Flutter: how to mock a stream - testing

Question

I´m using the bloc pattern and now I came to test the UI. My question is: how to mock streams?
This is the code that I have:
I give to the RootPage class an optional mockBloc value and I will set it to the actual _bloc if this mockBloc is not null
class RootPage extends StatefulWidget {
final loggedOut;
final mockBlock;
RootPage(this.loggedOut, {this.mockBlock});
#override
_RootPageState createState() => _RootPageState();
}
class _RootPageState extends State<RootPage> {
LoginBloc _bloc;
#override
void initState() {
super.initState();
if (widget.mockBlock != null)
_bloc = widget.mockBlock;
else
_bloc = new LoginBloc();
if (widget.loggedOut == false)
_bloc.startLoad.add(null);
}
...
#override
Widget build(BuildContext context) {
return StreamBuilder(
stream: _bloc.load,
builder: (context, snapshot) {
if (snapshot.hasData) {
return Column(
children: <Widget>
...
This is what I´ve tried:
testWidgets('MyWidget', (WidgetTester tester) async {
MockBloc mockBloc = new MockBloc();
MockTokenApi mockTokenApi = new MockTokenApi();
await tester.pumpWidget(new MaterialApp(
home: RootPage(false, mockBlock: mockBloc)));
when(mockBloc.startLoad.add(null)).thenReturn(mockBloc.insertLoadStatus.add(SettingsStatus.set)); //this will give in output the stream that the StreamBuilder is listening to (_bloc.load)
});
await tester.pump();
expect(find.text("Root"), findsOneWidget);
});
The result that I achieve is always to get:
The method 'add' was called on null
when _bloc.startLoad.add(null) is called

Answer 1

You can create a mock Stream by creating a mock class using mockito. Here's a sample on how one can be implemented.
import 'package:mockito/mockito.dart';
class MockStream extends Mock implements Stream<int> {}
void main() async {
var stream = MockStream();
when(stream.first).thenAnswer((_) => Future.value(7));
print(await stream.first);
when(stream.listen(any)).thenAnswer((Invocation invocation) {
var callback = invocation.positionalArguments.single;
callback(1);
callback(2);
callback(3);
});
stream.listen((e) async => print(await e));
}

Answer 2

Since the advent of null safety, you can do this as follows:
import 'package:mockito/annotations.dart';
import 'package:mockito/mockito.dart';
import 'package:test/test.dart';
import 'test_test.mocks.dart';
#GenerateMocks([Stream])
void main() {
test('foo', () async {
var stream = MockStream();
Stream<int> streamFunc() async* {
yield 1;
yield 2;
yield 3;
}
when(stream.listen(
any,
onError: anyNamed('onError'),
onDone: anyNamed('onDone'),
cancelOnError: anyNamed('cancelOnError'),
)).thenAnswer((inv) {
var onData = inv.positionalArguments.single;
var onError = inv.namedArguments[#onError];
var onDone = inv.namedArguments[#onDone];
var cancelOnError = inv.namedArguments[#cancelOnError];
return streamFunc().listen(onData, onError: onError, onDone: onDone, cancelOnError: cancelOnError);
});
stream.listen((e) async => print(await e));
});
}
source