_listeners() {
        /**
         * Listen on two events:
         *
         * - init: bootstrap or url reset
         * - added: resource was added to the database
         */
        this.queue.register({
            event: 'init',
            tolerance: 100
        }, this._onInit.bind(this));

        this.queue.register({
            event: 'added'
        }, this._onAdded.bind(this));
    }
@description Helper function to register listeners for each url generator instance.
@private