How to overwrite built in XPCOM component in Firefox addon?

Question

I'm taking a foray into Firefox extension development for the first time, and so far it's been pretty comfortable going, but I'm running into a problem; one of the things I need to do overwriting the built-in nsIPromptService and replacing it with something of my own instead.

I walked through the basic XPCOM component creation tutorial here and got the hello world one working:

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

And everything in that seems to work fine, but nothing I've been able to find or research shows how I can overwrite an interface from javascript. I've seen things in C++ and Java that seem to be able to overwrite the built-in components, but I can't find anything about doing this from javascript, and just trying to change the contract ID didn't work; when I try to get the service from the contract ID (as below), it just returns the original, built-in component version.

var myComponent = Components.classes['@mozilla.org/embedcomp/prompt-service;1']
                                               .getService(Components.interfaces.nsIPromptService);

Is there something really obvious here that I'm missing? Is this the wrong way to go about overriding components (I can't seem to find anything anywhere, so I'm not really sure what I should be doing..).

Answer 1

Neil, thanks for the suggestion. That's what I thought I was doing (and I was), but if you're actually overriding a contract (instead of defining a new one), it looks like the answer is that you have to go to the nsIComponentRegistrar and actually register your factory (rather than relying on the chrome.manifest to handle it for you). An example of this would be:

Components.manager.nsIComponentRegistrar.registerFactory(CLASS_ID, CLASS_NAME, CONTRACT_ID, MyPromptServiceFactory);

Where the constans were:

const CLASS_ID = Components.ID("{a2112d6a-0e28-421f-b46a-25c0b308cbd0}");

// description
const CLASS_NAME = "My Prompt Service";

// textual unique identifier
const CONTRACT_ID = "@mozilla.org/embedcomp/prompt-service;1";

Where the CLASS_ID/CONTRACT_ID were the IDs for the pre-existing service.

Answer 2

You need to register your component using the contract id of the service that you want to override.