Return data from a callback function for use in a different function

asked 2016-11-18 16:33:30 -0500

mscott gravatar image

updated 2016-11-18 16:42:46 -0500

I am trying to use ROS to control many different robots in a swarm simulation. I need to have access to all of the robots position and velocity. I am using the MORSE simulator, which publishes the position and velocity of each of the robots. What I need my function to do (Python) is to take all of those pieces of data and organizes it so that I can use it for my controller. What I have so far:

##Create robot class##
class robot():

    def __init__(self):
        self.robot_number = robot_number
        self.x_data = 0
        self.y_data = 0
        self.z = 0
        self.x_vel = 0
        self.y_vel = 0
        self.position = []
    def get_pose(self, pose_topic):
        self.x_pos = rospy.Subscriber(pose_topic, PoseStamped,self.pose_callback)
        return self.x_pos
    def pose_callback(self,data):
        self.x_data = data.pose.position.x
        return self.x_data

class move_feature():
    def __init__(self):
        self.x_position = 0
    def pose(self,pose_topic):
        main_robot = robot()
        self.x_position = main_robot.get_pose(pose_topic)  

def main(args):
    '''Initializes and cleanup ros node'''
    robot_dictionary = {}

    for i in range(0,robot_number+1):
        robot_name = 'robot%s' % (i)
        robot_dictionary[robot_name] = move_feature()
        pose_topic = '/robot/pose%s' % (i+1)
        vel_topic = '/velocity%s' % (i+1)
    except KeyboardInterrupt:
        print("Shutting down ROS Image feature detector module")

if __name__ == '__main__':
    rospy.init_node('robot', anonymous=True)

So I created a robot class which has the function get_pose. I would like get_pose to return the position (right now just the x position) from the topic so that it can be used within the move_feature class. So, in the move_feature, I should just be able to call pose with the position topic and have it return the current position.

I will eventually need it to expand to 100 robots, so organization is very important. Right now, the function does not return the x_value at all... it just returns information about the subscriber. Is this the right way to try to solve this problem?

Thanks in advance!

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Callback functions do not return data. Instead just set self.x_pos in the callback function.

JoshMarino gravatar image JoshMarino  ( 2016-11-18 21:24:28 -0500 )edit

ROS pub/sub is a dataflow system, so you should model dataflows. Your code seems to be based on a synchonous approach (ie: RPC). ROS supports that as well, but then you'd use services. Note that components != objects+RPC.

gvdhoorn gravatar image gvdhoorn  ( 2016-11-19 04:01:15 -0500 )edit

Thanks for the comments.
JoshMarino: Sure, I generally just set self.x_pos and then use it. The problem in this case is with organization. Since I need to many robots, I need to have them organized properly, not just specify self.x_pos. gvdhoorn: I'm not sure I understand. Could you elaborate?

mscott gravatar image mscott  ( 2016-11-19 18:43:22 -0500 )edit

You are wanting to get the position from a subscriber, but only whenever you want it (synchronous). Pub/sub in ROS do not work like this, they are continuously sending data at a specified frequency. If you want it when called from a function, a ROS service would be better implemented.

JoshMarino gravatar image JoshMarino  ( 2016-11-20 11:15:58 -0500 )edit

Okay I think that makes sense, thank you!

mscott gravatar image mscott  ( 2016-11-21 11:41:07 -0500 )edit