Stream transformer
- Dart course
- Advanced
- Lesson 14/83
- 5 min
Learn transformation with StreamTransformer.
StreamTransformer
A StreamTransformer is an abstraction that turns one stream into another. It can map, filter, group or retime each element.
The simplest transform is .map(fn): it replaces each element via fn. .where(test) filters, .take(n) takes the first n elements.
For more complex transforms, write a custom transformer with StreamTransformer.fromHandlers. This is the basis of stream pipelines.
Building a pipeline
Multiple transformers can be chained: s.where((x) => x > 0).map((x) => x * 2).take(3). Each step returns a new stream; data does not flow backwards.
At the end of the chain comes a terminal operator: forEach, listen, toList, first. This call consumes the stream.
Transformers are lazy: data does not flow until a terminal is called. This avoids unnecessary computation.
Practice Double each element with map
Make main async. Add var s = Stream.fromIterable([1, 2, 3]);. Add await s.map((x) => x * 2).forEach(print);.
Starter code
void main() {
// make async; build [1,2,3] stream; double via map; print via forEach
}Sample solution
void main() async {
var s = Stream.fromIterable([1, 2, 3]);
await s.map((x) => x * 2).forEach(print);
}Expected output
2 4 6
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Transform each element
var t = s.____((x) => x * 2);var t = s.map((x) => x * 2);Explanation: map transforms each element.
2 Which is a StreamTransformer?
- .toList()
- .map(fn) Correct answer
- .length
- .toString()
Explanation: map turns one stream into another.
3 Match the operators
| Term | Meaning |
|---|---|
.map(fn) |
Transform |
.where(test) |
Filter |
.take(n) |
First n elements |
.forEach(fn) |
Consume |