What are streams?

Learn the stream concept and event sequences.

What is a Stream?

Stream<T> represents a sequence of events over time. A Future gives one result; a Stream delivers multiple values in order. It is used for event-driven data.

Example: var s = Stream.fromIterable([1, 2, 3]);. This is a stream that produces 1, 2 and 3 in order. To receive the values you use await for: await for (var n in s) { print(n); }.

Streams are used for user input, sensor data, network packets and other continuously flowing data. Each event is processed as it arrives; the flow continues until the stream closes.

Listening with await for

To receive the events of a Stream you use the await for loop. It resembles a for loop; but at each step it waits for the next event.

Example: await for (var n in stream) { print(n); }. Each time the Stream sends an event n takes its value and print runs. When the Stream closes the loop ends.

await for works only in async functions. So you must write void main() async. The loop waits until the Stream completes.

Exercise Print Stream events

void main() async is already written. Create a stream with Stream.fromIterable([1, 2, 3]). Wait for each event with await for and print it with print.

Show example solution

void main() async {
  var s = Stream.fromIterable([1, 2, 3]);
  await for (var n in s) {
    print(n);
  }
}

Test yourself

What is the difference between Stream and Future?
- They are exactly the same
- Stream is faster
- Future delivers multiple values
- Stream delivers multiple values, Future one value
Show answer

Stream delivers multiple values, Future one value — Stream delivers multiple values in sequence, Future gives one result.

Creating a Stream

var s = ____.fromIterable([1, 2, 3]);

Show answer

var s = Stream.fromIterable([1, 2, 3]);

Match the Stream parts

Stream.fromIterable

Stream creation
await for

Listening
var n in stream

Event variable
print(n)

Print event

Next lesson: Listening to streams

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