What are StreamControllers in Dart?

A StreamController object in Dart does exactly what the name suggests, it controls Dart Streams. The object is used to create streams and send data, error, and done events on them. Controllers also help check a stream’s properties, such as how many subscribers it has or if it’s paused.

To understand how controllers work, let’s play around with a controller for a stream that supports a single subscriber. The code below shows a single subscriber listening for events sent on a stream through a stream controller:

import 'dart:convert';
import 'dart:async';
// Initializing a stream controller
StreamController<String> controller = StreamController<String>();
// Creating a new stream through the controller
Stream<String> stream = controller.stream;
void main() {
    // Setting up a subscriber to listen for any events sent on the stream
    StreamSubscription<String> subscriber = stream.listen((String data) {
        print(data);
    },
    onError: (error) {
        print(error);
    },
    onDone: () {
        print('Stream closed!');
    });
    // Adding a data event to the stream with the controller
    controller.sink.add('Hello!');
    // Adding an error event to the stream with the controller
    controller.addError('Error!');
    // Closing the stream with the controller
    controller.close();
}

import 'dart:convert';
import 'dart:async';
// Initializing a stream controller for a broadcast stream
StreamController<String> controller = StreamController<String>.broadcast();
// Creating a new broadcast stream through the controller
Stream<String> stream = controller.stream;
void main() {
    // Setting up a subscriber to listen for any events sent on the stream
    StreamSubscription<String> subscriber1 = stream.listen((String data) {
        print('Subscriber1: ${data}');
    },
    onError: (error) {
        print('Subscriber1: ${error}');
    },
    onDone: () {
        print('Subscriber1: Stream closed!');
    });
    // Setting up another subscriber to listen for any events sent on the stream
    StreamSubscription<String> subscriber2 = stream.listen((String data) {
        print('Subscriber2: ${data}');
    },
    onError: (error) {
        print('Subscriber2: ${error}');
    },
    onDone: () {
        print('Subscriber2: Stream closed!');
    });
    // Adding a data event to the stream with the controller
    controller.sink.add('Hello!');
    // Adding an error event to the stream with the controller
    controller.addError('Error!');
    // Closing the stream with the controller
    controller.close();
}