Streams and await for

A Stream<T> delivers many values over time, where a Future delivers one. Consume it with await for (final x in stream) inside an async function, or with stream.listen(onData).

async* functions return a Stream; yield v emits one value and yield* other forwards another stream. The body runs only while someone listens.

Streams support pipelines like iterables: where, map, take, asyncMap (await per item) and fold. stream.toList() collects everything into a Future.

A single-subscription stream (the default) allows one listener. asBroadcastStream() or StreamController.broadcast() allows many listeners, but late listeners miss earlier events.

await for ends when the stream closes. A break cancels the subscription, which stops the generator.

Stream<int> countdown(int from) async* {
  for (var i = from; i > 0; i--) {
    yield i;
  }
}

Future<void> main() async {
  await for (final n in countdown(3)) {
    print(n);
  }
  final evens = await countdown(6).where((n) => n.isEven).toList();
  print(evens);
}