event/target

Create objects that broadcast events. Users of the object can listen to the events using the standard on() and once() functions.

Usage

Many objects in the SDK can broadcast events. For example, a panel instance emits an show event when the panel is shown. With this module you can create your own objects that emit events.

This module provides an exemplar EventTarget object, that implements an interface for adding and removing event listeners of a specific type. EventTarget is the base class for all objects in SDK on which events are emitted.

Also see the tutorial on implementing event targets to get started with this API.

Instantiation

It's easy to create event target objects, no special arguments are required.

const { EventTarget } = require("sdk/event/target");
let target = EventTarget();

For a convenience though optional options arguments may be used, in which case all the function properties with keys like: onMessage, onMyEvent... will be auto registered for associated 'message', 'myEvent' events on the created instance. All other properties of options will be ignored.

Adding listeners

EventTarget interface defines on method, that can be used to register event listeners on them for the given event type:

target.on('message', function onMessage(message) {
  // Note: `this` pseudo variable is an event `target` unless
  // intentionally overridden via `.bind()`.
  console.log(message);
});

Sometimes event listener may care only about very first event of specific type. EventTarget interface defines convenience method for adding one shot event listeners via method once. Such listeners are called only once next time event of the specified type is emitted:

target.once('ready', function onReady() {
  // Do the thing once ready!
});

Removing listeners

EventTarget interface defines API for unregistering event listeners, via removeListener method:

target.removeListener('message', onMessage);

Emitting events

EventTarget interface intentionally does not define an API for emitting events. In majority of cases party emitting events is different from party registering listeners. In order to emit events one needs to use event/core module instead:

let { emit } = require('sdk/event/core');

target.on('hi', function(person) { console.log(person + ' says hi'); });
emit(target, 'hi', 'Mark'); // info: 'Mark says hi'

For more details see event/core documentation.

More details

Listeners registered during the event propagation (by one of the listeners) won't be triggered until next emit of the matching type:

let { emit } = require('sdk/event/core');

target.on('message', function onMessage(message) {
  console.log('listener triggered');
  target.on('message', function() {
    console.log('nested listener triggered');
  });
});

emit(target, 'message'); // info: 'listener triggered'
emit(target, 'message'); // info: 'listener triggered'
                         // info: 'nested listener triggered'

Exceptions in the listeners can be handled via 'error' event listeners:

target.on('boom', function() {
  throw Error('Boom!');
});
target.once('error', function(error) {
  console.log('caught an error: ' + error.message);
});
emit(target, 'boom');
// info: caught an error: Boom!

If there is no listener registered for error event or if it also throws exception then such exceptions are logged into a console.

Chaining

Emitters can also have their methods chained:

target.on('message', handleMessage)
  .on('data', parseData)
  .on('error', handleError);

EventTarget

EventTarget is an exemplar for creating an objects that can be used to add / remove event listeners on them. Events on these objects may be emitted via emit function exported by event/core module.

Methods

initialize()

Method initializes this event source. It goes through properties of a given options and registers listeners for the ones that look like event listeners.

on(type, listener)

Registers an event listener that is called every time events of specified type are emitted.

worker.on('message', function (data) {
  console.log('data received: ' + data)
});

Parameters

type : String
The type of event.

listener : Function
The listener function that processes the event.

Returns

EventTarget : Returns the EventTarget instance

once(type, listener)

Registers an event listener that is called only once: the next time an event of the specified type is emitted.

Parameters

type : String
The type of event.

listener : Function
The listener function that processes the event.

Returns

EventTarget : Returns the EventTarget instance

removeListener(type, listener)

Removes an event listener for the given event type.

Parameters

type : String
The type of event.

listener : Function
The listener function that processes the event.

Returns

EventTarget : Returns the EventTarget instance

off()

An alias for removeListener.