<\/span><\/h3>\nLet’s create one roscpp node with 2 simple callbacks for 2 simple topics.<\/p>\n
#include <ros\/ros.h>\r\n#include <std_msgs\/String.h>\r\n\r\nvoid callbackTalker1(const std_msgs::String::ConstPtr &msg)\r\n{\r\n ROS_INFO(\"In callback talker1.\");\r\n ROS_INFO(\"%s\", msg->data.c_str());\r\n}\r\n\r\nvoid callbackTalker2(const std_msgs::String::ConstPtr &msg)\r\n{\r\n ROS_INFO(\"In callback talker2.\");\r\n ROS_INFO(\"%s\", msg->data.c_str());\r\n}\r\n\r\n\r\nint main(int argc, char **argv)\r\n{\r\n ros::init(argc, argv, \"talker_subscribers\");\r\n ros::NodeHandle nh;\r\n\r\n ros::Subscriber counter1_sub = nh.subscribe(\"talker1\", 10, callbackTalker1);\r\n ros::Subscriber counter2_sub = nh.subscribe(\"talker2\", 10, callbackTalker2);\r\n\r\n ros::spin();\r\n}<\/pre>\nThat’s pretty simple here: the node subscribes to 2 topics, named “talker1” and “talker2”. Both topics use the std_msg\/String data type. The 2 callbacks do the same thing, which is displaying the received message.<\/p>\n
So, it seems like there is no problem with that!<\/p>\n
Now let’s create a node to publish on those topics.<\/p>\n
#!\/usr\/bin\/env python\r\n\r\nimport rospy\r\nfrom std_msgs.msg import String\r\n\r\nif __name__ == \"__main__\":\r\n rospy.init_node(\"talker_publishers\")\r\n\r\n talker1_pub = rospy.Publisher(\"talker1\", String, queue_size=10)\r\n talker2_pub = rospy.Publisher(\"talker2\", String, queue_size=10)\r\n\r\n rate = rospy.Rate(1.0)\r\n\r\n while not rospy.is_shutdown():\r\n talker1_pub.publish(\"Hey\")\r\n talker2_pub.publish(\"Oy\")\r\n rate.sleep()<\/pre>\nImportant note<\/strong>: you can create those publishers in Cpp or Python, it really doesn’t matter. Nothing is wrong with the publisher node, the issue will come (later) from the subscriber node. You could also publish directly from the terminal, but for simplicity, it’s better to just write a small and minimal node.<\/p>\nThis node will simply publish on both “talker1” and “talker2” topics at a 1 Hz frequency. The first publisher will send “Hey” and the second publisher will send “Oy”. That’s it. For the rest of this tutorial we won’t modify this node.<\/p>\n
Now, you can start roscore<\/code>, and then the publisher node with for example rosrun asyncspinner_tutorials talker_publishers.py<\/code>. You can keep it running for the rest of this tutorial or restart it for each test. For simplicity, in the following, when I write “start the subscriber” I’ll assume that the publisher is already running.<\/p>\nAnd, if you start the roscpp subscriber node, you’ll get this:<\/p>\n
$ rosrun asyncspinner_tutorials talker_subscribers \r\n[ INFO] [1572016597.728219447]: Hey\r\n[ INFO] [1572016597.730289973]: Oy\r\n[ INFO] [1572016598.728299107]: Oy\r\n[ INFO] [1572016598.728426453]: Hey\r\n[ INFO] [1572016599.728055430]: Hey\r\n[ INFO] [1572016599.728406145]: Oy\r\n[ INFO] [1572016600.728138572]: Hey\r\n[ INFO] [1572016600.728322249]: Oy\r\n[ INFO] [1572016601.728317492]: Oy\r\n[ INFO] [1572016601.728488483]: Hey<\/pre>\nIt’s working pretty well, no problem here! From the log timestamps you can see that each “Hey” and “Oy” are received by the subscriber and printed every second, which is the rate set on the publisher side.<\/p>\n
You can notice that sometimes, data from the “talker1” topic comes after data from the “talker2” topic. This is not an issue here. In fact, in the publisher we publish on those topics without any delay, so it’s like they’re sent at the exact same time. The subscriber node will receive them at almost the same time, and when waking up the callback functions, the second one may be called before the first one, that’s it.<\/p>\n
Now, let’s get to the problem.<\/p>\n