Windows watcher example for Custom Buttons (just a simplified demo)

windowsWatcher.js

// Windows watcher, based on code from
// https://github.com/Infocatcher/Custom_Buttons/tree/master/CB_Editor_Toggle_on_Top

const watcherId = "customButtonsWindowsWatcher_" + this.id;
var {Application, Components} = window; // Prevent garbage collection in Firefox 3.6 and older
var watcher = Application.storage.get(watcherId, null);
if(!watcher) {
	watcher = {
		REASON_STARTUP: 1,
		REASON_SHUTDOWN: 2,
		REASON_WINDOW_LOADED: 3,
		REASON_WINDOW_CLOSED: 4,

		get obs() {
			delete this.obs;
			return this.obs = Components.classes["@mozilla.org/observer-service;1"]
				.getService(Components.interfaces.nsIObserverService);
		},
		get ww() {
			delete this.ww;
			return this.ww = Components.classes["@mozilla.org/embedcomp/window-watcher;1"]
				.getService(Components.interfaces.nsIWindowWatcher);
		},
		get wm() {
			delete this.wm;
			return this.wm = Components.classes["@mozilla.org/appshell/window-mediator;1"]
				.getService(Components.interfaces.nsIWindowMediator);
		},
		init: function(reason) {
			this.obs.addObserver(this, "quit-application-granted", false);
			var ws = this.wm.getEnumerator(null);
			while(ws.hasMoreElements())
				this.initWindow(ws.getNext(), reason);
			this.ww.registerNotification(this);
		},
		destroy: function(reason) {
			this.obs.removeObserver(this, "quit-application-granted");
			var ws = this.wm.getEnumerator(null);
			while(ws.hasMoreElements())
				this.destroyWindow(ws.getNext(), reason);
			this.ww.unregisterNotification(this);
		},
		initWindow: function(window, reason) {
			// Do something with window
			window.addEventListener("keydown", this, true);
		},
		destroyWindow: function(window, reason) {
			if(reason == this.REASON_WINDOW_CLOSED)
				window.removeEventListener("load", this, false); // Window can be closed before "load" event
			// Cleanup
			window.removeEventListener("keydown", this, true);
		},
		observe: function(subject, topic, data) {
			if(topic == "quit-application-granted")
				this.destroy();
			else if(topic == "domwindowopened")
				subject.addEventListener("load", this, false);
			else if(topic == "domwindowclosed")
				this.destroyWindow(subject, this.REASON_WINDOW_CLOSED);
		},
		handleEvent: function(e) {
			switch(e.type) {
				case "load":    this.loadHandler(e);    break;
				case "keydown": this.keyDownHandler(e);
			}
		},
		loadHandler: function(e) {
			var window = e.currentTarget;
			window.removeEventListener("load", this, false);
			this.initWindow(window, this.REASON_WINDOW_LOADED);
		},
		keyDownHandler: function(e) {
			Services.console.logStringMessage("[watcher] " + e.type + " " + e.keyCode);
		}
	};
	Application.storage.set(watcherId, watcher);
	watcher.init(watcher.REASON_STARTUP);
}
function destructor(reason) {
	if(reason == "update" || reason == "delete") {
		watcher.destroy(watcher.REASON_SHUTDOWN);
		Application.storage.set(watcherId, null);
	}
}
if(
	typeof addDestructor == "function" // Custom Buttons 0.0.5.6pre4+
	&& addDestructor != ("addDestructor" in window && window.addDestructor)
)
	addDestructor(destructor, this);
else
	this.onDestroy = destructor;