Using MutationObserver in GeckoFx with C#?

Question

I am using GeckoFx to perform a login to a specific website. This website edits the page with new information should the login fail (or require additional authentication, such as a ReCaptcha). Unfortunately, it is vital that I have access an event when the page is updated. I have tried numerous approaches mainly

• A continual check if the uri is still the same upon each login attempt and a subsequent check on the specific element in question (to see if the display: none property was changed. (This resulted in an infinite loop as it seems GeckoFx updates the page in a nonblocking way, causing the program to go into an infinite loop)
• Sleeping for ~5 seconds between login requests and using the aforementioned uri check. All this did (predictably, I was grasping at straws) was freeze the browser for 5 seconds and still fail to update the page
• Searching the GeckoFx codebase for a specific event when the page is updated similar to the DocumentCompleted event (no such luck).

The most common approach I have read about (and one that makes the most sense) is to use a MutationObserver. It seems that all of the answers across the internet involve injecting Javascript in order to perform the requisite task. Seeing as all of my programming background has not touched web development whatsoever, I'm trying to stick to what I know.

Here is my approach so far, unfortunately, it is not much.

public class GeckoTestWebLogin
{
    private readonly string _user;
    private readonly string _pass;
    public GeckoWebBrowser Gweb;
    public Uri LoginUri { get; } = new Uri("https://website.com/login/");

    public bool LoginCompleted { get; private set; } = false;
    public bool Loaded { get; private set; } = false;

    public GeckoTestWebLogin(string user, string pass)
    {
        _user = user;
        _pass = pass;
        Xpcom.EnableProfileMonitoring = false;
        Xpcom.Initialize("Firefox");

        //this code is for testing purposes, it will be removed upon project completion
        CookieManager.RemoveAll();

        Gweb = new GeckoWebBrowser();
        Gweb.DocumentCompleted += DocLoaded;

        //right about here is where I get lost, where can I set a callback method for the observer to report back to? Is this even how it works?
        MutationObserver mutationObserver = new MutationObserver(Gweb.Window.DomWindow, (nsISupports)Gweb.Document.DomObject);
    }

    private void TestObservedEvent(string parms, object[] objs)
    {
        MessageBox.Show("The page was changed @ " + DateTime.Now);
    }

    public void DocLoaded(object obj, GeckoDocumentCompletedEventArgs e)
    {
        Loaded = true;
        if (Gweb.Url != LoginUri) return;
        AttemptLogin();
    }

    private void AttemptLogin()
    {
        GeckoElementCollection elements = Gweb.Document.GetElementsByTagName("input");
        foreach (GeckoHtmlElement element in elements)
        {
            switch (element.Id)
            {
                case "username":
                    element.SetAttribute("value", _user);
                    break;
                case "password":
                    element.SetAttribute("value", _pass);
                    break;
                case "importantchangedinfo":
                    GeckoHtmlElement authcodeModal =
                        (GeckoHtmlElement)
                            Gweb.Document.GetElementsByClassName("login_modal").First();
                    if (authcodeModal.Attributes["style"].NodeValue != "display: none")
                    {
                        InputForm form = new InputForm { InputDescription = "Captcha Required!" };
                        form.ShowDialog();
                        elements.FirstOrDefault(x => x.Id == "captchabox")?.SetAttribute("value", form.Input);
                    }
                    break;
            }
        }
        elements.FirstOrDefault(x => x.Id == "Login")?.Click();
    }

    public void Login()
    {
        //this will cause the DocLoaded event to fire after completion
        Gweb.Navigate(LoginUri.ToString());
    }
}

As stated in the above code in the comments, I am completely lost at

MutationObserver mutationObserver = new MutationObserver(Gweb.Window.DomWindow, (nsISupports)Gweb.Document.DomObject);

I can't seem to find anything in GeckoFx's source for MutationObserver that would allow me to set a callback/event/whathaveyou. Is my approach the correct way to go about things or am I left with no options other than to inject Javascript into the page?

Much appreciated, thank you in advance.

Here is my attempt at option 2 in Tom's answer:

(Added into GeckoTestWebLogin)

    public void DocLoaded(object obj, GeckoDocumentCompletedEventArgs e)
    {
        Loaded = true;
        if (Gweb.Url != LoginUri) return;

        MutationEventListener mutationListener = new MutationEventListener();
        mutationListener.OnDomMutation += TestObservedEvent;
        nsIDOMEventTarget target = Xpcom.QueryInterface<nsIDOMEventTarget>(/*Lost here*/);
        using (nsAString modified = new nsAString("DOMSubtreeModified"))
            target.AddEventListener(modified, mutationListener, true, false, 0);

        AttemptLogin();
    }

MutationEventListener.cs:

public delegate void OnDomMutation(/*DomMutationArgs args*/);

public class MutationEventListener : nsIDOMEventListener
{
    public event OnDomMutation OnDomMutation;

    public void HandleEvent(nsIDOMEvent domEvent)
    {
        OnDomMutation?.Invoke(/*new DomMutationArgs(domEvent, this)*/);
    }
}

Answer 1

I don't think Geckofx's webidl compiler is currently advanced enough to generate the callback constructor.

Option 1. - Enhance MutationObserver source.

You could modify MutationObserver source manually to add the necessary constructor callback. Then recompile Geckofx. (I haven't look to see how difficult this is)

Option 2. - Use old style Mutation events.

public class DOMSubtreeModifiedEventListener : nsIDOMEventListener
{
     ... // Implement HandleEvent 
}

Then something like (maybe in DocumentCompleted event handler):

    _domSubtreeModifiedEventListener = new DOMSubtreeModifiedEventListener(this);

    var target = Xpcom.QueryInterface<nsIDOMEventTarget>(body);
    using (nsAString subtreeModified = new nsAString("DOMSubtreeModified"))
        target.AddEventListener(subtreeModified, _domSubtreeModifiedEventListener, true, false, 0);

Option 3. - Use Idle + Check.

Add an winforms Application.idle event handler - and examine the document, to know when its ready.

Option 4. - Inject a javascript callback.

(As you have already mentioned) - This example is waiting until after a resize is done.

basically inject: "<body onresize=fireResizedEventAfterDelay()>" : then inject something like this:

 string fireResizedEventAfterDelayScript = "<script>\n" +
            "var resizeListner;" +
            "var msDelay = 20;" +
            "function fireResizedEventAfterDelay() {" +
            "clearTimeout(resizeListner);" +
            "resizeListner = setTimeout(function() { document.dispatchEvent (new MessageEvent('resized')); }, msDelay);" +
            "}\n" +
            "</script>\n";

Then in the C#:

browser.AddMessageEventListener("resized", (s) => runafterImDone())