event_bus

Package	Kind	Provides the role	Needs	Choose with
smf_event_bus	infrastructure	Events	nothing	-m event_bus

event_bus lets parts of the app that do not know each other, such as two features, exchange events through one service. The service is built on the event_bus package.

What it adds to the app

The module adds event_bus: ^2.0.1 to the dependencies, and lib/core/events/event_bus_communication_service.dart with the implementation of the service on an EventBus.

Whichever module provides it, the events role adds lib/core/events/communication_service.dart with:

• AppEvent, the base class of the events, such as final class ItemAdded extends AppEvent;
• the CommunicationService interface, where fire(event) sends an event and on<T>() is the stream of the events of type T;
• createCommunicationService(), which returns the service of the app.

In the guide for coding agents of the app, the events role describes the service under "Events", and the module adds when a listener gets an event.

How events are delivered

• An event goes to everyone listening to its type, or to a type it extends or implements, so on<AppEvent>() gets every event.
• Listeners get an event after the code that fires it has completed, not during fire.
• A listener gets the events fired after it starts listening.

Whichever module provides the events role, a listener of a type gets each event of that type once, in the order the events were fired, and a listener of an unrelated type gets none. SMF's CI checks this in every app with the role in the matrix of the built-in modules.

Getting the service

The service is created on first use, without waiting, so the app starts as fast as it would without it. With a module of dependency injection, the events role registers the service in the container as a lazy singleton, and a feature takes it with resolve in its composition file. Without a container, the code of the app gets it from createCommunicationService(), which always returns the same service. Don't create a second one with createEventBusCommunicationService(), because the listeners of one service do not get the events fired into another. See Services and state.

Choosing it

smf create asks which module provides the events of the app, and offers None as well. To choose it without the question:

smf create my_app -m event_bus