[d876f2]: addon-sdk / source / doc / module-source / sdk / event / target.md  Maximize  Restore  History

Download this file

175 lines (133 with data), 5.6 kB

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.

Chaining

Emitters can also have their methods chained:

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

<api name="EventTarget">
@class
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.

<api name="initialize">
@method
Method initializes this event source. It goes through properties of a
given options and registers listeners for the ones that look like
event listeners.
</api>

<api name="on">
@method
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)
});

@param type {String}
The type of event.
@param listener {Function}
The listener function that processes the event.
@returns {EventTarget}
Returns the EventTarget instance
</api>

<api name="once">
@method
Registers an event listener that is called only once:
the next time an event of the specified type is emitted.
@param type {String}
The type of event.
@param listener {Function}
The listener function that processes the event.
@returns {EventTarget}
Returns the EventTarget instance
</api>

<api name="removeListener">
@method
Removes an event listener for the given event type.
@param type {String}
The type of event.
@param listener {Function}
The listener function that processes the event.
@returns {EventTarget}
Returns the EventTarget instance
</api>

<api name="off">
@method
An alias for removeListener.
</api>

</api>

Get latest updates about Open Source Projects, Conferences and News.

Sign up for the SourceForge newsletter:





No, thanks