Lots of people seem to be playing with the new support for restartless extensions (also known as bootstrapped extensions) coming in Firefox 4. Nothing could make me happier really. I’m not sure I can remember ever helping implement something which is will hopefully turn out to be a game changer for extension development in the future. The technical details of restartless extensions are talked about in a few places but one thing that is missing is what I think is probably going to be the most common code pattern in these extensions.

When you’re developing an extension it’s almost certain that you’re going to want to interact with the main browser window in some way. With the XUL based extensions this was a pretty simple process, normally your extension would be overlaying browser.xul and so any script you add there would automatically run for every new browser window and would have direct access to it (in fact this sometimes confused new developers who assumed their script would run just once and be globally available, not for every browser window opened).

In restartless extensions things are different. You have one script that runs when your extension is started and runs in its own sandbox so you have to take steps to get access to windows. Complicating matters your extension may be started when Firefox starts up and there are no browser windows or after when some already exist. The best way to go is when your extension is started to look for any existing browser windows and make your necessary modifications and then wait for new windows to open and modify those too.

Since this is such a common case (and vlad was trying to figure it out on IRC) I put together a very simple bootstrap script that just does this. It calls the setupBrowserUI function for every window that needs to be initialised by your extension and tearDownBrowserUI for every window that must be cleaned up when your extension is being disabled. This code isn’t the only way to do all this but it is fairly straightforward and works, feel free to take it and modify it for use in your own extensions. I’ll probably get it put onto MDC too.

const Cc = Components.classes;
const Ci = Components.interfaces;

var WindowListener = {
  setupBrowserUI: function(window) {
    let document = window.document;

    // Take any steps to add UI or anything to the browser window
    // document.getElementById() etc. will work here
  },

  tearDownBrowserUI: function(window) {
    let document = window.document;

    // Take any steps to remove UI or anything from the browser window
    // document.getElementById() etc. will work here
  },

  // nsIWindowMediatorListener functions
  onOpenWindow: function(xulWindow) {
    // A new window has opened
    let domWindow = xulWindow.QueryInterface(Ci.nsIInterfaceRequestor)
                             .getInterface(Ci.nsIDOMWindow);

    // Wait for it to finish loading
    domWindow.addEventListener("load", function listener() {
      domWindow.removeEventListener("load", listener, false);

      // If this is a browser window then setup its UI
      if (domWindow.document.documentElement.getAttribute("windowtype") == "navigator:browser")
        WindowListener.setupBrowserUI(domWindow);
    }, false);
  },

  onCloseWindow: function(xulWindow) {
  },

  onWindowTitleChange: function(xulWindow, newTitle) {
  }
};

function startup(data, reason) {
  let wm = Cc["@mozilla.org/appshell/window-mediator;1"].
           getService(Ci.nsIWindowMediator);

  // Get the list of browser windows already open
  let windows = wm.getEnumerator("navigator:browser");
  while (windows.hasMoreElements()) {
    let domWindow = windows.getNext().QueryInterface(Ci.nsIDOMWindow);

    WindowListener.setupBrowserUI(domWindow);
  }

  // Wait for any new browser windows to open
  wm.addListener(WindowListener);
}

function shutdown(data, reason) {
  // When the application is shutting down we normally don't have to clean
  // up any UI changes made
  if (reason == APP_SHUTDOWN)
    return;

  let wm = Cc["@mozilla.org/appshell/window-mediator;1"].
           getService(Ci.nsIWindowMediator);

  // Get the list of browser windows already open
  let windows = wm.getEnumerator("navigator:browser");
  while (windows.hasMoreElements()) {
    let domWindow = windows.getNext().QueryInterface(Ci.nsIDOMWindow);

    WindowListener.tearDownBrowserUI(domWindow);
  }

  // Stop listening for any new browser windows to open
  wm.removeListener(WindowListener);
}