How to access a ros message in a launch file?

edit

Hello Is it possible to access a ros message in a launch file and based on the value of the message publish to another topic? Moreor like a conditional statement. I checked online and seems no method to access the ros messages in a launch file.

    #!/usr/bin/env python3

import ros_compatibility as roscomp from ros_compatibility.node import CompatibleNode from ros_compatibility.qos import QoSProfile, DurabilityPolicy

from carla_msgs.msg import CarlaEgoVehicleStatus from ra_comm.msg import DriverWish import rospy

import sys

class RAssistCondition(CompatibleNode):

def __init__(self):
    """
    Constructor
    """
    super(RAssistCondition, self).__init__("ra_assist_condition")

    self.role_name = self.get_param("role_name", "ego_vehicle")
    self.driver_wish_pub = rospy.Publisher('/ra/core/smach/driver_wish', DriverWish, queue_size=10)

    self.new_subscription(
        CarlaEgoVehicleStatus,
        "/carla/{}/vehicle_status".format(self.role_name),
        self.racondition,
        qos_profile=QoSProfile(depth=1, durability=DurabilityPolicy.TRANSIENT_LOCAL))

    self.published = False


def racondition(self, vehicle_status):

    if not vehicle_status.velocity:  # pylint: disable=no-member
        self.logerr("Cannot determine the vehicle velocity")
        sys.exit(1)

    self.vehicle_velocity = vehicle_status.velocity
    self.logerr("Vehicle velocity info received. velocity={}".format(self.vehicle_velocity))

    if (self.vehicle_velocity > 0) and not self.published:
        try:
            self.pubDriverWish()
            self.published = True
            self.logerr("publishing driver wish")

        except:
            self.logerr("Vehicle velocity info received. velocity={}".format(self.vehicle_velocity))
            self.logerr("Error while publishing driver wish")


# publish function
def pubDriverWish(self):
    curr_ts = rospy.Time.now()
    self.logerr("publishing driver wish")
    rospy.loginfo('[Display] Publish driver wish')
    msg = DriverWish()
    msg.Header.stamp = curr_ts
    msg.driver_wish = True
    self.driver_wish_pub.publish(msg)

def main(args=None): """ main function

:return:
"""
roscomp.init("ra_assit_condition", args)



ra_assit_condition = None

rospy.sleep(15)


try:
    ra_assit_condition = RAssistCondition()
    ra_assit_condition.spin()
    rospy.logerr("Condition for reverse assist")
except KeyboardInterrupt:
    pass
finally:
    if ra_assit_condition is not None:
        ra_assit_condition.logerr("Done, deleting twist to control")
        del ra_assit_condition
    roscomp.shutdown()

if __name__ == '__main__':

main()