Event Aggregator

The Event Aggregator is a pub/sub-event package that allows you to publish and subscribe to custom events inside your Aurelia applications. Some parts of the Aurelia framework use the event aggregator to publish certain events at various stages of the lifecycle and actions taking place.

The Event Aggregator is not for listening to native events. For those, you still use addEventListener and detachEventListener the event aggregator is a pub/sub package for publishing and subscribing to custom events.

Using the event aggregator

To use the Event Aggregator, we inject the IEventAggregator interface into our component. We inject it as IEventAggregator on our component class in the following code example.

import { ICustomElementViewModel, IEventAggregator } from 'aurelia';

export class MyComponent implements ICustomElementViewModel {    
    constructor(@IEventAggregator readonly ea: IEventAggregator) {

    }
 }   

Subscribing to events

The Event Aggregator provides a subscription method to subscribe to published events.

import { ICustomElementViewModel, IEventAggregator } from 'aurelia';

export class MyComponent implements ICustomElementViewModel {    
    constructor(@IEventAggregator readonly ea: IEventAggregator) {

    }
    
    bound() {
        this.ea.subscribe('event name', payload => {
            // Do stuff inside of this callback
        });
    }
 }   

Sometimes, you might only want to subscribe to an event once. To do that, we can use the subscribeOnce method to listen to the event and dispose of itself once it has been fired.

import { ICustomElementViewModel, IEventAggregator } from 'aurelia';

export class MyComponent implements ICustomElementViewModel {    
    constructor(@IEventAggregator readonly ea: IEventAggregator) {

    }
    
    bound() {
        this.ea.subscribeOnce('event name', payload => {
            // Do stuff inside of this callback just once
        });
    }
 }   

Publishing events

To publish (emit) an event, we use the publish method. You can provide an object to the publish method, which allows you to emit data via the event (accessible as a parameter on the subscribe method).

import { ICustomElementViewModel, IEventAggregator } from 'aurelia';

export class MyComponent implements ICustomElementViewModel {    
    constructor(@IEventAggregator readonly ea: IEventAggregator) {

    }
    
    bound() {
        const payload = {
            component: 'my-component',
            prop: 'value',
            child: {
                prop: 'value'
            }
        };
        
        this.ea.publish('component bound', payload);
    }
 }   

Disposing of event listeners

It's considered best practice to dispose of your event listeners when you are finished with them. Inside a component, you would usually do this inside of the unbinding method. The event will be of type IDisposable that we will use to type our class property strongly.

import { ICustomElementViewModel, IEventAggregator, IDisposable } from 'aurelia';

export class MyComponent implements ICustomElementViewModel {    
    private myEvent: IDisposable;
    
    constructor(@IEventAggregator readonly ea: IEventAggregator) {

    }
    
    bound() {
        this.myEvent = this.ea.subscribe('event name', payload => {
            // Do stuff inside of this callback
        });
    }
    
    unbinding() {
        this.myEvent.dispose();
    }
 }   

Always clean up after yourself. Like native Javascript events, you should dispose of any events properly to avoid memory leaks and other potential performance issues in your Aurelia applications.