For my flutter project, I am using the following multiple providers below:
#override
Widget build(BuildContext context) {
return MultiProvider(
providers: [
ChangeNotifierProvider<FirstProvider>(
create: (context) => FirstProvider(),
),
ChangeNotifierProvider<SecondProvider>(
create: (context) => SecondProvider(),
),
ChangeNotifierProvider<ThirdProvider>(
create: (context) => ThirdProvider(),
),
ChangeNotifierProvider<FourthProvider>(
create: (context) => FourthProvider(),
),
],
child: const MainApp(),
);
}
Because sometimes I need to either get data or call functions from different providers from another provider, I am using it like this:
//First Provider
class FirstProvider with ChangeNotifier {
void callFunctionFromSecondProvider({
required BuildContext context,
}) {
//Access the SecondProvider
final secondProvider= Provider.of<SecondProvider>(
context,
listen: false,
);
secondProvider.myFunction();
}
}
//Second Provider
class SecondProvider with ChangeNotifier {
bool _currentValue = true;
void myFunction(){
//Do something
}
}
The callFunctionFromSecondProvider()of the FirstProvider is called from a widget and it will call myFunction() successfully, most of times.
Depending on the complexity of the function, I am sometimes experiencing that I can't access the SecondProvider, presumably due to context being null, when the widget state changes.
I am reading some documents online regarding provider, and they are suggesting changenotifierproxyprovider for what I understood as 1 to 1 provider relationship.
However, in my case, one provider needs to be accessed by multiple providers and vice versa.
Question:
Is there a more appropriate way that I can approach my case where one provider can be accessed by multiple providers?
EDIT:
Accessing provider should also be able to access different variable values without creating a new instance.
Instead of passing context to the callFunctionFromSecondProvider function add the second provider as the parameter. So the function looks like the below.
Not sure this is the correct way of doing that but my context null issue was fixed this way.
void callFunctionFromSecondProvider({
required SecondProvider secondProvider,
}) {
secondProvider.myFunction();
}
}
Alright.
So it looks like Riverpod by the same author is the way to go as it addresses alot of flaws such as Provider being dependent on the widget tree, in my case, where the underlying issue came from.
—--------
For the time being, I still need to use the provider and for a quick and dirty solution, I am providing the context of not only the current widget that I am trying to access the provider, but also passing the parent context of the widget directly, so that in case a modal (for example) is closed, then any subsequent provider call can still be executed using the parent context.
Hope this helps.
Related
I just arrived on a flutter project for a web app, and all developers have a problem using flutter provider for state management.
What is the problem
When you arrive on a screen, the variables of the corresponding provider are initialised by calling a function of the provider. This function calls an api, and sets the variables in the provider.
Problem : This function is called in the build section of the widget. Each time the window is resized, the widget is rebuilt, and the function is called again.
What we want
We want to call an api when the page is first displayed, set variables with the result, and not call the api again when the widget is rebuilt.
What solution ?
We use a push from the first screen to go to the second one. We can call the function of the provider at this moment, to initialise the provider just before the second screen.
→ But a refresh on the second page will clear the provider variables, and the function to initialise them will not be called again.
We call the function to initialise the provider in the constructor of the second screen. Is it a good pattern ?
Thank you for your help in my new experience with flutter :)
I think you're mixing a couple different issues here:
How do you correctly initialize a provider
How do you call a method on initialization (only once)
For the first question:
In your main.dart file you want to do something like this:
#override
Widget build(BuildContext context) {
return MultiProvider(
providers: [
ChangeNotifierProvider(create: (context) => SomeProvider()),
ChangeNotifierProvider(create: (context) => AnotherProvider()),
],
child: YourRootWidget();
);
}
Then in a widget (that probably represents a "screen" in your app), you need to do something like this to consume state changes from that provider:
#override
Widget build(BuildContext context) {
return Container(
child: Consumer<SomeProvider>(
builder: (context, provider, child) {
return Text(provider.someState);
}
),
)
}
And you need to do something like this to get access to the provider to mutate state:
#override
Widget build(BuildContext context) {
SomeProvider someProvider = Provider.of<SomeProvider>(context, listen: false);
return Container(
child: TextButton(
child: Text('Tap me'),
onPressed: () async {
await someProvider.mutateSomeState();
}
),
)
}
Regarding the second question... You can (I think) just use the initState() method on a widget to make the call only 1 time. So...
#override
void initState() {
super.initState();
AnotherProvider anotherProvider = Provider.of<AnotherProvider>(context, listen: false);
Future.microtask(() {
anotherProvider.doSomethingElse();
});
}
If I'm off on any of that, I'm sorry. That mirrors my implementation and works fine/well.
A caveat here is that I think RiverPod is likely the place you really want to go (it's maybe easier to work with and has additional features that are helpful, etc.) but I've not migrated to RiverPod yet and do not have that figured out all the way.
Anyway... Good luck!
As far as I understood, you can wrap your application with MultiProvider and call the API before going to the second screen.
I'm using Flutter provider, and I want to access a provider from another provider without using context.
I found many documentations on the internet explaining how to use ProxyProvider, but no one of the implementations was working (I think it's related to the last update 3 months ago).
I posted this question in StackOverflow after encountering a problem in ProxyProvider, but I didn't get an answer.
So now I'm just searching for any way to use a provider inside another one without using context.
You mean a Provider or the service that the Provider encapsulates? If it's the latter, the way that I've done it is by calling a provided service inside another provided service, given that they are all under the same MultiProvider. For example, I have two provided services:
MultiProvider(
providers: [
ChangeNotifierProvider(
create: (_) => FirstService()
),
ChangeNotifierProvider(
create: (_) => SecondService()
),
]
)
FirstService looks like this:
class FirstService extends ChangeNotifier {
List<String> getListOfValues() {
return ['a', 'b', 'c'];
}
}
and for example, I need to access those values from the SecondService, then I do this:
class SecondService extends ChangeNotifier {
void pullDataFromService(BuildContext context) {
FirstService firstService = Provider.of<FirstService>(context, listen: false);
var values = firstService.getListOfValues();
}
}
So pretty much you pass a context to a method where you need to access a provided service and you extract it from the Provider as normal, then access its functionality from the other service where you fetch it.
I want to share some data across different widgets so I decided to use a ChangeNotifierProvider<Example> with its relative Consumer<Example>. I have already used Providers before but never in this way (in fact I got some errors).
ChangeNotifierProvider<Example> has been defined in menu page while Consumer<Example> in an other widget defined in menu too.
Menu page :
class Menu extends StatefulWidget {
//...SOme code
ChangeNotifierProvider<Example>(
create: (context) => Example(),
child: ShowMultipleAnswers()
//...some code
And now I would like to use Consumer<Example> inside ShowMultipleAnswers() widget consuming data created in menu like :
class ShowMultipleAnswers extends StatefulWidget {
//...some code
Widget build(BuildContext context) {
return Consumer<Example>(builder: (context, handler, child) {
//some code
But I got these errors :
Error: Could not find the correct Provider<Example> above this Consumer<Example> Widget
This happens because you used a `BuildContext` that does not include the provider
of your choice. There are a few common scenarios:
- You added a new provider in your `main.dart` and performed a hot-reload.
To fix, perform a hot-restart.
- The provider you are trying to read is in a different route.
Providers are "scoped". So if you insert of provider inside a route, then
other routes will not be able to access that provider.
- You used a `BuildContext` that is an ancestor of the provider you are trying to read.
consider using `builder` like so:
Make sure that Consumer<Example> is under your MultiProvider/Provider<Example>.
This usually happens when you are creating a provider and trying to read it immediately.
For example, instead of:
```
Widget build(BuildContext context) {
return Provider<Example>(
create: (_) => Example(),
// Will throw a ProviderNotFoundError, because `context` is associated
// to the widget that is the parent of `Provider<Example>`
child: Text(context.watch<Example>()),
),
}
```
```
Widget build(BuildContext context) {
return Provider<Example>(
create: (_) => Example(),
// we use `builder` to obtain a new `BuildContext` that has access to the provider
builder: (context) {
// No longer throws
return Text(context.watch<Example>()),
}
),
}
```
I think the most valid options are 2 :
- The provider you are trying to read is in a different route.
But I don't know because they should be in the same, I mean ShowMultipleAnswers() is the Provider child.
Or the second
- You used a `BuildContext` that is an ancestor of the provider you are trying to read.
In this case, following suggestions above, I should use a builder:(context){} instead of directly calling child : .. but I read that after provider 5 builder has been substituted by create so I'm confused.
If I'm using wrong widgets tell me please!
The error message exactly specifies and describe your error
ShowMultipleAnswers got built using Menu context which doesn't have the Example provider, thus it throws this error.
you can either use builder attribute instead of child or wrap your MaterialApp with the provider
your code should be something like this:
class Menu extends StatefulWidget {
//...SOme code
ChangeNotifierProvider<Example>(
create: (context) => Example(),
builder: (context) {
return ShowMultipleAnswers();
}
//...some code
I forgot to say that there was an other page between Menu and ShowMultipleAnswers, like a bridge between them so the real flow was :
Menu Page -> Bridge Page -> ShowMultipleAnswers Page.
I removed this Bridge Page and it worked! But I still don't understand why it didn't work, maybe because Bridge Page didn't have any references of its Provider?
I'm trying to understand multiproviders in Flutter. In my App, one Provider need to change based on a value from an other Provider.
AuthProvider gets initiated higher up in the widget tree on build. Works like a charm with automatic sign in if possible...
In a lower placed widget, I try to initiate two other Providers. One, WhatEver, is not depended on other data and gets initiated on build like it is supposed to using ChangeNotifierProvider.
ProductList however is depended on AuthProvider. If log in status is changed, the ProducList should update accordingly.
In my attempts, I've found out, ie found on SO, that ChangeNotifierProxyProvider is the right way to go. But when I run the App, it seems like the 'create'-part of ChangeNotifierProxyProvider is not initiated when the widget gets build. It seems like the ProductList provider is not initiated until it's read or written to.
What have I misunderstood using MultiProviders and ChangeNotifierProxyProvider?
return MultiProvider(
providers: [
ChangeNotifierProvider<WhatEver>(create: (context) => WhatEver()),
ChangeNotifierProxyProvider<AuthProvider, ProductList>(
create: (_) => ProductList(Provider.of<AuthProvider>(context, listen: false)),
update: (_, auth, productList) => productList..reloadList(auth)
),
],
The ProductList looks like this:
final AuthProvider _authProvider;
static const String _TAG = "Shop - product_list.dart : ";
ProductList(this._authProvider) {
print(_TAG + "ProductList Provider initiated");
reloadList(this._authProvider);
}
void reloadList(AuthProvider authProvider) {
print(_TAG + "ProductList reload started");
if (authProvider.user==null) {
print(_TAG + "ProductList: _authProvider == null");
_loadBuiltInList();
} else {
print(_TAG + "ProductList: user = " + authProvider.user.displayName);
_loadFirestoreList();
}
}
I have code that does this:
ChangeNotifierProxyProvider<AuthService, ProfileService>(
create: (ctx) => ProfileService(),
update: (ctx, authService, profileService) =>
profileService..updateAuth(authService),
),
My ProfileService() does not rely on AuthService being available when it is constructed. The code works fine :)
The ChangeNotifierProxyProvider documentation explicitly describes this approach:
Notice how MyChangeNotifier doesn't receive MyModel in its constructor
anymore. It is now passed through a custom setter/method instead.
have a problem that I'm sitting on couple of days now.
have an app where:
depending of AUTH state, 'LoginScreen' or 'MainScreen' is Shown.
in MainScreen I setUp bottomNavigation with screens (HomeScreen, ShoppingScreen,MyFavorites)
I set up there as well my StreamProviders(those depend on Auth) by using MultiProvider
on HomeScreen when I User Provider.of(context) it works like it should
but when I use :
`Navigator.push(
context,
MaterialPageRoute(
builder: (_) => ProfileScreen(),
),
);
` and use Provider.of(context) there I get "Could not find correct Provider....above this...widget"
I read some issues on that and solution there was to decler providers above MaterailApp which in my case I can not do because I can set up thoese only after Auth is successfull.
Tryed passing context(from HomeScreen) to ProfileScreen(through constructor) and that work but when value changed of UserData it did not update the screen (guessing beacause of diffrent 'contexts')
What am I doing wrong in here,any Ideas?:S
Providers are "scoped".
This means that if they are placed inside a screen, they aren't accessible outside that screen.
Which means that if a provider is scoped but needs to be accessed outside of the route it was created in, we have two solutions:
un-scope the provider. This involves moving the provider to a common ancestor of both widgets that needs to obtain the value.
If those two widgets are on two different Routes, then it basically mean "move the provider above MaterialApp/CupertinoApp.
manually pass the provider to the new screen (needed when using Navigator.push)
The idea is, instead of having one provider, we have two of them, both using the same value as explained here See How to scope a ChangeNotifier to some routes using Provider? for a practical example.
For Navigator.push, this can look like:
final myModel = Provider.of<MyModel>(context);
Navigator.push(
context,
MaterialPageRoute(
builder: (_) =>
ChangeNotifierProvider.value(
value: myModel,
child: MyScreen(),
),
),
);
Please make sure that you application's root widget is Provider Widget, it should event be the parent of MaterialWidget. If this is already the case I will need your code to look into. Something like this
class AppState {
User loggedInUser;
bool get isLoggedIn {
return loggedInUser != null;
}
// Other states as per the requirements
// ...
}