Module provides core (low level) API for working with the application
observer service, also known as
[nsIObserverService](https://developer.mozilla.org/en-US/docs/XPCOM_Interface_Reference/nsIObserverService).
You can find a list of events dispatched by firefox codebase
[here](https://developer.mozilla.org/en-US/docs/Observer_Notifications).
## Example
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);
@function
Send an event to observer service
@param type {string}
The event type.
@param [event] {object}
An optional object 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).
@function
Listen to events of a given type
@param type {string}
The event type name to watch.
@param 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
@param 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 of weak reference
to the listener method.
@function
Listen only once to a particular event type
@param type {string}
The event type name to watch.
@param listener {function}
Function that will be called when an event is fired.
@param 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 of weak reference
to the listener method.
@function
Stop listening for an event
@param type {string}
The event type name to unsubscribe to.
@param listener {function}
The function we registered to listen for events.