Dart: how to append a transformer to an existing stream? - stream

Question

I'm looking a for a way to programmatically add a transformer to an existing stream that's already being listen to.
Example:
Stream numbers = new Stream.fromIterable([0,1,2,3]);
numbers.listen((number) => print(number));
Now in response to some UI event, I'd like to modify this stream by adding a mapping transformer, as if I originally wrote:
numbers.where((number) => number % 2 == 0);
All existing listeners should from now own only receive even numbers, without interruption. How can this be done?

Answer 1

Instead of thinking about it like "how do I dynamically insert a transformer into a stream", one possible way is to think about it like "how do I dynamically control a transformer that I already injected".
Here's an example of using a StreamTransformer:
var onlySendEvenNumbers = false; // controlled by some UI event handler
var originalStream = makeStreamOfStuff();
originalStream = originalStream.transform(new StreamTransformer.fromHandlers(
handleData: (int value, EventSink<int> sink) {
if (onlySendEvenNumber) {
if (value.isEven) {
sink.add(value);
}
} else {
sink.add(value);
}
}));
originalStream.listen(print); // listen on events like normal

Answer 2

One way I can think of doing that is filtering the Stream with a function that calls another function:
var filter = (n) => true;
Stream numbers = new String.fromIterable([0, 1, 2, 3]).where((n) => filter(n));
Then, when you want to change the filtering:
filter = (n) => n % 2 == 0;
A concrete example:
import 'dart:async';
main() {
var filter = (n) => true;
Stream numbers = new Stream.periodic(new Duration(seconds: 1), (n) => n)
.where((n) => filter(n));
numbers.listen((n) => print(n));
new Future.delayed(new Duration(seconds: 4)).then((_) {
filter = (n) => n % 2 == 0;
});
}
This will print:
0
1
2
3
4
6
8
10
12
And so on, for even numbers only, after 4 seconds.

Answer 3

What about rxdart's combineLatest2 ?
It combine two streams, and emit each time when changed both streams.
You can use Switch class for switch on/off with conditions.
class XsBloc {
Api _api = Api();
BehaviorSubject<List<X>> _xs = BehaviorSubject();
BehaviorSubject<Switcher> _switcher =
BehaviorSubject<Switcher>.seeded(Switcher(false, []));
XsBloc() {
Observable.combineLatest2<List<X>, Switcher, List<X>>(
_api.xs(), _switcher, (xs, s) {
if (s.isOn == true) {
return xs.where((x) => s.conditions.contains(x.id)).toList();
} else {
return xs;
}
}).listen((x) => _xs.add(x));
}
Stream<List<X>> get xs => _xs;
ValueObservable<Switcher> get switcher =>
_switcher.stream;
Function(Switcher) get setSwitcher => _switcher.sink.add;
}
class Switcher {
final bool isOn;
final List<String> conditions;
Switcher(this.isOn, this.conditions);
}
var bloc = XsBloc();
bloc.setSwitcher(true, ['A', 'B']);
bloc.setSwitcher(false, []);
bloc.setSwitcher(true, []);