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Flutter Mobile Development · Lesson

Streams & Reactive Data

Learn how to work with Dart Streams in Flutter to handle continuous asynchronous data such as live updates, sockets and reactive UI.

Streams & Reactive Data is a free Flutter Mobile Development lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Flutter Mobile Development learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Beyond a Single Future

A Future gives you one value later. A Stream gives you many values over time.

  • WebSocket messages
  • Sensor readings
  • Live database updates

Streams are the reactive backbone of async Dart.

Listening to a Stream

Subscribe to a stream with listen, supplying callbacks for data, errors and completion.

stream.listen(
  (data) => print('Got: ' + data.toString()),
  onError: (e) => print('Error: ' + e.toString()),
  onDone: () => print('Done'),
);

Creating Streams with async*

An async generator uses async* and yield to emit values.

Stream<int> countTo(int n) async* {
  for (var i = 1; i <= n; i++) {
    await Future.delayed(const Duration(seconds: 1));
    yield i;
  }
}

await for

Inside an async function you can iterate a stream with await for, processing each value as it arrives.

Future<void> run() async {
  await for (final value in countTo(3)) {
    print(value);
  }
}

Single vs Broadcast

A single-subscription stream allows one listener. A broadcast stream allows many. Convert with asBroadcastStream().

final broadcast = controller.stream.asBroadcastStream();
broadcast.listen((v) => print('A: ' + v.toString()));
broadcast.listen((v) => print('B: ' + v.toString()));

StreamController

A StreamController lets you push values into a stream from your own code.

final controller = StreamController<String>();
controller.add('hello');
controller.add('world');
controller.close();

Transforming Streams

Streams have functional operators like map, where and take.

stream
  .where((n) => n.isEven)
  .map((n) => n * 10)
  .take(5)
  .listen(print);

StreamBuilder Widget

In the UI, StreamBuilder rebuilds whenever the stream emits a new value.

StreamBuilder<int>(
  stream: countTo(5),
  builder: (context, snapshot) {
    if (!snapshot.hasData) return const CircularProgressIndicator();
    return Text('Value: ' + snapshot.data.toString());
  },
);

Reading the Snapshot

The AsyncSnapshot tells you the connection state and whether data or an error is present.

  • snapshot.connectionState
  • snapshot.hasData / snapshot.data
  • snapshot.hasError / snapshot.error
if (snapshot.hasError) return Text('Error: ' + snapshot.error.toString());

Cancelling Subscriptions

Always cancel subscriptions in dispose to avoid memory leaks.

late StreamSubscription sub;

@override
void dispose() {
  sub.cancel();
  super.dispose();
}

Backpressure & Errors

Streams can emit errors mid-flow. Handle them with handleError so one bad event does not kill the whole pipeline.

stream
  .handleError((e) => print('Skipped: ' + e.toString()))
  .listen(print);

Quick Check

Which widget rebuilds your UI automatically as a stream emits new values?

Recap

You learned to work with Streams:

  • async* / yield and StreamController to create them
  • map / where / take to transform
  • StreamBuilder to drive reactive UI
  • Cancelling subscriptions and handling errors

Streams let your app react to continuous async data in real time.

Frequently asked questions

Is the “Streams & Reactive Data” lesson free?

Yes — the full text of “Streams & Reactive Data” is free to read here on the web, and the Flutter Mobile Development course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Flutter Mobile Development course, upgrade to CoddyKit PRO.

What will I learn in “Streams & Reactive Data”?

Learn how to work with Dart Streams in Flutter to handle continuous asynchronous data such as live updates, sockets and reactive UI. You practise Flutter Mobile Development with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Flutter Mobile Development?

No prior experience is required. Flutter Mobile Development on CoddyKit is structured for beginners through advanced learners; this is — lesson 4 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “Streams & Reactive Data” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Flutter Mobile Development lesson?

Yes. Every Flutter Mobile Development lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

All lessons in this course

  1. Futures & Async/Await
  2. HTTP Requests & JSON
  3. Error Handling in Async
  4. Streams & Reactive Data
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