Ashita.org

There are lots of cases when it is desirable to implement one of the XPCOM interfaces in use by Firefox, or other mozilla-based browsers. There are three cases where PirateQuesting does so, but once you see the concept, it should be easy to adapt to your situation.

• You must have a QueryInterface to enjoy this ride

  First off, all XPCOM interfaces in Firefox inherit from nsISupports (Also see details on oxymoronical.com here). Only one method is scriptable and part of XPCOM — QueryInterface — and it must be present in all implementations of XPCOM interfaces.

  
  //"implements" nsISupports
  var InterfaceImplementation = function() {
    QueryInterface: function (aIID) {
        if (aIID.equals(Components.interfaces.nsISupports))
        {
           return this;
        }
        throw Components.results.NS_NOINTERFACE;
    }
  }
  

  The above is an example of the very minimum required to support any interface. QueryInterface requires a first parameter which is an aIID from Components.interfaces.*. There is also a second, optional, parameter, but as I have never come across this in use, it’s not worth pursuing here.

• Now what?

  A very common (and useful) use of XPCOM interface implementation is creating your own observers, for example:

  
  var myObserver = {
  
    observe: function(request, aTopic, aData){
      if (aTopic == "http-on-examine-response")
      {
        //response has come back, now what?
      }
      else if (aTopic == "http-on-modify-request")
      {
        //opportunity to modify headers on request
      }
    },
  
    QueryInterface: function(aIID){
       if (aIID.equals(Components.interfaces.nsIObserver) ||
           aIID.equals(Components.interfaces.nsISupports))
      {
        return this;
      }
      throw Components.results.NS_NOINTERFACE;
    },
  };
  

  The nsIObserver interface is fairly simple as it only adds one new method. As you can see now, though, QueryInterface now checks for both nsIObserver and nsISupports. Remember: any interface you implement must have a QueryInterface supporting all interfaces in the inheritance chain.

• Observer registration

  If you then wanted to register your observer, it’s as easy as:

  
  var observerService = Components.classes["@mozilla.org/observer-service;1"]
                                 .getService(Components.interfaces.nsIObserverService);
  
  observerService.addObserver(myObserver,"http-on-examine-response", false);
  observerService.addObserver(myObserver,"http-on-modify-request", false);
  

  Then to unregister, you do:

  
  observerService.removeObserver(myObserver, "http-on-examine-response");
  observerService.removeObserver(myObserver, "http-on-modify-request");
  

  When I have a chance, I’ll add a more complete example of using observers to watch http requests, but in the meantime check out the list of Observer Notifications at MDC.

Tags: Firefox Extension, JavaScript, XPCOM