'Retrieve latest state.'

def update(self):

    if (self._russ.get_power('1', self._zone_id) == 0):
        self._state = STATE_OFF
    else:
        self._state = STATE_ON
    self._volume = (self._russ.get_volume('1', self._zone_id) / 100.0)
    index = self._russ.get_source('1', self._zone_id)
    try:
        self._source = self._sources[index]
    except IndexError:
        self._source = None
