def __init__(self, state_name, topic, topic_class, field=None, msgeval=None):
Condition.__init__(self, state_name)
self._topic = topic
self._field = field
self._subscriber = rospy.Subscriber(topic, topic_class, self._callback)
if msgeval is None:
assert field is not None
msgeval = rostopic.msgevalgen(field)
self._msgeval = msgeval
self._value = None
