Communicating using "port"

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("https://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.

For example, here's an add-on that creates a page-worker and a button. The page-worker loads https://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: "https://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]);
    });
  }
});