What is a Stream?

Learn that a Stream emits multiple values over time.

What is Stream?

Stream is a data flow that can produce multiple values over time. While a Future produces a single value, a Stream keeps emitting values.

Stream<int> numbers() async* {
  yield 1;
  yield 2;
  yield 3;
}

You produce a Stream with async* and yield.

async* and yield

To produce a Stream, you write a function marked with async*. You publish each value with yield.

Stream<int> numbers() async* {
  yield 1;
  yield 2;
}

Each yield adds a value to the stream.

Exercise Produce a number stream with async*

Write an async* function named sayiAkisi returning Stream<int>. Produce 0, 1 and 2 with yield. In main, listen to the stream and print each value with print('değer: $deger').

Show example solution

import 'package:flutter/material.dart';

Stream<int> sayiAkisi() async* {
  yield 0;
  yield 1;
  yield 2;
}

void main() {
  final stream = sayiAkisi();
  stream.listen((deger) {
    print('değer: $deger');
  });
}

Test yourself

What is the main difference between Stream and Future?
- Stream only throws errors
- Future produces one value, Stream many
- Stream produces one value, Future many
- Neither produces any value
Show answer

Future produces one value, Stream many — Future produces a single value and completes; Stream keeps producing multiple values over time.

Produce a value with yield

Stream<int> sayilar() async* {
  ____ 1;
  yield 2;
}

Show answer

Stream<int> sayilar() async* {
  yield 1;
  yield 2;
}

Stream parts

Stream

Flow producing multiple values over time
async*

Marker for Stream-producing function
yield

Adding a value to the stream
Future

Async result producing a single value

Next lesson: StreamController

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