Event Emitter package (
@nestjs/event-emitter) provides a simple observer implementation, allowing you to subscribe and listen for various events that occur in your application. Events serve as a great way to decouple various aspects of your application, since a single event can have multiple listeners that do not depend on each other.
EventEmitterModule internally uses the eventemitter2 package.
First install the required package:
Once the installation is complete, import the
EventEmitterModule into the root
AppModule and run the
forRoot() static method as shown below:
.forRoot() call initializes the event emitter and registers any declarative event listeners that exist within your app. Registration occurs when the
onApplicationBootstrap lifecycle hook occurs, ensuring that all modules have loaded and declared any scheduled jobs.
To configure the underlying
EventEmitter instance, pass the configuration object to the
.forRoot() method, as follows:
To declare an event listener, decorate a method with the
@OnEvent() decorator preceding the method definition containing the code to be executed, as follows:
The first argument can be a
symbol for a simple event emitter and a
string | symbol | Array<string | symbol> in a case of a wildcard emitter. The second argument (optional) is a listener options object (read more).
To use namespaces/wildcards, pass the
wildcard option into the
EventEmitterModule#forRoot() method. When namespaces/wildcards are enabled, events can either be strings (
foo.bar) separated by a delimiter or arrays (
['foo', 'bar']). The delimiter is also configurable as a configuration property (
delimiter). With namespaces feature enabled, you can subscribe to events using a wildcard:
To dispatch an event, first inject
EventEmitter2 using standard constructor injection:
info Hint Import the
Then use it in a class as follows.
EventEmitter2class provides several useful methods for interacting with events, like
onAny. You can read more about them here.