Streams in Dart

What is a Stream?

A Stream<T> is a sequence of asynchronous events over time — like a pipe delivering values one by one. Where a Future delivers a single result, a Stream delivers zero or more results over time. Examples: user clicks, file bytes, WebSocket messages, real-time sensor data.

Stream Types

• Single-subscription (default) — can only be listened to once; like reading a file
• Broadcast — multiple listeners at once; like a radio broadcast

Creating Streams

// From an iterable:
Stream<int> counter = Stream.fromIterable([1, 2, 3, 4, 5]);

// Periodic stream:
Stream<int> ticker = Stream.periodic(
  Duration(seconds: 1),
  (count) => count,
).take(5);  // stop after 5 events

// Async generator function:
Stream<int> countdown(int from) async* {
  for (int i = from; i >= 0; i--) {
    await Future.delayed(Duration(milliseconds: 500));
    yield i;
  }
}

// From a Future:
Stream<String> fromFuture = Stream.fromFuture(
  Future.delayed(Duration(seconds: 1), () => 'Done')
);

Listening to Streams

var stream = Stream.fromIterable([1, 2, 3, 4, 5]);
stream.listen(
  (data) => print('Data: $data'),
  onError: (e) => print('Error: $e'),
  onDone: () => print('Stream closed'),
);

// Full async generator example:
Future<void> main() async {
  await for (var n in countdown(5)) {
    print(n);
  }
  // 5, 4, 3, 2, 1, 0
}

Transforming Streams

Stream<int> numbers = Stream.fromIterable(
  List.generate(10, (i) => i + 1)
);

numbers
  .where((n) => n.isEven)          // keep even numbers
  .map((n) => n * n)               // square them
  .take(3)                          // first 3 only
  .listen(print);                   // 4, 16, 36

async for-in

Stream<String> messageStream() async* {
  var messages = ['Hello', 'World', 'From', 'Dart', 'Streams'];
  for (var msg in messages) {
    await Future.delayed(Duration(milliseconds: 200));
    yield msg;
  }
}

Future<void> main() async {
  await for (var message in messageStream()) {
    print(message);
  }
  print('All messages received');
}

Hello
World
From
Dart
Streams
All messages received

🧠 Quiz

Summary

A Stream<T> delivers multiple values over time. Create streams with async* generators using yield, or with factory constructors like Stream.fromIterable and Stream.periodic. Listen with .listen() or await for. Transform with .map(), .where(), .take() and other stream methods.