var Widget = new Class({
    Implements: Events,
    initialize: function(arg1){
      //Initialize me
    },
    run: function(){
      this.fireEvent('onStart', maybe_some_args)
      //Do a bunch of shit
      this.fireEvent('onComplete', maybe_some_args)
    }
});

var Loading = {
  setup: function(class_that_implements_events) {
    class_that_implements_events.addEvents({
      'onStart': this.startLoading,
      'onComplete': this.stopLoading
    })
    class_that_implements_events.run()
  },
  startLoading: function(args) {
    //set some styles
  },
  stopLoading: function(args) {
    //set some styles
  }
}

Loading.setup(new Widget("nice"))