event/target
Many objects in the SDK can broadcast events. For example, a
panel
instance emits an show
event when the
panel is shown.
The event/target
module enables you to create objects that broadcast
events. Users of the object can listen to the events using the standard
on()
and once()
functions.
Also see the tutorial on implementing event targets to get started with this API.
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.
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 defines 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.
API Reference
Classes
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)
});
The type of event.
The listener function that processes the event.
once(type, listener)
Registers an event listener
that is called only once:
the next time an event of the specified type
is emitted.
The type of event.
The listener function that processes the event.
removeListener(type, listener)
Removes an event listener
for the given event type
.
The type of event.
The listener function that processes the event.