how to use callback() with python

asked 2018-09-11 21:15:37 -0500

lanlan
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can anyone explain how to use callback in writing a subscriber? the tutorial uses callback(data),but what it data? what content dose it have? thank you very much!

answered 2018-09-12 01:15:56 -0500

mgruhler

12390 ●32 ●193 ●173 https://github.com/mgruhler

updated 2018-09-12 08:12:58 -0500

data is just the variable name for the message that is passed in. You could name it msg, foo or whateveryoulike.

The content of it depends on the message type defined in the according instantiation of the subscriber. In the tutorial this is rospy.Subscriber("chatter", String, callback) type is std_msgs::String (check the import statements). You can find the data fields of ROS messages on the docs pages, e.g. here or using rosmsg show PKG MSG.

EDIT

Well the first data in callback(data) is a ROS message, so if the ROS message is an std_msgs/Int16, data refers to that. However, this is actually a class and the actual int16 is found in the data.data member. Again, the first data is just a name, the second one is the variable of the std_msgs/Int16 which is "hardcoded".

As a more intuitive example, as it gets rid of this double data thing is e.g. std_msgs/ColorRGBA. data is the full message and you can access the respective field r, g, b and a via data.r, data.g and so on...