code: function(callback) {
          var self = this;
          var guid = guidFor(this);
  
          Ember.$(window).on('hashchange.ember-location-'+guid, function() {
            run(function() {
              var path = self.getURL();
              if (get(self, 'lastSetURL') === path) { return; }
  
              set(self, 'lastSetURL', null);
  
              callback(path);
            });
          });
        }
docs: Register a callback to be invoked when the hash changes. These
  callbacks will execute when the user presses the back or forward
  button, but not after `setURL` is invoked.
  
  @private
  @method onUpdateURL
  @param callback {Function}