How to access instance of class in ROS2 subscriber callback?

I have the following code

classdef MTMSApiSimple

%MTMSAPI Summary of this class goes here

% Detailed explanation goes here

properties

data

node

status_subscriber

end

methods

function obj = MTMSApiSimple()

obj.node = ros2node("mtms_matlab_api");

obj.status_subscriber = ros2subscriber(obj.node, "/fpga/status_monitor_state", "fpga_interfaces/SystemState", @obj.subscriber_callback);

end

function subscriber_callback(message)

% access and set obj.data

end

end

end

I create a class with a subscriber that subscribes to a topic. My problem is that I would like to access obj.data inside the subscriber_callback method, but since the callback is called when a new message is received, I don't know how to give obj as a parameter for it. subscriber_callback receives only the message as an argument.

Answer 1

[Josh Chen]

Hello Kyösti,

Using a single object to manage both data and callback for subscriber is feasible. In this case, subscriber callback can have one additional argument, i.e., obj. This will be considered as a member function of the MTMSApiSimple class:

classdef MTMSApiSimple < handle

%MTMSAPI Summary of this class goes here

% Detailed explanation goes here

properties

data

node

status_subscriber

end

methods

function obj = MTMSApiSimple()

obj.node = ros2node("mtms_matlab_api");

obj.status_subscriber = ros2subscriber(obj.node, "/fpga/status_monitor_state", "std_msgs/Int32", @obj.subscriber_callback);

end

function subscriber_callback(obj,message)

% access and set obj.data

fprintf('data received is: %d\n', message.data);

obj.data = message.data;

end

end

end

>> nd = ros2node('aaa');

>> [pub, pubmsg] = ros2publisher(nd,"/fpga/status_monitor_state", "std_msgs/Int32");

>> msa = MTMSApiSimple();

>> pubmsg.data = int32(5);

>> send(pub,pubmsg)

data received is: 5

>> msa.data

ans =

int32

5

I am using "std_msgs/Int32" as the message type since I do not have the custom message on my end, but the original message type should also work.

Hope this helps,

Josh