Intercepting Links in Firefox

While working on a Firefox extension i needed to intercept links Firefox is about to open and stop it on certain conditions. While it seems like an easy task it took more time then i thought due to not much information was available online. I saw the question asked multiple times with no definitive answer. Correct recipe turns out to be using a observer and listen for an "http-on-examine-response".

Following script will listen all request and you will have a chance to stop the transmission based on your rules.

var observer = {
  observe: function(subject,topic,data){

    var httpChannel = 
    subject.QueryInterface(Components.interfaces.nsIHttpChannel);
    var contentType = httpChannel.getResponseHeader("Content-Type");

    var channel = subject.QueryInterface(Components.interfaces.nsIChannel);
    var url = channel.URI.spec;
    url = url.toString();

    if ( isDownloadable( url ) == true 
         &&  contentType.indexOf("html") == -1 ){

        window.getBrowser().stop();

        download( url );
        //alert("Wait a moment!\n"+ url );
    }

    //alert("Topic sent: " + topic);
  }
};

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

I am no extension guru, so maybe it is not the best way to do it but it gets the job done.

Tags: firefox