Dev : Extensions : Example Code

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XulPlanet has an article on preferences

Progress Listeners

Progress Listeners alllow extensions to be notified of events assosiated with documents loading in the browser and with tab switching events. Progress Listeners implement the nsIWebProgressListener interface.

  1. Create an object which implements nsIWebProgressListener:
const STATE_START = Components.interfaces.nsIWebProgressListener.STATE_START;
const STATE_STOP = Components.interfaces.nsIWebProgressListener.STATE_STOP; 
var myListener =
  QueryInterface : function(aIID)
    if (aIID.equals(Components.interfaces.nsIWebProgressListener) ||
        aIID.equals(Components.interfaces.nsISupportsWeakReference) ||
      return this;
    throw Components.results.NS_NOINTERFACE;
    if(aFlag & STATE_START)
     //This fires when the load event is initiated
    if(aFlag & STATE_STOP)
     //This fires when the load finishes
   return 0;
  onLocationChange:function(aProgress webProgress , aRequest, aURI)
     //This fires when the location bar changes i.e load event is confirmed or when the user switches tabs
     //return 0;
  //For defnitiions of the remaining functions see XulPlanet
  1. Attach the progressListener to a <browser> object, e.g. for firefox:
const NOTIFY_STATE_DOCUMENT = Components.interfaces.nsIWebProgress.NOTIFY_STATE_DOCUMENT; 
window.getBrowser().addProgressListener(myListener , NOTIFY_STATE_DOCUMENT);

The second argument is a status filter which determines the type of events that will be recieved.