Event Dispatcher

A event dispatcher for dart.

Installation

Follow the steps described on this page: https://pub.dev/packages/event_dispatcher_builder/install

You also need to install the build_runner package.

Configuration

To customize the builder, create a build.yaml beside your pubsepc.yaml with this content:

targets:
  $default:
    auto_apply_builders: true
    builders:
      event_dispatcher_builder|buildEventDispatcher:
        options:
          eventDispatcherClassName: 'DefaultEventDispatcher' # class name of the event dispatcher
          includePackageDependencies: false # True if subscribers from dependencies should be included.

Annotations

@Subscribe()

This annotation applies on methods. It is used to tell the generator that this method handles specific events. The method MUST HAVE one parameter with the type of the event. What type this is, is up to you.

Example:

// The event
class TestEvent {
  final String name;

  TestEvent({
    required this.name,
  });
}

class FakeHandler {
  List<String> eventTexts = [];

  // The method that listens to the event
  @Subscribe()
  void onTestEvent(TestEvent event) {
    eventTexts.add(event.name);
  }
}

@GenerateEventDispatcher()

This annotation SHOULD only occur once at the entry point of your application. It is used for generating the output file that contains your event dispatcher.

Usage

After annotating your event subscriber methods with @Subscribe and adding the @GenerateEventDispatcher() to the main function, you need to run dart run build_runner build or flutter pub run build_runner build.

You should find a new file named *.event_dispatcher_builder.g.dart next to the file you added the annotation.

This generated file should not be version controlled.

Import this file and create/set up a new instance of your event dispatcher.

void main() {
  // The class name depends on your configuration
  var eventDispatcher = DefaultEventDispatcher();

  eventDispatcher.addHandler(FakeHandler());
}

To dispatch events you can use the EventDispatcher.dispatch(event) method.

void main() {
  var eventDispatcher = DefaultEventDispatcher();

  eventDispatcher.addHandler(FakeHandler());

  // Dispatch a new event
  var event = TestEvent(name: 'Foo Bar');
  dispatcher.dispatch(event);
}

That's it 🙌

Automating the addHandler stuff

In large projects it can be tedious to manage all the addHandler stuff. Especially if the event handlers require additional services.

To optimize this, you can install the catalyst_builder package which generates a dependency injection container.

After installing and configuring it, you can create a HandlerRegistry class which is preloaded and add all this kind of code from above:

@Service()
@Preload()
class HandlerRegistry {

  HandlerRegistry(
    EventDispatcher dispatcher,
    @Inject(tag: #eventListener) List<Object> listeners,
  ) {

    for (var listener in listeners) {
      dispatcher.addHandler(listener, listener.runtimeType);
    }

  }
}

Your event subscriber classes need an additional annotation:

@Service(tags: [#eventListener]) // new
class FakeHandler {
  List<String> eventTexts = [];

  @Subscribe()
  void onTestEvent(TestEvent event) {
    eventTexts.add(event.name);
  }
}

Finally, you need to update the main annotations:

@GenerateEventDispatcher()
// New
@GenerateServiceProvider()
@ServiceMap(services: {
  // Register your event dispatcher inside the service container as a EventDispatcher 
  MyEventDispatcher: Service(
    lifetime: ServiceLifetime.singleton,
    exposeAs: EventDispatcher,
  )
})
void main(List<String> arguments) {
  // Load the provider 
  var provider = DefaultServiceProvider();
  // boot it
  provider.boot();

  // get the event dispatcher
  var dispatcher = provider.resolve<EventDispatcher>();

  // dispatch a event
  var event = TestEvent(name: 'Foo Bar');
  dispatcher.dispatch(event);
}