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The easiest way is to use an existing dynamic reconfigure client which does the work of converting to or from messages on that topic into the cfg type which is delivered to a callbac, or published out via update_configuration(): http://wiki.ros.org/dynamic_reconfigure/Tutorials/UsingTheDynamicReconfigurePythonClient or http://wiki.ros.org/dynamic_reconfigure/Tutorials/SettingUpDynamicReconfigureForANode%28cpp%29

Or you can do the same on the server side.

But if you want to use the raw message (implementing your own custom client or server) you can:

$ rostopic info /example_server_node/parameter_updates
Type: dynamic_reconfigure/Config
...

$ rosmsg show dynamic_reconfigure/Config
dynamic_reconfigure/BoolParameter[] bools
  string name
  bool value
dynamic_reconfigure/IntParameter[] ints
  string name
  int32 value
dynamic_reconfigure/StrParameter[] strs
  string name
  string value
dynamic_reconfigure/DoubleParameter[] doubles
  string name
  float64 value
dynamic_reconfigure/GroupState[] groups
  string name
  bool state
  int32 id
  int32 parent

Using the above information you can populate a Config message for publishing or decipher it in a subscriber callback. Use push_back for C++ or append in python to add IntParameters or any of the other types. You just need to use the same names in the right type slots as in the cfg file. (I believe if you only partially populate it still works, and extra names that don't match any in the cfg are ignored silently, but I haven't tried that)

I'm not sure how the groups work but if you rostopic echo an example with a group in it maybe it is self explanatory.