Progress listeners

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Progress Listeners

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

Note that if you just want to execute your code each time a page loads, you can use an an easier method.


  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;
      onStateChange: function(aProgress, aRequest, aFlag, aStatus)
       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, aRequest, aURI)
       // This fires when the location bar changes i.e load event is confirmed
       // or when the user switches tabs
       return 0;
      // For definitions of the remaining functions see
      onProgressChange: function() {return 0;},
      onStatusChange: function() {return 0;},
      onSecurityChange: function() {return 0;},
      onLinkIconAvailable: function() {return 0;}
  2. Attach the progress listener to a <browser> or a <tabbrowser> element, e.g. for Firefox put the following code in a load listener of a main window:
    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.

Remember to change myListener to an unique identifier.