Can you explain async* and yield in Dart?

In Dart, the async* and yield keywords are used together to create asynchronous generators, which allow you to produce a sequence of asynchronous values — essentially, to create a Stream easily without manually managing a StreamController.

• async* marks a function as one that will emit multiple values asynchronously, returning a Stream.
• yield is used inside that function to emit values one by one over time.

Here’s a simple example:

Stream numberStream() async* {
  for (int i = 1; i <= 5; i++) {
    await Future.delayed(Duration(seconds: 1)); // simulate delay
    yield i; // emit one value at a time
  }
}

void main() async {
  await for (var number in numberStream()) {
    print('Received: $number');
  }
}

What happens here:

1. The function numberStream() is declared with async*, so it returns a Stream<int>.
2. Each time the loop runs, it waits for one second (because of the await) and then yields one value to the stream.
3. The await for loop in main() listens to this stream and prints each value as it arrives.

This is different from a normal async function, which can only return one Future value.
An async* function, on the other hand, can yield many values over time — just like a continuous data source.

I’ve applied this concept in a Flutter project where I needed to emit real-time timer values for a countdown feature. Instead of using a StreamController, I used async* with yield, which made the code cleaner and easier to maintain.

For example:

Stream countdown(int start) async* {
  for (int i = start; i >= 0; i--) {
    await Future.delayed(Duration(seconds: 1));
    yield i;
  }
}