I am losing stream data when navigating to another screen - flutter

Question

I am new to Dart/Flutter and after "attending" a Udemy course,
everything has been going well.
Until now ;-)
As in the sample application in the Udemy course i am using the BLOC pattern.
Like this:
class App extends StatelessWidget {
Widget build(context) {
return AppBlocProvider(
child: MaterialApp(
(See "AppBlocProvider" which I later on use to get the "AppBloc")
The App as well as all the screens are StatelessWidget's.
The AppBlocProvider extends the InheritedWidget.
Like this:
class AppBlocProvider extends InheritedWidget {
final AppBloc bloc;
AppBlocProvider({Key key, Widget child})
: bloc = AppBloc(),
super(key: key, child: child);
bool updateShouldNotify(_) => true;
static AppBloc of(BuildContext context) {
return (context.inheritFromWidgetOfExactType(AppBlocProvider) as AppBlocProvider).bloc;
}
}
The AppBlocProvider provides an "AppBloc" containing two further bloc's to separate the different data a bit.
Like this:
class AppBloc {
//Variables holding the continuous state
Locale appLocale;
final UserBloc userBloc;
final GroupsBloc groupsBlock;
In my application I have a "GroupSearchScreen" with just one entry field, where you can enter a fragment of a group name. When clicking a button, a REST API call is done and list of group names is returned.
As in the sample application, I put the data in a stream too.
In the sample application the data fetching and putting it in the stream is done in the bloc itself.
On the next line, the screen that uses the data, is created.
Like this:
//Collecting data and putting it in the stream
storiesBloc.fetchTopIds();
//Creating a screen ths shows a list
return NewsList();
In my case however, there are two major differences:
After collecting the data in the GroupSearchScreen, I call/create the GroupsListScreen, where the list of groups shall be shown, using regular routing.
Like this:
//Add data to stream ("changeGroupList" privides the add function of the stream!)
appBloc.groupsBlock.changeGroupList(groups);
//Call/create screen to show list of groups
Navigator.pushNamed(context, '/groups_list');
In the GroupsListScreen, that is created, I fetch the bloc.
Like this:
Widget build(context) {
final AppBloc appBloc = AppBlocProvider.of(context);
These are the routes:
Route routes(RouteSettings settings) {
switch (settings.name) {
case '/':
return createLoginScreen();
case '/search_group':
return createSearchGroupScreen();
case '/groups_list':
return createGroupsListScreen();
default:
return null;
}
}//routes
And "/groups_list" points to this function:
Route createSearchGroupScreen() {
return MaterialPageRoute(
builder: (context) {
//Do we need a DashboardScreen BLOC?
//final storiesBloc = StoriesProvider.of(context);
//storiesBloc.fetchTopIds();
return GroupSearchScreen();
}
);
}
As you can see, the "AppBlocProvider" is only used once.
(I ran into that problem too ;-)
Now to the problem:
When the GroupsListScreen starts rendering the list, the stream/snapshot has no data!
(See "if (!snapshot.hasData)" )
Widget buildList(AppBloc appBloc) {
return StreamBuilder(
stream: appBloc.groupsBlock.groups,
builder: (context, AsyncSnapshot<List<Map<String, dynamic>>>snapshot) {
if (!snapshot.hasData) {
In order to test if all data in the bloc gets lost, I tried not to put the data in the stream directly, but in a member variable (in the bloc!).
In GroupSearchScreen I put the json data in a member variable in the bloc.
Now, just before the GroupsListScreen starts rendering the list, I take the data (json) out of the bloc, put it in the stream, which still resides in the bloc, and everything works fine!
The snapshot has data...
Like this (in the GroupsListScreen):
//Add data to Stream
appBloc.groupsBlock.changeGroupList(appBloc.groupsBlock.groupSearchResult);
Why on earth is the stream "losing" its data on the way from "GroupSearchScreen" to "GroupsListScreen" when the ordinary member variable is not? Both reside in the same bloc!
At the start of the build method of the GroupsListScreen, I have a print statement.
Hence I can see that GroupsListScreen is built twice.
That should be normal, but could that be the reason for not finding data in the stream?
Is the Stream listened on twice?
Widget buildList(AppBloc appBloc) {
return StreamBuilder(
stream: appBloc.groupsBlock.groups,
I tried to explain my problem this way, not providing tons of code.
But I don't know if it's enough to give a hint where I can continue to search...
Update 16.04.2019 - SOLUTION:
I built up my first app using another app seen in a Udemy course...
The most important difference between "his" app and mine is that he creates the Widget that listens to the stream and then adds data to the stream.
I add data to the stream and then navigate to the Widget that shows the data.
Unfortunately I used an RX-Dart "PublishSubject." If you listen to that one you will get all the data put in the stream starting at that time you started listening!
An RX-Dart "BehaviorSubject" however, will also give you the last data, just before you started listening.
And that's the behavior I needed here:
Put data on stream
Create Widget and start listening
I can encourage all Flutter newbies to read both of these very good tutorials:
https://medium.com/flutter-community/reactive-programming-streams-bloc-6f0d2bd2d248
https://www.didierboelens.com/2018/12/reactive-programming---streams---bloc---practical-use-cases/
In the first one, both of the streams mentioned, are explained very well.