Stream transformation

Learn stream transformation with map and where.

Stream transformation

Streams can be transformed with map and where. map transforms each event; where lets through events that match a condition. These methods return a new Stream.

Example: var filtered = stream.where((n) => n.isEven);. This lets only even number events through. var doubled = filtered.map((n) => n * 2); doubles each event.

To listen to the transformed Stream you use await for. Chaining is possible: stream.where(...).map(...). Each transform is lazy.

Chaining transforms

Stream transforms can be chained. Writing stream.where(...).map(...) you do multiple transforms in one flow. Each transform feeds the next.

Example: on a [1, 2, 3, 4] stream first where keeps evens (2, 4), then map doubles them (4, 8). Result: 4, 8. You listen with await for.

Chaining is a readable way to process data step by step. Each step has one responsibility; the code stays clear.

Exercise Filter and transform a Stream

void main() async is already written. Stream.fromIterable([1, 2, 3, 4]) is already created. First filter evens with .where((n) => n.isEven), then double with .map((n) => n * 2). Listen to the result with await for and print with print.

Show example solution

void main() async {
  var stream = Stream.fromIterable([1, 2, 3, 4]);
  var filtered = stream.where((n) => n.isEven);
  var doubled = filtered.map((n) => n * 2);
  await for (var n in doubled) {
    print(n);
  }
}

Test yourself

Match the transform methods

.where((n) => n.isEven)

Filter evens
.map((n) => n * 2)

Double
await for

Listen to result
print(n)

Print event
What does stream.where(...) return?
```
var s = Stream.fromIterable([1, 2, 3]);
var f = s.where((n) => n.isEven);
```
- A list
- A new Stream
- A Future
- void
Show answer

A new Stream — where returns a new Stream.

The map call

var filtered = stream.where((n) => n.isEven);
var doubled = filtered.____((n) => n * 2);

Show answer

var filtered = stream.where((n) => n.isEven);
var doubled = filtered.map((n) => n * 2);

Next lesson: asyncExpand

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