Channel primitive in dart?

Question

Is there a channel primitive in Dart as there is in Go for example? The closest I found is StreamIterator.

The use case would be to allow a consumer to asynchronously process values one by one. Additionally, if there is no value, it should just wait.

Answer 1

You can do this with streams

import 'dart:async' show Stream, StreamController;

main() {
  StreamController<int> sc = new StreamController<int>();

  sc.stream.listen((e) => print(e));

  for(int i = 0; i < 10; i++) {
    sc.add(i);
  }
}

Try it in DartPad

See also

• https://www.dartlang.org/docs/tutorials/streams/
• https://www.dartlang.org/articles/creating-streams/

You can't "just wait" in Dart, especially in the browser.

There are also async* generator functions available in Dart to create streams. See for example Async/Await feature in Dart 1.8

A "naive" translation form await for(var event in someStream)

import 'dart:async';

main() {
  StreamController<int> sc = new StreamController<int>();

  startProcessing(sc.stream);

  for(int i = 0; i < 10; i++) {
    sc.add(i);
  }
}

Future startProcessing(Stream stream) async {
  StreamSubscription subscr;
  subscr = stream.listen((value) {
    subscr.pause();
    new Future.delayed(new Duration(milliseconds: 500)).then((_) {
      print('Processed $value');
      subscr.resume();
    });
  });
  //await for(int value in stream) {
  //  await new Future.delayed(new Duration(milliseconds: 500)).then((_) {
  //    print('Processed $value');
  //  });
  //} 
}

The actual implementation takes a different approach

The way the await-for loop is implemented uses a one-event buffer (to avoid repeatedly pausing and resuming).

See https://github.com/dart-lang/sdk/issues/24956#issuecomment-157328619