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  6. Working with Events

Working with Events

In This Article
  1. Adding Listeners
    1. Listening to all events
    2. Adding Listeners in Constructors
  2. Removing Event Listeners

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.

The Add-on SDK supports event-driven programming.

Objects emit events on state changes that might be of interest to add-on code, such as browser windows opening, pages loading, network requests completing, and mouse clicks. By registering a listener function to an event emitter an add-on can receive notifications of these events.

We talk about content scripts in more detail in the Working with Content Scripts guide.

Additionally, if you're using content scripts to interact with web content, you can define your own events and use them to communicate between the main add-on code and the content scripts. In this case one end of the conversation emits the events, and the other end listens to them.

So there are two main ways you will interact with the EventEmitter framework:

  • listening to built-in events emitted by objects in the SDK, such as tabs opening, pages loading, mouse clicks

  • sending and receiving user-defined events between content scripts and add-on code

This guide only covers the first of these; the second is explained in the Working with Content Scripts guide.

Adding Listeners

You can add a listener to an event emitter by calling its on(type, listener) method.

It takes two parameters:

  • type: the type of event we are interested in, identified by a string. Many event emitters may emit more than one type of event: for example, a browser window might emit both open and close events. The list of valid event types is specific to an event emitter and is included with its documentation.

  • listener: the listener itself. This is a function which will be called whenever the event occurs. The arguments that will be passed to the listener are specific to an event type and are documented with the event emitter.

For example, the following add-on registers a listener with the tabs module to listen for the ready event, and logs a string to the console reporting the event:

var tabs = require("sdk/tabs");
tabs.on("ready", function () {
  console.log("tab loaded");
});

It is not possible to enumerate the set of listeners for a given event.

The value of this in the listener function is the object that emitted the event.

Listening to all events

You can pass the wildcard "*" as the type argument. If you do this, the listener will be called for any event emitted by that object, and its argument will be the name of the event:

var ui = require("sdk/ui");
var panels = require("sdk/panel");
var self = require("sdk/self");
var panel = panels.Panel({
  contentURL: self.data.url("panel.html")
});
panel.on("*", function(e) {
  console.log("event " + e + " was emitted");
});
var button = ui.ActionButton({
  id: "my-button",
  label: "my button",
  icon: "./icon-16.png",
  onClick: handleClick
});
function handleClick(state) {
  panel.show({
    position: button
  });
}

This wildcard feature does not yet work for the tabs or windows modules.

Adding Listeners in Constructors

Event emitters may be modules, as is the case for the ready event above, or they may be objects returned by constructors.

In the latter case the options object passed to the constructor typically defines properties whose names are the names of supported event types prefixed with "on": for example, "onOpen", "onReady" and so on. Then in the constructor you can assign a listener function to this property as an alternative to calling the object's on() method.

For example: the ActionButton object emits an event when the button is clicked.

The following add-on creates a button and assigns a listener to the onClick property of the options object supplied to the button's constructor. The listener loads https://developer.mozilla.org/:

require("sdk/ui/button/action").ActionButton({
  id: "visit-mozilla",
  label: "Visit Mozilla",
  icon: "./icon-16.png",
  onClick: function() {
    require("sdk/tabs").open("https://developer.mozilla.org/");
  }
});

This is exactly equivalent to constructing the button and then calling the button's on() method:

var button = require("sdk/ui/button/action").ActionButton({
  id: "visit-mozilla",
  label: "Visit Mozilla",
  icon: "./icon-16.png"
});
button.on("click", function() {
  require("sdk/tabs").open("https://developer.mozilla.org/");
});

Removing Event Listeners

Event listeners can be removed by calling removeListener(type, listener), supplying the type of event and the listener to remove.

The listener must have been previously been added using one of the methods described above.

In the following add-on, we add two listeners to the tabs module's ready event. One of the handler functions removes the listener again.

Then we open two tabs.

var tabs = require("sdk/tabs");
function listener1() {
  console.log("Listener 1");
  tabs.removeListener("ready", listener1);
}
function listener2() {
  console.log("Listener 2");
}
tabs.on("ready", listener1);
tabs.on("ready", listener2);
tabs.open("https://www.mozilla.org");
tabs.open("https://www.mozilla.org");

We should see output like this:

info: tabevents: Listener 1
info: tabevents: Listener 2
info: tabevents: Listener 2

Listeners will be removed automatically when the add-on is unloaded.

Document Tags and Contributors

 Contributors to this page: wbamberg, evold
 Last updated by: wbamberg, Dec 1, 2016, 10:12:21 AM
See also
  1. WebExtensions
  2. Getting started
    1. What are WebExtensions?
    2. Your first WebExtension
    3. Your second WebExtension
    4. Anatomy of a WebExtension
    5. Example WebExtensions
  3. How to
    1. Intercept HTTP requests
    2. Modify a web page
    3. Add a button to the toolbar
    4. Implement a settings page
  4. Concepts
    1. Using the JavaScript APIs
    2. User interface components
    3. Content scripts
    4. Match patterns
    5. Internationalization
    6. Content Security Policy
    7. Native messaging
  5. Porting
    1. Porting a Google Chrome extension
    2. Porting a legacy Firefox add-on
    3. Embedded WebExtensions
    4. Comparison with the Add-on SDK
    5. Comparison with XUL/XPCOM extensions
    6. Chrome incompatibilities
  6. Firefox workflow
    1. Temporary Installation in Firefox
    2. Debugging
    3. Getting started with web-ext
    4. web-ext command reference
    5. WebExtensions and the Add-on ID
    6. Publishing your WebExtension
  7. JavaScript APIs
    1. Browser support for JavaScript APIs
    2. alarms
    3. bookmarks
    4. browserAction
    5. browsingData
    6. commands
    7. contextMenus
    8. contextualIdentities
    9. cookies
    10. downloads
    11. events
    12. extension
    13. extensionTypes
    14. history
    15. i18n
    16. identity
    17. idle
    18. management
    19. notifications
    20. omnibox
    21. pageAction
    22. runtime
    23. sessions
    24. sidebarAction
    25. storage
    26. tabs
    27. topSites
    28. webNavigation
    29. webRequest
    30. windows
  8. Manifest keys
    1. applications
    2. author
    3. background
    4. browser_action
    5. chrome_url_overrides
    6. commands
    7. content_scripts
    8. content_security_policy
    9. default_locale
    10. description
    11. developer
    12. homepage_url
    13. icons
    14. manifest_version
    15. name
    16. omnibox
    17. options_ui
    18. page_action
    19. permissions
    20. short_name
    21. sidebar_action
    22. version
    23. web_accessible_resources
  9. Add-on SDK
  10. Getting started
    1. Installation
    2. Getting started
    3. Troubleshooting
  11. High-Level APIs
    1. addon-page
    2. base64
    3. clipboard
    4. context-menu
    5. hotkeys
    6. indexed-db
    7. l10n
    8. notifications
    9. page-mod
    10. page-worker
    11. panel
    12. passwords
    13. private-browsing
    14. querystring
    15. request
    16. selection
    17. self
    18. simple-prefs
    19. simple-storage
    20. system
    21. tabs
    22. timers
    23. ui
    24. url
    25. webextension
    26. widget
    27. windows
  12. Low-Level APIs
    1. /loader
    2. chrome
    3. console/plain-text
    4. console/traceback
    5. content/content
    6. content/loader
    7. content/mod
    8. content/symbiont
    9. content/worker
    10. core/heritage
    11. core/namespace
    12. core/promise
    13. dev/panel
    14. event/core
    15. event/target
    16. frame/hidden-frame
    17. frame/utils
    18. fs/path
    19. io/byte-streams
    20. io/file
    21. io/text-streams
    22. lang/functional
    23. lang/type
    24. loader/cuddlefish
    25. loader/sandbox
    26. net/url
    27. net/xhr
    28. places/bookmarks
    29. places/favicon
    30. places/history
    31. platform/xpcom
    32. preferences/event-target
    33. preferences/service
    34. remote/child
    35. remote/parent
    36. stylesheet/style
    37. stylesheet/utils
    38. system/child_process
    39. system/environment
    40. system/events
    41. system/runtime
    42. system/unload
    43. system/xul-app
    44. tabs/utils
    45. test/assert
    46. test/harness
    47. test/httpd
    48. test/runner
    49. test/utils
    50. ui/button/action
    51. ui/button/toggle
    52. ui/frame
    53. ui/id
    54. ui/sidebar
    55. ui/toolbar
    56. util/array
    57. util/collection
    58. util/deprecate
    59. util/list
    60. util/match-pattern
    61. util/object
    62. util/uuid
    63. window/utils
  13. Firefox for Android
  14. Getting started
    1. Walkthrough
    2. Debugging
    3. Code snippets
  15. APIs
    1. Accounts.jsm
    2. BrowserApp
    3. HelperApps.jsm
    4. Home.jsm
    5. HomeProvider.jsm
    6. JavaAddonManager.jsm
    7. NativeWindow
    8. Notifications.jsm
    9. PageActions.jsm
    10. Prompt.jsm
    11. RuntimePermissions.jsm
    12. Snackbars.jsm
    13. Sound.jsm
    14. Tab
  16. Legacy
  17. Restartless extensions
    1. Overview
  18. Overlay extensions
    1. Overview
  19. Themes
  20. Lightweight themes
    1. Overview
  21. Complete themes
    1. Overview
  22. Publishing add-ons
  23. Guides
    1. Signing and distribution overview
    2. Submit an add-on
    3. Review policies
    4. Developer agreement
    5. Featured add-ons
    6. Contact addons.mozilla.org
  24. Community and support
  25. Channels
    1. Add-ons blog
    2. Add-on forums
    3. Stack Overflow
    4. Development newsgroup
    5. IRC Channel