Add-ons using the techniques described in this document are considered a legacy technology in Firefox. Don't use these techniques to develop new add-ons. Use WebExtensions instead. If you maintain an add-on which uses the techniques described here, consider migrating it to use WebExtensions.
From Firefox 53 onwards, no new legacy add-ons will be accepted on addons.mozilla.org (AMO).
From Firefox 57 onwards, WebExtensions will be the only supported extension type, and Firefox will not load other types.
Even before Firefox 57, changes coming up in the Firefox platform will break many legacy extensions. These changes include multiprocess Firefox (e10s), sandboxing, and multiple content processes. Legacy extensions that are affected by these changes should migrate to WebExtensions if they can. See the "Compatibility Milestones" document for more.
A wiki page containing resources, migration paths, office hours, and more, is available to help developers transition to the new technologies.
Unstable
API for working with the application observer service.
Usage
The system/events module provides core (low level) API for working with the application observer service, also known as nsIObserverService. You can find a list of events dispatched by the Firefox codebase here.
var events = require("sdk/system/events");
var { Ci } = require("chrome");
function listener(event) {
var channel = event.subject.QueryInterface(Ci.nsIHttpChannel);
channel.setRequestHeader("User-Agent", "MyBrowser/1.0", false);
}
events.on("http-on-modify-request", listener);
Globals
Functions
emit(type, event)
Send an event to observer service
Parameters
type : string
The event type.
event : object
An optional object with data and subject attributes. data refers to a string that you would like to pass through this event. subject should refer to the actual actor/subject of this event (ie: the object emitting the event).
on(type, listener, strong)
Listen to events of a given type
Parameters
type : string
The event type name to watch.
listener : function
Function that will be called when an event is fired, with a single event object as argument. This object has three attributes:
- type: the event type name
- subject: the event subject object
- data: the event data string
strong : boolean
Default is false, a weak reference, which means it can be garbage collected at any time if there are no other references to it. Determines if we should keep a strong or weak reference to the listener method.
once(type, listener, strong)
Listen only once to a particular event type
Parameters
type : string
The event type name to watch.
listener : function
Function that will be called when an event is fired.
strong : boolean
Default is false, a weak reference, which means it can be garbage collected at any time if there are no other references to it. Determines if we should keep a strong or weak reference to the listener method.
off(type, listener)
Stop listening for an event
Parameters
type : string
The event type name to unsubscribe to.
listener : function
The function we registered to listen for events.