How to know if the network is (dis)connected?

Question

How can I know, in Xul, if the network is (dis)connected?

--update

Using:

    function observe(aSubject, aTopic, aState) {
        if (aTopic == "network:offline-status-changed") {
            write("STATUS CHANGED!");
        }
    }
    var os = Components.classes["@mozilla.org/observer-service;1"].getService(Components.interfaces.nsIObserverService);
    os.addObserver(observe, "network:offline-status-changed", false);

and the preference:

pref("network.manage-offline-status", true);

it's not working.. There's a bug report here, but I don't think it has something to do with it.

--

Actually I think it's not possible to be notified, as even in Firefox we're never notified, and the user need to manually mark "work offline" if he wants the browser to know that it's offline..

--

Screenshot my of Firefox "about:config" filtering for "offline" string, unfortunately, there no "network.manage-offline-status":

Answer 1

You should be able to use navigator.onLine. Here is the help page

https://developer.mozilla.org/en/Online_and_offline_events

navigator.onLine is a property that maintains a true/false value (true for online, false for offline). This property is updated whenever the user switches into "Offline Mode" by selecting the corresponding menu item (File -> Work Offline in Firefox).

Another solution (as commented by @Neil):

Components.classes["@mozilla.org/observer-service;1"]
    .getService(Components.interfaces.nsIObserverService)
    .addObserver(myF­unction, "network:offline-status-changed", false);

Answer 2

The best way I found is to use the following javascript code, that behaves like a ping, and make the test with some big websites, and assume that if none of them answers, so the network must be disconnected.

var ping = {};
ping = {
    img:null,
    imgPreload:null,
    timer:null,
    init:function() {
        var sess = new Date();
        var nocache = sess.getTime();
        var imguri = ping.img+"?time="+nocache;
        var ping.imgPreload = new Image();
        ping.imgPreload.onload = function() {
            clearTimeout(ping.timer);
            ping.timer = null;
            alert("Domain is available");
        };
        ping.imgPreload.src = imguri;
        ping.timer = setTimeout("ping.fail_to_ping()",60000);
    },
    fail_to_ping:function() {
        clearTimeout(ping.timer);
        ping.timer = null;
        ping.imgPreload = null;
        alert("Ping to domain failed!");
    }
};

(from http://crynobone.com/ci/index.php/archive/view/852)

--update

But, as it's not a reliable solution (as you can't rely that the image will be in the website forever), the best solution might be to develop a new XPCom component.

Answer 3

Eh... as per HTML5 (read echmascript 5), the on-/offline events are available.

See it here at Mozilla Hacks

Edit 20/4/2011:
I just encountered an update for this answer, when i was watching a podcast from MS MIX11:
http://channel9.msdn.com/Events/MIX/MIX11/HTM14 around time 43:36, the lecturer is actually talking about the window.navigator.onLine property, where he uses it for detecting if the browser (and the computer) is online. Then he uses the online event to do something when he gets online again.

This method is only available in modern browsers, however. So IE 8 and below have to poll for the connection.