would be greatly appreciated!
#override
Widget build(BuildContext context) {
return BlocProvider<HomeBloc>(
create: (context) {
return HomeBloc(homeRepo: HomeRepository());
},
child: BlocProvider.of<HomeBloc>(context).state is HomeStateLoading
? CircularProgressIndicator()
: Container());
}
I am confused with the error:
BlocProvider.of() called with a context that does not contain a Bloc of type HomeBloc.
No ancestor could be found starting from the context that was passed to
BlocProvider.of<HomeBloc>().
Didn't I just create the HomeBloc at its immediate parent? What does it want?
You are using the context passed into the build method of your widget class to look for a parent BlocProvider. However, that context is the widget tree as far as your widget class sees it. Because of this, your BlocProvider.of is looking for a BlocProvider that is a parent of your widget class. If you want to get the provider that is the immediate parent, you need a new context object in which the BlocProvider is an ancestor in the widget tree. The easiest way to do this is with a Builder widget:
#override
Widget build(BuildContext context) {
return BlocProvider<HomeBloc>(
create: (context) {
return HomeBloc(homeRepo: HomeRepository());
},
child: Builder(
builder: (newContext) => BlocProvider.of<HomeBloc>(newContext).state is HomeStateLoading
? CircularProgressIndicator()
: Container(),
),
);
}
That being said, it's pretty redundant to create a provider and then immediately reverence the provider. Providers are for retrieving stuff further down the widget tree, not typically for immediate descendants. In this case, using a provider is overkill and there isn't really any reason to not just have the bloc be a field of your class and reference it directly.
From the documentation :
The easiest way to read a value is by using the static method
Provider.of(BuildContext context).
This method will look up in the widget tree starting from the widget
associated with the BuildContext passed and it will return the nearest
variable of type T found (or throw if nothing is found).
In your case it starts looking up the widget tree from your whole widget (associated to the BuildContext).
So you need to move your BlocProvider to be an ancestor of this widget.
If for some reason this is not possible, you can use Consumer, which allows obtaining a value from a provider when you don't have a BuildContext that is a descendant of the said provider.
Read https://pub.dev/documentation/provider/latest/provider/Consumer-class.html
I am running into a globalKey error after I navigate from Screen A to Screen B and click a "Cancel" button to go back to Screen A.
It seems like the issue is that Screen B is either
A) Not being disposed of correctly
B) Is not doing something that it otherwise could
And I don't actually know:
What bad things are happening if I just remove the use of a globalKey? (as to get a better understanding of the fundamentals)
How can I correctly resolve this issue?
StatefulWidget documentation states:enter link description here
A StatefulWidget keeps the same State object when moving from one
location in the tree to another if its creator used a GlobalKey for
its key. Because a widget with a GlobalKey can be used in at most one
location in the tree, a widget that uses a GlobalKey has at most one
associated element. The framework takes advantage of this property
when moving a widget with a global key from one location in the tree
to another by grafting the (unique) subtree associated with that
widget from the old location to the new location (instead of
recreating the subtree at the new location). The State objects
associated with StatefulWidget are grafted along with the rest of the
subtree, which means the State object is reused (instead of being
recreated) in the new location. However, in order to be eligible for
grafting, the widget must be inserted into the new location in the
same animation frame in which it was removed from the old location.
Console Error Output:
══╡ EXCEPTION CAUGHT BY WIDGETS LIBRARY ╞═══════════════════════════════════════════════════════════
The following assertion was thrown while finalizing the widget tree:
Duplicate GlobalKey detected in widget tree.
The following GlobalKey was specified multiple times in the widget tree. This will lead to parts of
the widget tree being truncated unexpectedly, because the second time a key is seen, the previous
instance is moved to the new location. The key was:
- [LabeledGlobalKey<FormFieldState<String>>#3c76d]
This was determined by noticing that after the widget with the above global key was moved out of its
previous parent, that previous parent never updated during this frame, meaning that it either did
not update at all or updated before the widget was moved, in either case implying that it still
thinks that it should have a child with that global key.
The specific parent that did not update after having one or more children forcibly removed due to
GlobalKey reparenting is:
- Column(direction: vertical, mainAxisAlignment: start, crossAxisAlignment: center, renderObject:
RenderFlex#7595c relayoutBoundary=up1 NEEDS-PAINT)
A GlobalKey can only be specified on one widget at a time in the widget tree.
So this part of the error output:
previous parent never updated during this frame, meaning that it
either did not update at all or updated before the widget was moved
makes me think there was some opportunity for my old Stateful widget to do something (either reposition itself or release something as to be disposed correctly.
This seems to be failing in framework.dart on assert(_children.contains(child)):
#override
void forgetChild(Element child) {
assert(_children.contains(child));
assert(!_forgottenChildren.contains(child));
_forgottenChildren.add(child);
}
In my case, it likes a hot reload bug. Just restart debugging works for me.
Remove the static and final type from the key variable so if
static final GlobalKey<FormState> _abcKey = GlobalKey<FormState>();
change it to
GlobalKey<FormState> _abcKey = GlobalKey<FormState>();
Thanks to Gunter's commments, I determined that this is because the Screens are not being properly disposed.
Flutter's pushReplacement makes a call to Route.dispose which will ultimately dispose the screen.
I am still unsure as to this comes into play:
widget must be inserted into the new location in the same animation
frame
I'm not sure what situation would benefit from such trickery. However, my problem is solved. I just need to make a call to pop or replace.
Here are the available options:
Use push from A to B and just Navigator.pop from B
Use pushReplacement from A to B and from B to A
I've recently started playing with Fluro for routing and there are a few more ways to to handle these situations (Note the optional argument replace):
Use router.navigateTo(context, route, replace: false) from A to B and Navigator.pop from B
Use router.navigateTo(context, route, replace: true) from A to B the same from B to A (the key is replace: true)
Make sure that you don't have a Form parent and a Form child with the same key
I had this issue too.
I had a four screen bottom tabbed application and a 'logout' method.
However, that logout method was calling a pushReplacementNamed.
This prevented the class that held the global keys (different from the logout function) from calling dispose.
The resolution was to change pushReplacementNamed with popAndPushNamed to get back to my 'login' screen.
Best way to solve that, which worked for me:
class _HomeScreenState extends State<HomeScreen> {
GlobalKey<FormState> _homeKey = GlobalKey<FormState>(debugLabel: '_homeScreenkey');
#override
Widget build(BuildContext context) {
return Container(
key: _homeKey,
);
}
}
In my case I wanted to use the static GlobalKey<ScaffoldState> _scaffoldKey but when I used the same widget multiple times it gave this duplicate error.
I wanted to give it a unique string and still use this scaffold state.
So I ended up using:
static GlobalObjectKey<ScaffoldState> _scaffoldKey
and in the initState:
_scaffoldKey = new GlobalObjectKey<ScaffoldState>(id);
Edit:
Actually, silly me. I just simply removed the static and made it GlobalKey again :)
please take SingleChildScrollview:
and after if you use the bloc pettern then use strem with broadcast
code is here:
body: Container(
decoration: BoxDecoration(
image: DecorationImage(
image: AssetImage('assets/images/abcd.jpg'),
fit: BoxFit.cover,
),
),
child: Container(child:Form(key: _key,
child: Padding(
padding: EdgeInsets.symmetric(vertical: 100.0, horizontal: 20.0),
child: SingleChildScrollView(child:Column(
children: <Widget>[
Padding(
padding: const EdgeInsets.all(10.0),
child: Image.asset('assets/images/logo.png', height: 80, width:80,),
),
email(),
Padding(
padding: EdgeInsets.all(5.0),
),
password(),
row(context),
],
),
),
),
),
),
),
resizeToAvoidBottomPadding: false,
);
}
and the bloc pettern code is here:
final _email = StreamController<String>.broadcast();
final _password = StreamController<String>.broadcast();
Stream<String> get email => _email.stream.transform(validateEmail);
Stream<String> get password=> _password.stream.transform(validatepassword);
Function(String) get changeEmail=> _email.sink.add;
Function(String) get changePassword => _password.sink.add;
dispose(){
_email.close();
_password.close();
}
}
final bloc=Bloc();
I had similar issue on a StatelessWidget class, Converted it to StatefulWidget and error is gone.
If you have multiple forms with different widgets, you must use separate GlobalKey for each form. Like I have two forms, one with Company signup & one with Profile. So, I declared
GlobalKey<FormState> signUpCompanyKey = GlobalKey<FormState>();
GlobalKey<FormState> signUpProfileKey = GlobalKey<FormState>();
This happened to me, what I did was enclosed the whole view into a navigator using an extension I made
Widget addNavigator() => Navigator(
onGenerateRoute: (_) => MaterialPageRoute(
builder: (context2) => Builder(
builder: (context) => this,
),
),
);
I also got this error. There was a static bloc object in a class and I removed the static keyword which fixed the error.
Events should be added by using the BlocProvider anyway.
I also had a similar error. My answer was that after I updated Flutter some widgets no longer had child or children properties. In my case it was the CircleAvatar. The build doesn't error out initially, but when navigating back and forth through the app it will fail.
*Please review all widgets that require a child then review the updated documentation and make sure you're parameters are still correct.