Firefox extension and nsIMutationObserver - c++

Question

I am a Firefox C++ extension newbie. I need to get access to DOM mutation events in my extension during page load. Firefox by default doesn't send DOM mutation events during page load to improve page load performance.
I understand the reason, but understanding the consequences I still need access to the DOM mutation events. I read somewhere that nsIMutationObserver still gets invoked during page load (and is bit more efficient then DOM mutation events as don't have to walk up the DOM tree looking for listeners), however it's only available to native code.
So I have following questions :
Is nsIMutationObserver and nsIMutationObserver2 available to Firefox extensions?
If yes, how can I write a simple Firefox extension in C++ to get access to it and expose it to Javascript?
If folks can point me to a existing extension that does this forwarding from C++ land to JS, that will be highly appreciated.
Or can I use JS-CTypes to get access to that functionality from my Javascript based extension?
BTW, I asked this question in Firefox's extension forum, but no replies there.
Thanks in advance

Answer 1

Is nsIMutationObserver and nsIMutationObserver2 available to Firefox extensions?
Yes, binary Firefox extensions can use it. Of course, the drawback is that your binary XPCOM component will only work with one Firefox release - it will have to be recompiled for each new release.
If yes, how can I write a simple Firefox extension in C++ to get access to it and expose it to Javascript?
You create an XPCOM component (see example code) and implement nsIMutationObserver interface. You then attach this mutation observer to documents like this:
NS_IMETHODIMP
MyMutationObserver::AttachToDocument(nsIDOMDocument* document)
{
nsCOMPtr<nsINode> node(do_QueryInterface(document));
node->AddMutationObserver(this);
}
For reference: nsINode interface
If folks can point me to a existing extension that does this forwarding from C++ land to JS, that will be highly appreciated.
Sorry, don't know any. But your XPCOM component can expose an additional interface that your JavaScript code will use - e.g. to register a callback. You have to consider that it might not be safe to run JavaScript when the mutation observer gets called. Important methods here: nsContentUtils::IsSafeToRunScript() and nsContentUtils::AddScriptRunner() (see nsContentUtils.h).
Or can I use JS-CTypes to get access to that functionality from my Javascript based extension?
No, you cannot. These are Gecko internals, they aren't exposed to js-ctypes.

Answer 2

A binary extension can use nsIMutationObserver, but unless it's very very careful about what it does when it's notified (see the big WARNING above the interface declaration) it'll cause crashes and various other broken behavior and is likely to introduce security bugs. Like any other internal API, this is a footgun; probably a fatal one if not used extremely carefully. Things that are fine to do in a DOM mutation listener are NOT OK in an nsIMutationObserver.