Communicating using "postMessage"

As an alternative to port, content modules support the built-in message event. In most cases port is preferable to message events. However, the context-menu module does not support port, so to send messages from a content script to the add-on via a context menu object, you must use message events.

Handling Message Events in the Content Script

To send a message from a content script, you use the postMessage function of the global self object:

self.postMessage(contentScriptMessage);

This takes a single parameter, the message payload, which may be any JSON-serializable value.

To receive a message from the add-on script, use self's on function:

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

Like all event-registration functions, this takes two parameters: the name of the event, and the handler function. The handler function is passed the message payload.

Handling Message Events in the Add-on Script

To send a message to a content script, use the worker's postMessage function. Again, panel and page integrate worker directly:

// Post a message to the panel's content scripts
panel.postMessage(addonMessage);

However, for page-mod objects you need to listen to the onAttach event and use the worker supplied to that:

var pageMod = require('sdk/page-mod').PageMod({
  include: ['*'],
  contentScript: pageModScript,
  onAttach: function(worker) {
    worker.postMessage(addonMessage);
  }
});

To receive messages from a content script, use the worker's on function. To simplify this most content modules provide an onMessage property as an argument to the constructor:

panel = require("sdk/panel").Panel({
  onMessage: function(contentScriptMessage) {
    // Handle message from the content script
  }
});

Message Events Versus User-Defined Events

You can use message events as an alternative to user-defined events:

var pageModScript = "window.addEventListener('mouseover', function(event) {" +
                    "  self.postMessage(event.target.toString());" +
                    "}, false);";
var pageMod = require('sdk/page-mod').PageMod({
  include: ['*'],
  contentScript: pageModScript,
  onAttach: function(worker) {
    worker.on('message', function(message) {
      console.log('mouseover: ' + message);
    });
  }
});

The reason to prefer user-defined events is that as soon as you need to send more than one type of message, then both sending and receiving messages gets more complex.

Suppose the content script wants to send mouseout events as well as mouseover. Now we have to embed the event type in the message payload, and implement a switch function in the receiver to dispatch the message:

var pageModScript = "window.addEventListener('mouseover', function(event) {" +
                    "  self.postMessage({" +
                    "    kind: 'mouseover'," +
                    "    element: event.target.toString()" +
                    "  });" +
                    "}, false);" +
                    "window.addEventListener('mouseout', function(event) {" +
                    "  self.postMessage({" +
                    "    kind: 'mouseout'," +
                    "    element: event.target.toString()" +
                    "  });" +
                    " }, false);"
var pageMod = require('sdk/page-mod').PageMod({
  include: ['*'],
  contentScript: pageModScript,
  onAttach: function(worker) {
    worker.on('message', function(message) {
    switch(message.kind) {
      case 'mouseover':
        console.log('mouseover: ' + message.element);
        break;
      case 'mouseout':
        console.log('mouseout: ' + message.element);
        break;
      }
    });
  }
});

Implementing the same add-on with user-defined events is shorter and more readable:

var pageModScript = "window.addEventListener('mouseover', function(event) {" +
                    "  self.port.emit('mouseover', event.target.toString());" +
                    "}, false);" +
                    "window.addEventListener('mouseout', function(event) {" +
                    "  self.port.emit('mouseout', event.target.toString());" +
                    "}, false);";
var pageMod = require('sdk/page-mod').PageMod({
  include: ['*'],
  contentScript: pageModScript,
  onAttach: function(worker) {
    worker.port.on('mouseover', function(message) {
      console.log('mouseover :' + message);
    });
    worker.port.on('mouseout', function(message) {
      console.log('mouseout :' + message);
    });
  }
});