How to combine multiple providers in Flutter - flutter

Question

I am new to Flutter development.
I am building application where once users login they are shown list of posts.
If the user is not login still they are shown some random post.
I got parts of the application various posts in the internet.
This is what I did
class TestApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MultiProvider(
providers: [
Provider<AuthBase>(
create: (context) => Auth(),
),
StreamProvider<User>.value(
value: Auth().onAuthStateChanged,
),
ProxyProvider<User, Database>(
update: (BuildContext context, User user, Database db) {
return user == null
? FirestoreDatabase(uid: null)
: FirestoreDatabase(uid: user.uid);
}),
ChangeNotifierProxyProvider<Database, PostProvider>(
create: (context) => PostProvider(),
update: (ctx, database, previousProvider) {
print("at ChangeNotifierProxyProvider $previousProvider");
return previousProvider.update(database);
},
),
],
child: MaterialApp(
title: 'Test Project',
home: LandingPage(),
),
);
}
My idea is;
if the user is logged in StreamProvider<User>.value will give that user to
ProxyProvider<User, Database> and it will create database with that user.
And that database in turn will be used by ChangeNotifierProxyProvider<Database, PostProvider> to create the provider that actually get posts. It uses the database to get the posts.
I noticed even if the user is logged in at the start I get a null value for the user then immediately i get the actual user.
In the landing page, I only have the following line and it generate an error
final provider = Provider.of<PostProvider>(context);
I noticed the cause for the error was this line;
previousProvider.update(database);
First time time Postprovider constructor and update methods get called without any problem. (here we get null user)
When the FirestoreDatabase get created with the actual user "previousProvider" is null.
This is the reason for the error.
Not sure why this is happening or how to fix it.
Is there a better way of doing this?
Initially getting a null value even when there is a logged in user may be the reason. How to prevent it?

Answer 1

I will try to explain your code a bit so we are on the same page
Provider<AuthBase>(
create: (context) => Auth(), //this subscribe an Auth Class to the provider
),
StreamProvider<User>.value(
value: Auth().onAuthStateChanged,
/// This subscribe a stream from another Auth Class instance
/// Unless Auth its a singleton class, this is not related to the previous Provider
/// and changes to Provider<AuthBase> won't affect this Stream
),
I noticed even if the user is logged in at the start I get a null
value for the user then immediately i get the actual user.
That happens when using Stream, while the first value is captured (sometimes is delayed a tick) its null, you can use initialData parameter if you know the first value while the stream subscription is done
Finally
ChangeNotifierProxyProvider<Database, PostProvider>(
create: (context) => PostProvider(),
update: (ctx, database, previousProvider) {
print("at ChangeNotifierProxyProvider $previousProvider");
return previousProvider.update(database);
},
),
update expects to return a value of type PostProvider, but you're returning whatever the method previousProvider.update(database) returns (which I think it just updates some inner variable or something, I believe it's some void method maybe?)
Change the update like this
update: (ctx, database, previousProvider) {
print("at ChangeNotifierProxyProvider $previousProvider");
return previousProvider..update(database);
/// It's the same as writing this
/// previousProvider..update(database);
/// return previousProvider;
},