Accessing global variables in Firefox extensions

Question

I'm writing a Firefox extension which takes a custom switch from the command line and sets a variable inside my cmdline.js in the components directory, we'll call the variable switchDetected which is a boolean. Now based upon this variable I want actions to be carried out in my overlay.js file in the chrome/content directory. The problem I'm having is I can't seem to be able to access the variable switchDetected that is declared in components/cmdline.js from within chrome/contents/overlay.js.

I've tried numerous ways of doing this but nothing seems to work. So I'm just wondering if anyone knows how this can be achieved.

Answer 1

A script loaded in an overlay runs in the context of the browser window - its global variables are stored as properties of the window object corresponding with the browser. If you open a second browser window the same script will load a second time and run in the context of the new browser window - it will have different global variables. The scripts containing XPCOM components on the other hand only load once and they have their independent context that isn't bound to a window. So their global variables cannot be accessed from a browser window directly, just like two browser windows cannot access each others global variables directly.

Instead the browser window should communicate with the XPCOM component using the usual approach: get a component instance and call its method. If you don't want to define your own interface for that (you probably don't) you can use a trick, something like this:

CommandLineHandler.prototype = {
  handle: function(commandLine) {...},
  get helpInfo() {...},
  isSwitchDetected: function()
  {
    return switchDetected;
  },
  get wrappedJSObject()
  {
    return this;
  },
  QueryInterface: XPCOMUtils.generateQI(["nsICommandLineHandler"]);
};

The wrappedJSObject property makes sure that your component can be unwrapped - all its methods and properties will become accessible then and not just the ones defined in the interface. So your overlay script needs to do the following:

var cmdLineHandler = Components.classes["@myself.com/my-command-line-handler;1"]
                               .getService()
                               .wrappedJSObject;
var switchDetected = cmdLineHandler.isSwitchDetected();