How to make the connection to waiting state by using StreamBuilder in flutter - flutter

Question

My requirement is to make that StreamBuilder connection state to waiting.
I'm using publish subject, whenever I want to load data in stream builder I'm just adding data to the sink by calling postStudentsToAssign() method, here this method making an API call which takes some time, in that time I to want make that streamBuilder connection state to waiting
Stream Builder:
StreamBuilder(
stream: studentsBloc.studentsToAssign,
// initialData: [],
builder: (context, snapshot) {
switch (snapshot.connectionState) {
case ConnectionState.waiting:
// While waiting for the data to load, show a loading spinner.
return getLoader();
default:
if (snapshot.hasError)
return Center(child: Text('Error: ${snapshot.error}'));
else
return _getDrawer(snapshot.data);
}
}),
Initializing Observable:
final _assignStudentSetter = PublishSubject<dynamic>();
Observable<List<AssignMilestoneModel>> get studentsToAssign =>
_studentsToAssignFetcher.stream;
Method that add's data to Stream:
postStudentsToAssign(int studyingClass, String milestoneId, String subject,
List studentList) async {
var response = await provider.postAssignedStudents(
studyingClass, milestoneId, subject, studentList);
_assignStudentSetter.sink.add(response);
}

Answer 1

You can send null to the stream, so the snapshot.connectionState changes to active. I don't know why and whether it's official solution, but it works (at least now). I found this accidentally.
I would like the Flutter team to explain how to set snapshot's connectionState. It's not clear from StreamBuilder documentation. It seems you should replace the stream with a new one to have snapshot in waiting state. But it's agains the logic you want to implement.
I checked StreamBuilder source to find out that the AsyncSnapshot.connectionState starts as waiting (after stream is connected), after receiving data changes to active. snapshot.hasData returns true if snapshot.data != null. That's how following code works.
class SearchScreen extends StatelessWidget {
final StreamController<SearchResult> _searchStreamController = StreamController<SearchResult>();
final SearchService _service = SearchService();
void _doSearch(String text) async {
if (text?.isNotEmpty ?? false) {
_searchStreamController.add(null);
_searchService.search(text)
.then((SearchResult result) => _searchStreamController.add(result))
.catchError((e) => _searchStreamController.addError(e));
}
}
#override
Widget build(BuildContext context) {
return Scaffold(
body: Column(children: <Widget>[
SearchBar(
onChanged: (text) => _doSearch(text),
),
StreamBuilder<SearchResult>(
stream: _searchStreamController.stream,
builder: (BuildContext context, AsyncSnapshot<SearchResult> snapshot) {
Widget widget;
if (snapshot.hasData) {
widget = Expanded(
// show search result
);
}
else if (snapshot.hasError) {
widget = Expanded(
// show error
);
}
else if(snapshot.connectionState == ConnectionState.active){
widget = Expanded(
// show loading
);
}
else {
// empty
widget = Container();
}
return widget;
},
),
]),
);
}
}