How do I properly generate code to set the topic name for a ROS 2 publisher using the value of the getParameter(...) function?

A basic test example:

function testROS2Node()

% Create ROS 2 Node
nodeParams.topicName = 'test';
node = ros2node("/test", 0, Parameters=nodeParams);
% Get the parameter
topicName = getParameter(node, "topicName", Datatype="string");
% Create the publisher
pub = ros2publisher(node, topicName, "std_msgs/Bool");
% Loop rate to 1 Hz
r = ros2rate(node, 1);

while(1)
    % Create and send a message every cycle.
    msg = ros2message("std_msgs/Bool");
    msg.data = true;
    send(pub, msg);
    waitfor(r);
end
end

The coder commands:

cfg = coder.config('exe');
cfg.Hardware = coder.hardware('Robot Operating System 2 (ROS 2)');
cfg.Hardware.DeployTo = 'Localhost';
cfg.Hardware.BuildAction = 'Build and Load';
cfg.TargetLang = 'C++';
cfg.InlineBetweenUserFunctions = 'Readability';
codegen('-config', cfg, 'testROS2Node');

The error:

Expression could not be reduced to a constant.

Error in ==> ros2publisher Line: 183 Column: 22

Code generation failed: View Error Report

Answer 1

[Josh Chen]

Hi Sam,

Unfortunately, the topicName input for ros2publisher has to be a constant. Since parameter value is not constant, you cannot create the publisher with it.

Can you please share more about the workflow? Is there a reason you would like to create a publisher using the parameter?

Thanks,

Josh

[Sam]

Im using Matlab's deploy to a remote device, and the topic changes based on that device. It was a curiosity.

[Erwin Mathew Louis]

Hello,

Just a quick question to this. In case it was a regular parameter and not a topic name, would I be able to change the value from the terminal when the generated C++ code of the node created in matlab is running? There isnt much documentation about how matlab handles such changes to parameters. I guess I am mainly asking about the parameter callback functionality available in ros2. are such callback available in the ROS toolbox?

[Josh Chen]

Hi Erwin,

If I understand you correctly, you are referring to ROS 2 parameters associated with ROS 2 nodes. Yes, this is supported for code generation. You can specify all parameters with the "Parameters" name-value pair when creating the node object.

When generating code from MATLAB, those parameters will be generated as ROS 2 parameters for the associated ROS 2 node.

You may refer to this example for the syntax.

As for parameter callbacks, could you please give some more information about what you are planning to achieve/do inside each callback?

Thanks,

Josh