Transforming Streams: map, where, take
Reshape an event sequence.
Transforming Streams: map, where, take is a free Dart Academy lesson on CoddyKit — lesson 3 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 Dart Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.
Streams Are Transformable
Just like lists, streams have handy methods that reshape their events. You can transform a stream into a new one without touching the source.
map Transforms Each Event
The map method runs a function on every event and emits the result. It turns one stream of values into a stream of transformed values.
final doubled = numbers.map((n) => n * 2);map Keeps the Count
With map, the number of events stays the same; only their shape changes. Three inputs always produce exactly three outputs. ✨
Stream.fromIterable([1, 2, 3])
.map((n) => 'Item $n');where Filters Events
The where method keeps only events that pass a test. Anything returning false is dropped from the new stream.
final evens = numbers.where((n) => n.isEven);where Can Shrink the Stream
Unlike map, where may emit fewer events than it received. If nothing passes the test, the resulting stream stays empty.
take Limits the Count
The take method emits only the first n events, then stops listening. It is perfect when you need just a few values from a long stream. 🎯
final firstThree = bigStream.take(3);take Cancels Early
After delivering its n events, take automatically cancels its subscription to the source. The original stream stops producing for you.
Chaining Transformations
These methods return new streams, so you can chain them. Read the chain left to right: filter, then transform, then limit.
numbers
.where((n) => n.isEven)
.map((n) => n * 10)
.take(2);Lazy by Nature
Transformations are lazy: nothing runs until you listen. The chain only processes events as they arrive, one at a time.
More Tools: takeWhile and skip
Beyond the core three, takeWhile emits until a test fails, and skip ignores the first n events. The same chaining style applies.
stream.skip(2).takeWhile((n) => n < 100);Consuming the Result
After transforming, read the new stream with await for or listen. Each emitted value has already passed through your whole chain.
await for (final v in transformed) {
print(v);
}Quick Check
Which method can reduce the event count?
Recap
You can reshape streams with map to transform, where to filter, and take to limit, all chainable and lazy until you listen. 🎉
Frequently asked questions
Is the “Transforming Streams: map, where, take” lesson free?
Yes — the full text of “Transforming Streams: map, where, take” is free to read here on the web, and the Dart Academy 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 Dart Academy course, upgrade to CoddyKit PRO.
What will I learn in “Transforming Streams: map, where, take”?
Reshape an event sequence. You practise Dart Academy 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 Dart Academy?
No prior experience is required. Dart Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 3 of 4, so you can start here or from the beginning and move at your own pace.
How long does the “Transforming Streams: map, where, take” 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 Dart Academy lesson?
Yes. Every Dart Academy 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
- Listening With await for
- Single vs Broadcast Streams
- Transforming Streams: map, where, take
- Creating Streams With async*