onTicker ({ symbol = '', cbGID } = {}, cb) {
    const s = this.getSocketWithDataChannel('ticker', { symbol })

    if (!s) {
      throw new Error('no data socket available; did you provide a symbol?')
    }

    s.ws.onTicker({ symbol, cbGID }, cb)
  }
@param {object} opts - options
@param {string} [opts.symbol] - symbol for ticker
@param {string} [opts.cbGID] - callback group id
@param {Function} cb - callback
@throws an error if no data socket is available
@see https://docs.bitfinex.com/v2/reference#ws-public-ticker