Notification when a parameter in param server is changed

edit

It doesn't appear to be a conventionally exposed interface for doing so, and it's probably not a recommended route, but if you have a look in stacks/ros_comm/clients/cpp/roscpp/src/libros you'll find param.cpp where ros does it's parameter handling for each node. (edit) You can try to emulate what they did.

In:

   void init(const M_string& remappings)

you'll see that they set up a callback:

   XMLRPCManager::instance()->bind("paramUpdate", paramUpdateCallback);

Which is used to update the local parameter table. You might be able to set up your own callback to do this, but I'm unsure if it would interfere with the node's own parameter updating.

The signature of the callback looks like this btw:

    void callback(XmlRpc::XmlRpcValue& params, XmlRpc::XmlRpcValue& result)

You can look at param.cpp for details on how to use the arguments.

I was looking for a way to accomplish this without the useful but cumbersome dynamic_reconfigure package as I think we have enough things to deal with, just considering ROS messages/actions/services, launch files (that support YAML syntax in XML...) and catkin specific CMakeLists file without adding another layer of complexity which is often unneeded.

So here is my working but *flawed example (I followed Asormeville's hints):

#include <ros/ros.h>
#include <ros/xmlrpc_manager.h>
#include <std_msgs/String.h>

void callback(XmlRpc::XmlRpcValue& params, XmlRpc::XmlRpcValue& result)
{
    ROS_ERROR("Updated parameters!");
}

int main(int argc, char **argv)
{
    XmlRpc::XmlRpcValue params, result, payload;

    ros::init(argc, argv, "test_node");

    ros::NodeHandle nh;

    ros::XMLRPCManager::instance()->unbind("paramUpdate");
    ros::XMLRPCManager::instance()->bind("paramUpdate", callback);

    ros::param::set("~test_param", true);

    ROS_INFO("Created new parameter through XmlRpc API call.");

    params[0] = ros::this_node::getName();
    params[1] = ros::XMLRPCManager::instance()->getServerURI();
    params[2] = ros::names::resolve(std::string("~test_param"));

    if (ros::master::execute("subscribeParam", params, result, payload, false)) {
        ROS_INFO("Subscribed to parameter.");
    }
    else {
        ROS_ERROR("Failed to subscribe to the parameter.");
    }

    while (ros::ok()) {
        ros::spinOnce();
    }

    return 0;
}

The next steps would be to have a specific callback per parameter. But I don't know if that would be possible and it would begin to look like Publishers/Subscribers mechanisms.

* The ROS node redefining this callback won't be able to use the ros::param::getCached() function anymore.