How can I merge multiple Streams into a higher level Stream?

import 'dart:async' show Stream;
import 'package:async/async.dart' show StreamGroup;

main() async {
  var s1 = stream(10);
  var s2 = stream(20);
  var s3 = StreamGroup.merge([s1, s2]);
  await for(int val in s3) {
    print(val);
  }
}

Stream<int> stream(int min) async* {
  int i = min;
  while(i < min + 10) {
    yield i++;
  }
}

See also http://news.dartlang.org/2016/03/unboxing-packages-async-part-2.html

prints

10
20
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You can use StreamZip in package:async to combine two streams into one stream of pairs, then create the C objects from that.

import "package:async" show StreamZip;
...
Stream<C> createCs(Stream<A> as, Stream<B> bs) =>
  new StreamZip([as, bs]).map((ab) => new C(ab[0], ab[1]));

If you need to react when either Stream<A> or Stream<B> emits an event and use the latest value from both streams, use combineLatest.

Stream<C> merge(Stream<A> streamA, Stream<B> streamB) {
  return streamA
    .combineLatest(streamB, (a, b) => new C(a, b));
}

For people that need to combine more than two streams of different types and get all latest values on each update of any stream.

import 'package:stream_transform/stream_transform.dart';

Stream<List> combineLatest(Iterable<Stream> streams) {
  final Stream<Object> first = streams.first.cast<Object>();
  final List<Stream<Object>> others = [...streams.skip(1)];
  return first.combineLatestAll(others);
}

The combined stream will produce:

streamA:  a----b------------------c--------d---|
streamB:  --1---------2-----------------|
streamC:  -------&----------%---|
combined: -------b1&--b2&---b2%---c2%------d2%-|

Why not StreamZip? Because StreamZip would produce:

streamA:  a----b------------------c--------d---|
streamB:  --1---------2-----------------|
streamC:  -------&----------%---|
combined: -------a1&-------b2%--|

Usage:

Stream<T> sA;
Stream<K> sB;
Stream<Y> sC;
combineLatest([sA, sB, sC]).map((data) {
  T resA = data[0];
  K resB = data[1];
  Y resC = data[2];
  return D(resA, resB, resC);
});