How to use a provider inside of another provider in Flutter - flutter

Question

I want to create an app that has an authentication service with different permissions and functions (e.g. messages) depending on the user role.
So I created one Provider for the user and login management and another one for the messages the user can see.
Now, I want to fetch the messages (once) when the user logs in. In Widgets, I can access the Provider via Provider.of<T>(context) and I guess that's a kind of Singleton. But how can I access it from another class (in this case another Provider)?

Answer 1

From version >=4.0.0, we need to do this a little differently from what #updatestage has answered.
return MultiProvider(
providers: [
ChangeNotifierProvider(builder: (_) => Auth()),
ChangeNotifierProxyProvider<Auth, Messages>(
update: (context, auth, previousMessages) => Messages(auth),
create: (BuildContext context) => Messages(null),
),
],
child: MaterialApp(
...
),
);

Answer 2

Thanks for your answer. In the meanwhile, I solved it with another solution:
In the main.dart file I now use ChangeNotifierProxyProvider instead of ChangeNotifierProvider for the depending provider:
// main.dart
return MultiProvider(
providers: [
ChangeNotifierProvider(builder: (_) => Auth()),
ChangeNotifierProxyProvider<Auth, Messages>(
builder: (context, auth, previousMessages) => Messages(auth),
initialBuilder: (BuildContext context) => Messages(null),
),
],
child: MaterialApp(
...
),
);
Now the Messages provider will be rebuilt when the login state changes and gets passed the Auth Provider:
class Messages extends ChangeNotifier {
final Auth _authProvider;
List<Message> _messages = [];
List<Message> get messages => _messages;
Messages(this._authProvider) {
if (this._authProvider != null) {
if (_authProvider.loggedIn) fetchMessages();
}
}
...
}

Answer 3

Passing another provider in the constructor of the ChangeNotifierProxyProvider may cause you losing the state, in that case you should try the following.
ChangeNotifierProxyProvider<MyModel, MyChangeNotifier>(
create: (_) => MyChangeNotifier(),
update: (_, myModel, myNotifier) => myNotifier
..update(myModel),
);
class MyChangeNotifier with ChangeNotifier {
MyModel _myModel;
void update(MyModel myModel) {
_myModel = myModel;
}
}

Answer 4

It's simple: the first Provider provides an instance of a class, for example: LoginManager. The other Provides MessageFetcher. In MessageFetcher, whatever method you have, just add the Context parameter to it and call it by providing a fresh context.
Perhaps your code could look something like this:
MessageFetcher messageFetcher = Provider.of<ValueNotifier<MessageFetcher>>(context).value;
String message = await messageFetcher.fetchMessage(context);
And in MessageFetcher you can have:
class MessageFetcher {
Future<String> fetchMessage(BuildContext context) {
LoginManager loginManager = Provider.of<ValueNotifier<LoginManager>>(context).value;
loginManager.ensureLoggedIn();
///...
}
}

Answer 5

Seems like this would be a lot easier with Riverpod, especially the idea of passing a parameter into a .family builder to use the provider class as a cookie cutter for many different versions.