How to unit test Flutter's StreamSubscription onData? - flutter

Question

This is what I tried so far.
Let's say result is a StreamSubscription.
This is my flutter file
try {
result.listen(
(event) async {
// This converts the JSON data
final news = NewsModel.fromJSON(jsonDecode(event));
// This saves the data to local database
await localDataSource.saveNews([news]);
},
onError: (e) {
debugPrint('$e');
},
);
} catch (e) {
debugPrint('$e');
}
this is my flutter test since I want to test if method localDataSource.saveNews() fails
await newsRepository.subscribe(); calls the try catch above
controller is a StreamController to add new data to the stream
news is a dummy data, it doesn't matter because whatever the localDataSource do it will throw a LocalDBException
also I am using Mockito https://pub.dev/packages/mockito to mock the localDataSource above
test(
'should handle fail save news method',
() async {
// arrange
when(mockLocalDataSource.saveNews(any)).thenThrow(LocalDBException());
// act
await newsRepository.subscribe();
controller.add(news)
// assert
},
);
As you can see I don't have any condition to pass the flutter test, but that's beyond the point as this flutter test already breaks the stream even if I have a onError on my listener.
if I use controller.addError(LocalDBException()) the onError works, but if I deliberately throw an exception from the method localDataSource.saveNews() it breaks the stream.
Given this context I want to know 2 things:
I want to know how to handle the error inside the onData of StreamSubscription, if a method / function throw an exception, as it ignores the onError if a method / function throw an exception inside the listener.
Is adding an error through addError() function the same as throwing an exception inside the stream?