StreamController
- Dart course
- Intermediate
- Lesson 68/116
- 5 min
Learn to create streams with StreamController.
StreamController
StreamController lets you create your own Stream manually. You send events with the add method; you close the stream with close. Listeners subscribe through controller.stream.
Example: var controller = StreamController<int>();. You send an event with controller.add(10);. You close the stream with controller.close();. You listen with await for (var n in controller.stream).
StreamController lives in the dart:async library; import 'dart:async'; is required. In real apps it is used to produce Streams from user input, timers and similar sources.
Controller flow
The flow of using a StreamController is: first create with StreamController<T>(). Send events with add. Close the stream with close. Finally listen through controller.stream with await for.
Example: var controller = StreamController<int>(); controller.add(10); controller.add(20); controller.close(); await for (var n in controller.stream) { print(n); }. Output: 10, 20.
The controller buffers events; when a listener subscribes it sends them in order. After close the stream ends and await for exits.
Practice Create a stream with StreamController
import 'dart:async' is already added. void main() async is already written. Create a StreamController<int>(). Send two events with controller.add(10) and controller.add(20). Close with controller.close(). Listen with await for (var n in controller.stream) and print with print.
Starter code
import 'dart:async';
void main() async {
// create a StreamController<int>, add two values, close
// print elements with await for on the stream
}Sample solution
import 'dart:async';
void main() async {
var controller = StreamController<int>();
controller.add(10);
controller.add(20);
controller.close();
await for (var n in controller.stream) {
print(n);
}
}Expected output
10 20
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Creating a StreamController
var controller = ____<int>();var controller = StreamController<int>();Explanation: StreamController<int>() creates a stream manually.
2 Which method is called to add an event to a StreamController?
var c = StreamController<int>();
c.???(10);- emit
- push
- send
- add Correct answer
Explanation: the add method sends an event to the controller.
3 Match the StreamController parts
| Term | Meaning |
|---|---|
StreamController<int>() |
Creation |
controller.add(10) |
Send event |
controller.close() |
Close stream |
controller.stream |
Stream access |