Btukfyl

I've been trying to find a nice way of doing this but I had no luck.

Here is a simplified version of the problem:

import 'package:rxdart/rxdart.dart';





/// Input a list of integers [0,1,2,3,4]

/// map each of those integers to the corresponding index in the map

/// if the map updates, the output should update too.

///

/// The output should be a list of Strings:

/// ["Hi from 1", "Hi from 2"; "Hi from 3", "Hi from 4", "Hi from 5"]

BehaviorSubject<Map<int, String>> subject = BehaviorSubject(

    seedValue: {

      1: "Hi from 1",

      2: "Hi from 2",

      3: "Hi from 3",

      4: "Hi from 4",

      5: "Hi from 5",

    }

);



void main() {

  Observable.fromIterable([1, 2, 3, 4, 5])

      .flatMap((index) => subject.stream.map((map) => map[index]))

      .toList().asObservable()

      .listen((data) {

    print("List of data incoming $data");

  });

}

When running this, nothing is printed. This is because the subject never completes and thus the toList() never finishes building the list.

Replacing the subject with for example an Observable.just(index + 2) does work because the Observable completes and the toList() is able to collect them.

But the intended behavior is that the example should emit the new list of strings each time the subject is changed.

Any help would be appreciated,

Thanks!

You probably want to use combineLatest instead

BehaviorSubject<Map<int, String>> subject = BehaviorSubject(seedValue: {

  1: "Hi from 1",

  2: "Hi from 2",

  3: "Hi from 3",

  4: "Hi from 4",

  5: "Hi from 5",

});



void main() {

  Observable.combineLatest2(

      Observable.just([1, 2, 3, 4, 5]), subject.stream, combiner)

    ..listen(

      (data) {

        print("List of data incoming $data");

      },

    );

}



Iterable<String> combiner(List<int> indexes, Map<int, String> map) {

  return indexes.map((index) => map[index]);

}

Edit: shortly after posting this I realized that it completely depends on how the data is exposed:

If you have a method which has a signature like Observable<String> getStringForInt(int number) and you have to call it for every item in the list, my solution is the one for you. Note if the above method always subscribes to the same stream, changing that stream will result in multiple emissions (because combineLatest updates every item).

However, if you have access to the whole data container (like a Map

Original Answer

Ok, it turns out, it wasn't quite possible to achieve that with the implementation of rxDart.

The combineLatest constructor only supported up to 9 streams.

But since then, combineLatest with n-streams has been implemented and the solution to this problem would look something like this:

Map<int, BehaviorSubject<String>> subject2 = {

  1: BehaviorSubject<String>(seedValue: "Subject 1"),

  2: BehaviorSubject<String>(seedValue: "Subject 2"),

  3: BehaviorSubject<String>(seedValue: "Subject 3"),

  4: BehaviorSubject<String>(seedValue: "Subject 4"),

  5: BehaviorSubject<String>(seedValue: "Subject 5"),

};



void main() async {



  Observable.fromIterable([1, 2, 3, 4, 5])

      .toList().asObservable()

      .flatMap((numbers) =>

      Observable.combineLatest<String, List<String>>(numbers.map((index) => subject2[index]), (strings) => strings))

      .listen((data) {

    print("List of data incoming $data");

  });



  await Future.delayed(Duration(seconds: 2));

  subject2[1].add("I'm 42 now");

}

This will print:

List of data incoming [Subject 1, Subject 2, Subject 3, Subject 4, Subject 5]

List of data incoming [I'm 42 now, Subject 2, Subject 3, Subject 4, Subject 5]