I have a Dart problem when I use map in a stream - dictionary

Question

I create a simple stream, add a few elements and listen to the stream. But there is a problem when I use the map on the stream. For simplicity I just map the value to the same value (I guess?).
When I try to run the program I get an map-error:
Uncaught Error: TypeError: Instance of '_MapStream<String, dynamic>': type '_MapStream<String, dynamic>' is not a subtype of type 'Stream<String>'
import 'dart:async';
void main() async {
StreamController controller = StreamController<String>.broadcast();
final StreamTransformer transformer = StreamTransformer<String, String>.fromHandlers(
handleData: (data, EventSink sink) {
sink.add(data);
}
);
Stream stream = controller.stream;
stream
.map((value) => value) // <-- Problem in this line
.transform(transformer)
.listen(
(data) {
print('listen: $data');
},
onError: (err) => print(err));
controller.add('foo');
controller.add('baa');
controller.close();
}

Answer 1

Your problem is you are forcing non-generic types from generic types. If you write the code like this where you are not use explicit typing:
import 'dart:async';
void main() async {
final controller = StreamController<String>.broadcast(); // <-- fixed here
final StreamTransformer transformer =
StreamTransformer<String, String>.fromHandlers(
handleData: (data, EventSink sink) {
sink.add(data);
});
final stream = controller.stream; // <-- fixed here
stream
.map((value) => value) // <-- Problem in this line
.transform(transformer)
.listen((data) {
print('listen: $data');
}, onError: (err) => print(err));
controller.add('foo');
controller.add('baa');
controller.close();
}
Dart will then automatically determine that the type is StreamController<String> and Stream<String> and not StreamController and Stream which you are enforcing.
When you are removing the generic part of the type you are also removing Dart's ability to guess the type for all methods you are calling.
If using explicit typing you should write:
import 'dart:async';
void main() async {
StreamController<String> controller = StreamController<String>.broadcast();
final StreamTransformer transformer =
StreamTransformer<String, String>.fromHandlers(
handleData: (data, EventSink sink) {
sink.add(data);
});
Stream<String> stream = controller.stream;
stream
.map((value) => value) // <-- Problem in this line
.transform(transformer)
.listen((data) {
print('listen: $data');
}, onError: (err) => print(err));
controller.add('foo');
controller.add('baa');
controller.close();
}