How to organize a two-way communication with the Firefox extension code injection?

Question

There is an extension for Firefox, which makes the injected JS code into the page.

Code for XUL:

window.addEventListener("load", function load(event){
document.addEventListener("TestEvnt", function (e) {
    var info = e.detail;
    if(info.cmd=='account_list') {
        consoleLog('... event account_list');
    }
}, false, true);
document.addEventListener("DOMContentLoaded", function (e) {
  var window = e.originalTarget.defaultView.wrappedJSObject;
  var inject = window.document.createElement('script');
  inject.type = 'text/javascript';
  inject.setAttribute('src','resource://apptab-notify/injscr.js');
  window.document.getElementsByTagName('head')[0].appendChild(inject);
}, false);
},false);

Code injscr.js:

var additionalInfo = {
        "cmd": 'account_list',
        "url": document.location.href
    };
var evt = document.createEvent('CustomEvent');
evt.initCustomEvent('TestEvnt', true, false, additionalInfo);
document.dispatchEvent(evt);

How to provide bi-directional communication, something to listen to code injection events from the extension? Without using Addon-SDK!

Answer 1

In injsrc.js:

var additionalInfo = {
    "cmd": 'account_list',
    "url": document.location.href
};

var request = document.createTextNode(JSON.stringify(additionalInfo));
request.addEventListener("something-response", function(event) {
    console.log('... something-response');
    var info=JSON.parse(event.target.nodeValue);
    $('body').css('background-color', info.color);
}, false);
document.head.appendChild(request);
var event = document.createEvent("HTMLEvents");
event.initEvent("something-query", true, false);
request.dispatchEvent(event);

In XUL:

window.addEventListener('load', function(event) {
   document.addEventListener("something-query", function(event) {
        consoleLog('... something-query');
        var node = event.target;
        if (!node || node.nodeType != Node.TEXT_NODE) return;
        var info = JSON.parse(node.nodeValue);
        var doc = node.ownerDocument;

        if (info.cmd == 'account_list') {
            consoleLog('... event account_list');
            var event = doc.createEvent("HTMLEvents");
            event.initEvent("something-response", true, false);
            node.nodeValue = JSON.stringify({
                'color': '#999'
            });
            node.dispatchEvent(event);
        } else if (info.cmd == 'page_options') {
            consoleLog('page_options');
        }
    }, false, true);
}, false);

Article helped MDN