can not retrieve model correctly through ScanStreamTransformer - flutter

Question

I'm trying to retrieve an ItemModel class from my repository using streams implementing the BLoC logic in Flutter.
The problem is that the ItemModel is found but not retrieved . Therefore there is another loop triggered and tries to return a different Object, which is not retrieve on the Widgets.
Sample output:
I/flutter ( 4520): trying to get 25314126 from cache
I/flutter ( 4520): 25314126 in transformer
I/flutter ( 4520): 25314126 found !! returning: {id: 25314126, type: story, by: xucheng, time: 1607172267, text: , parent: null, kids: [25314805,25314781,25314684,25314664], dead: 0, deleted: 0, url: https://bitbashing.io/std-visit.html, score: 24, title: std::visit is everything wrong with modern C++ (2017), descendants: 4}
I/flutter ( 4520): ----------item to find: 25314126 and found: Instance of 'ItemModel'
I/flutter ( 4520): returned item Instance of 'ItemModel'
Here the correct Object is found and retrieved but not received on the UI and therefore there is another search right away:
I/flutter ( 4520): trying to get 25314805 from cache
I/flutter ( 4520): 25314805 found !! returning: {id: 25314805, type: comment, by: vasama, time: 1607178963, text: We got here because adding library features is a lot easier and less risky than adding language features. Work on pattern matching [1] is ongoing, but the bar for entry for a language feature is much higher and as such takes a longer time.<p>1. http://wg21.link/p1371, parent: 25314126, kids: [], dead: 0, deleted: 0, url: null, score: null, title: null, descendants: 0}
I/flutter ( 4520): ----------item to find: 25314805 and found: Instance of 'ItemModel'
I/flutter ( 4520): returned item Instance of 'ItemModel'
Which is not the Object I'm trying to retrieve!!!
There is always 'Loading 1' String shown
Here is my main Widget
class NewsDetail extends StatelessWidget{
final int itemId;
NewsDetail({ this.itemId}) ;
#override
Widget build(BuildContext context) {
final bloc = CommentsProvider.of(context);
// TODO: implement build
return Scaffold(
appBar: AppBar(
title: Text('Detail'),
),
body: buildBody(bloc),
);
}
Widget buildBody(CommentsBloc bloc){
return StreamBuilder(
stream: bloc.itemWithComments,
builder:
(context, AsyncSnapshot<Map<int,Future<ItemModel>>> snapshot){
if (!snapshot.hasData){
return Text('Loading 1');
}
final itemFuture = snapshot.data[itemId];
return FutureBuilder(
future: itemFuture,
builder: (context,AsyncSnapshot<ItemModel> itemSnapshot){
if(!itemSnapshot.hasData){
return Text('Loading 2');
}
return buildTitle(itemSnapshot.data);
}
);
},);
}
which indicates that is something wrong with the stream: bloc.itemWithComments,
Here is my BLOC class:
class CommentsBloc{
final _repository = Repository();
final _commentsFetcher = PublishSubject<int>();
final _commentsOutput = BehaviorSubject<Map<int,Future<ItemModel>>>();
//Streams
get itemWithComments => _commentsOutput.stream;
//Sink
Function(int) get fetchItemWithComments => _commentsFetcher.sink.add;
CommentsBloc(){
_commentsFetcher.stream.transform(_commentsTransformer()).pipe(_commentsOutput);
}
_commentsTransformer (){
return ScanStreamTransformer(
(cache,int id,index){
cache[id] = _repository.fetchItem(id);
print('$id in transformer');
cache[id].then((ItemModel item){
item.kids.forEach((kidId)=>fetchItemWithComments(kidId));
});
},
<int,Future<ItemModel>>{},
);
}
dispose(){
_commentsFetcher.close();
_commentsOutput.close();
}
}
And here is how i fetch items in my Repository class
List<Source> sources = <Source>[
newsDbProvider,
NewsApiProvider(),
];
List<Cache> caches = <Cache>[
newsDbProvider,
];
Future<ItemModel> fetchItem(int id) async{
ItemModel item;
var source;
for(source in sources) {
item = await source.fetchItem(id);
print('----------item to find: $id and found: ${item}');
if(item!=null){
break;
}
}
for(var cache in caches) {
if(cache!=source) {
cache.addItem(item);
}
}
print('returned item ${item}');
return item;
}
Even though there is an Instance returned every time why is the snapshot.hasData false?
Also please note that the widget gets invoked just once with the correct id by using onTap method. What have i completely missed?

Answer 1

Even though i used the fetchItemWithComments to sink the items, I still had to add a return statement in the ScanStreamTransformer
_commentsTransformer (){
return ScanStreamTransformer<int,Map<int,Future<ItemModel>>>(
(Map<int,Future<ItemModel>>cache,int id,index){
print(index);
cache[id] = _repository.fetchItem(id);
cache[id].then((ItemModel item){
item.kids.forEach( (kidId) => fetchItemWithComments(kidId) );
});
return cache; //was missing this
},
<int,Future<ItemModel>>{},
);
}
This had been bugging me for days, I went over everything and couldn't find anything so if you are using a ScanStreamTransformer class
Always have a return statement