event_bus 0.3.0+2

  • Readme
  • Changelog
  • Example
  • Installing
  • 49

Event Bus #

A simple Event Bus using Dart Streams for decoupling applications.

Demo #

See Dart Event Bus in action.

All examples are also available in the example directory on GitHub.

Event Bus Pattern #

An Event Bus follows the publish/subscribe pattern. It allows listeners to subscribe for events and publishers to fire events. This enables objects to interact without requiring to explicitly define listeners and keeping track of them.

Event Bus and MVC #

The Event Bus pattern is especially helpful for decoupling MVC (or MVP) applications.

One group of MVC is not a problem.


But as soon as there are multiple groups of MVCs, those groups will have to talk to each other. This creates a tight coupling between the controllers.

Multi Model-View-Controllers

By communication through an Event Bus, the coupling is reduced.

Event Bus

Usage #

1. Create an Event Bus #

Create an instance of EventBus and make it available to other classes.

Usually there is just one Event Bus per application, but more than one may be set up to group a specific set of events.

import 'package:event_bus/event_bus.dart';

EventBus eventBus = new EventBus();

You can alternatively use the HierarchicalEventBus that filters events by event class including its subclasses.

EventBus eventBus = new EventBus.hierarchical();

Note: The default constructor will create an asynchronous event bus. To create a synchronous you must provide the optional sync: true attribute.

2. Define Events #

Any Dart class can be used as an event.

class UserLoggedInEvent {
  User user;

class NewOrderEvent {
  Order order;

3. Register Listeners #

Register listeners for a specific events:

eventBus.on(UserLoggedInEvent).listen((UserLoggedInEvent event) {

Register listeners for all events:

eventBus.on().listen((event) {
  // Print the runtime type. Such a set up could be used for logging.

About Dart Streams

EventBus uses Dart Streams as its underlying mechanism to keep track of listeners. You may use all functionality available by the Stream API. One example is the use of StreamSubscriptions to later unsubscribe from the events.

StreamSubscription loginSubscription = eventBus.on(UserLoggedInEvent).listen((UserLoggedInEvent event) {


4. Fire Events #

Finally, we need to fire an event.

User myUser = new User('Mickey');
eventBus.fire(new UserLoggedInEvent(myUser));

License #

The MIT License (MIT)

Changelog #

Version 0.3.0 (2014-09-08) #

  • BREAKING CHANGE: Changed and simplified the EventBus API. We can now dispatch any Dart object as an event. Before, we had to create an EventType for every type of event we wanted to fire. Now we can use any class as an event. Listeners can (optionally) filter events by that class.
  • Added a way to create a hierarchical event bus that filters events by class and their subclasses. This currently only works with classes extending other classes and not with implementing an interface. We might have to wait for https://code.google.com/p/dart/issues/detail?id=20756 to enable interfaces.
  • BREAKING CHANGE: The EventBus constructor defaults to async instead of sync!!. This matches the constructor of the Dart Streams and an async event bus might also be the more common use case.
  • BREAKING CHANGE: Removed LoggingEventBus. Reason is that logging can easily be implemented with a event listener that listens for all events and logs them.

Version 0.2.5 (2014-09-03) #

  • Update example.
  • Update readme with new links.
  • Update dependencies.

Version 0.2.4 (2013-11-11) #

  • Update to dart libraries 0.9.0.

Version 0.2.3 (2013-09-16) #

  • Fix issue #8: Add logging of events that flow through event bus

Version 0.2.2 (2013-09-16) #

  • Change default of SimpleEventBus to sync (same as factory in EventBus)

Version 0.2.1 (2013-07-01) #

  • Fix issue #6: Fire should accept null as data

Version 0.2.0 (2013-06-06) #

  • Update to new Dart SDK v0.5.13.1_r23552.
  • Using Darts new Stream.broadcast() factory.
  • Provide option for synchronous broadcasting of events.
  • Update unit tests and example.
  • Create demo page.

Version 0.1.3 (2013-05-19) #

  • Removed all occurrences of @override

Version 0.1.2 (2013-05-17) #

  • Change in README: contained wrong license (Apache instead of MIT).
  • Remove import 'package:meta/meta.dart' in event_bus.dart as it is not needed and may cause an error if used as pub package.

Version 0.1.1 (2013-04-29) #

  • Minor change in README to fix image links.

Version 0.1.0 (2013-04-29) #

  • Initial Version.


import 'dart:html';

import 'dart:async';
import 'events.dart';
import 'package:logging/logging.dart';

int counterA = 1;
int counterB = 1;

final _log = new Logger('event_bus_example');

void main() {
  // Init logging.
  // Log all events.
  eventBus.on().listen((event) => _log.finest('event fired:  ${event.runtimeType}'));
  // Initialize the listener boxes.
  Listener listener1 = new Listener(querySelector('#listener-1'));
  Listener listener2 = new Listener(querySelector('#listener-2'));
  // Init Event fields.
  LabelElement fireLabelA = querySelector('#fire-label-a');
  LabelElement fireLabelB = querySelector('#fire-label-b');
  ButtonElement fireButtonA = querySelector("#fire-button-a");
  ButtonElement fireButtonB = querySelector("#fire-button-b");
  fireButtonA.onClick.listen((_) {
    // -------------------------------------------------
    // Fire Event A
    // -------------------------------------------------
    eventBus.fire(new MyEventA('Received Event A [$counterA]'));
    fireLabelA.text = '--> fired [$counterA]';
  fireButtonB.onClick.listen((_) {
    // -------------------------------------------------
    // Fire Event B
    // -------------------------------------------------
    eventBus.fire(new MyEventB('Received Event B [$counterB]'));
    fireLabelB.text = '--> fired [$counterB]';

initLogging() {
  // Print output to console.
  Logger.root.onRecord.listen((LogRecord r) {
  // Root logger level.
  Logger.root.level = Level.FINEST;

class Listener {
  Element element;
  TextAreaElement output;
  StreamSubscription<String> subscription;
  Listener(this.element) {
    output = element.querySelector('textarea');
    // Init buttons.
    element.querySelector('.listen-a').onClick.listen((_) => listenForEventA());
    element.querySelector('.listen-b').onClick.listen((_) => listenForEventB());
    element.querySelector('.pause').onClick.listen((_) => pause());
    element.querySelector('.resume').onClick.listen((_) => resume());
    element.querySelector('.cancel').onClick.listen((_) => cancel());
  void listenForEventA() {
    if (subscription != null) {
      appendOuput('Already listening for an event.');
    } else {
      // -------------------------------------------------
      // Listen for Event A
      // -------------------------------------------------
      subscription = eventBus.on(MyEventA).listen((event) {
      appendOuput('Listening for Event A');
  void listenForEventB() {
    if (subscription != null) {
      appendOuput('Already listening for an event.');
    } else {
      // -------------------------------------------------
      // Listen for Event B
      // -------------------------------------------------
      subscription = eventBus.on(MyEventB).listen((MyEventB event) {
      appendOuput('Listening for Event B');
  void pause() {
    if (subscription != null) {
      appendOuput('Subscription paused.');
    } else {
      appendOuput('No subscription, cannot pause!');
  void resume() {
    if (subscription != null) {
      appendOuput('Subscription resumed.');
    } else {
      appendOuput('No subscription, cannot resume!');
  void cancel() {
    if (subscription != null) {
      subscription = null;
      appendOuput('Subscription canceled.');
    } else {
      appendOuput('No subscription, cannot cancel!');
  void appendOuput(String text) {
    output.scrollTop = output.scrollHeight;

Use this package as a library

1. Depend on it

Add this to your package's pubspec.yaml file:

  event_bus: ^0.3.0+2

2. Install it

You can install packages from the command line:

with pub:

$ pub get

Alternatively, your editor might support pub get. Check the docs for your editor to learn more.

3. Import it

Now in your Dart code, you can use:

import 'package:event_bus/event_bus.dart';
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The SDK constraint in pubspec.yaml doesn't allow the Dart 2.0.0 release. For information about upgrading it to be Dart 2 compatible, please see https://dart.dev/dart-2#migration.