_listeners() {
        this._onQueueStartedListener = this._onQueueStarted.bind(this);
        this.queue.addListener('started', this._onQueueStartedListener);

        this._onQueueEndedListener = this._onQueueEnded.bind(this);
        this.queue.addListener('ended', this._onQueueEndedListener);
    }
@description Helper function to register the listeners for this instance.
@private