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  7. Communicating using "port"

Communicating using "port"

In This Article
  1. Accessing port
    1. Accessing port in the Content Script
    2. Accessing port in the Add-on Script
  2. port.emit()
  3. port.on()
  4. port.removeListener()
  5. port.once()
  6. JSON-Serializable Values

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.

This page is now obsolete, and its content has been incorporated into the main page on content scripts.

To enable add-on scripts and content scripts to communicate with each other, each end of the conversation has access to a port object.

  • to send messages from one side to the other, use port.emit()
  • to receive messages sent from the other side, use port.on()

Messages are asynchronous: that is, the sender does not wait for a reply from the recipient but just emits the message and continues processing.

Here's a simple add-on that sends a message to a content script using port:

var tabs = require("sdk/tabs");
var alertContentScript = "self.port.on('alert', function(message) {" +
                         "  window.alert(message);" +
                         "})";
tabs.on("ready", function(tab) {
  worker = tab.attach({
    contentScript: alertContentScript
  });
  worker.port.emit("alert", "Message from the add-on");
});
tabs.open("http://www.mozilla.org");

In total, the port object defines four functions:

  • emit(): emit a message.
  • on(): listen to a message.
  • removeListener(): stop listening to a message.
  • once(): listen to only the first occurrence of a message.

Accessing port

Accessing port in the Content Script

Note that the global self object is completely different from the self module, which provides an API for an add-on to access its data files and ID.

In the content script the port object is available as a property of the global self object. Thus, to emit a message from a content script:

self.port.emit("myContentScriptMessage", myContentScriptMessagePayload);

To receive a message from the add-on code:

self.port.on("myAddonMessage", function(myAddonMessagePayload) {
  // Handle the message
});

Compare this to the technique used to receive built-in messages in the content script. For example, to receive the context message in a content script associated with a context menu object, you would call the on function attached to the global self object:

self.on("context", function() {
  // Handle the message
});

So the port property is essentially used here as a namespace for user-defined messages.

Accessing port in the Add-on Script

In the add-on code, the channel of communication between the add-on and a particular content script context is encapsulated by the worker object. Thus the port object for communicating with a content script is a property of the corresponding worker object.

However, the worker is not exposed to add-on code in quite the same way in all modules. The panel and page-worker objects integrate the worker API directly. So to receive messages from a content script associated with a panel you use panel.port.on():

var panel = require("sdk/panel").Panel({
  contentScript: "self.port.emit('showing', 'panel is showing');"
});
panel.port.on("showing", function(text) {
  console.log(text);
});
panel.show();

Conversely, to emit user-defined messages from your add-on you can just call panel.port.emit():

var panel = require("sdk/panel").Panel({
  contentScript: "self.port.on('alert', function(text) {" +
                 "  console.log(text);" +
                 "});"
});
panel.show();
panel.port.emit("alert", "panel is showing");

The panel and page-worker objects only host a single page at a time, so each distinct page object only needs a single channel of communication to its content scripts. But some modules, such as page-mod, might need to handle multiple pages, each with its own context in which the content scripts are executing, so it needs a separate channel (worker) for each page.

So page-mod does not integrate the worker API directly: instead, each time a content script is attached to a page, the worker associated with the page is supplied to the page-mod in its onAttach function. By supplying a target for this function in the page-mod's constructor you can register to receive messages from the content script, and take a reference to the worker so as to emit messages to the content script.

var pageModScript = "window.addEventListener('click', function(event) {" +
                    "  self.port.emit('click', event.target.toString());" +
                    "  event.stopPropagation();" +
                    "  event.preventDefault();" +
                    "}, false);" +
                    "self.port.on('warning', function(message) {" +
                    "window.alert(message);" +
                    "});"
var pageMod = require('sdk/page-mod').PageMod({
  include: ['*'],
  contentScript: pageModScript,
  onAttach: function(worker) {
    worker.port.on('click', function(html) {
      worker.port.emit('warning', 'Do not click this again');
    });
  }
});

In the add-on above there are two user-defined messages:

  • click is sent from the page-mod to the add-on, when the user clicks an element in the page
  • warning sends a silly string back to the page-mod

port.emit()

The port.emit() function sends a message from the "main.js", or another add-on module, to a content script, or vice versa.

It may be called with any number of parameters, but is most likely to be called with a name for the message and an optional payload. The payload can be any value that is serializable to JSON.

From the content script to the main add-on code:

var myMessagePayload = "some data";
self.port.emit("myMessage", myMessagePayload);

From the main add-on code (in this case a panel instance) to the content script:

var myMessagePayload = "some data";
panel.port.emit("myMessage", myMessagePayload);

port.on()

The port.on() function registers a function as a listener for a specific named message sent from the other side using port.emit().

It takes two parameters: the name of the message and a function to handle it.

In a content script, to listen for "myMessage" sent from the main add-on code:

self.port.on("myMessage", function handleMyMessage(myMessagePayload) {
  // Handle the message
});

In the main add-on code (in this case a panel instance), to listen for "myMessage" sent from a a content script:

panel.port.on("myMessage", function handleMyMessage(myMessagePayload) {
  // Handle the message
});

port.removeListener()

You can use port.on() to listen for messages. To stop listening for a particular message, use port.removeListener(). This takes the same two parameters as port.on(): the name of the message, and the name of the listener function.

This example uses the action button API, which is only available from Firefox 29 onwards.

For example, here's an add-on that creates a page-worker and a button. The page-worker loads http://en.wikipedia.org/wiki/Chalk alongside a content script "listener.js". The button sends the content script a message called "get-first-para" when it is clicked:

pageWorker = require("sdk/page-worker").Page({
  contentScriptFile: require("sdk/self").data.url("listener.js"),
  contentURL: "http://en.wikipedia.org/wiki/Chalk"
});
require("sdk/ui/button/action").ActionButton({
  id: "get-first-para",
  label: "get-first-para",
  icon: "./icon-16.png",
  onClick: function() {
    console.log("sending 'get-first-para'");
    pageWorker.port.emit("get-first-para");
  }
});

The content script "listener.js" listens for "get-first-para". When it receives this message, the script logs the first paragraph of the document and then calls removeListener() to stop listening.

function getFirstParagraph() {
  var paras = document.getElementsByTagName('p');
  console.log(paras[0].textContent);
  self.port.removeListener("get-first-para", getFirstParagraph);
}
self.port.on("get-first-para", getFirstParagraph);

The result is that the paragraph is only logged the first time the button is clicked.

Due to bug 816272 the page-mod's removeListener() function does not prevent the listener from receiving messages that are already queued. This means that if "main.js" sends the message twice in successive lines, and the listener stops listening as soon as it receives the first message, then the listener will still receive the second message.

port.once()

Often you'll want to receive a message just once, then stop listening. The port object offers a shortcut to do this: the once() method.

This example rewrites the "listener.js" content script in the port.removeListener() example so that it uses once():

function getFirstParagraph() {
  var paras = document.getElementsByTagName('p');
  console.log(paras[0].textContent);
}
self.port.once("get-first-para", getFirstParagraph);

JSON-Serializable Values

The payload for an message can be any JSON-serializable value. When messages are sent their payloads are automatically serialized, and when messages are received their payloads are automatically deserialized, so you don't need to worry about serialization.

However, you do have to ensure that the payload can be serialized to JSON. This means that it needs to be a string, number, boolean, null, array of JSON-serializable values, or an object whose property values are themselves JSON-serializable. This means you can't send functions, and if the object contains methods they won't be encoded.

For example, to include an array of strings in the payload:

var pageModScript = "self.port.emit('loaded'," +
                    "  [" +
                    "  document.location.toString()," +
                    "  document.title" +
                    "  ]" +
                    ");"
var pageMod = require('page-mod').PageMod({
  include: ['*'],
  contentScript: pageModScript,
  onAttach: function(worker) {
    worker.port.on('loaded', function(pageInfo) {
      console.log(pageInfo[0]);
      console.log(pageInfo[1]);
    });
  }
});

Document Tags and Contributors

 Contributors to this page: wbamberg, SimDev, evold
 Last updated by: wbamberg, Nov 30, 2016, 2:06:16 PM
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