How to access instance of class in ROS2 subscriber callback?

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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.

Answers (1)

Josh Chen
Josh Chen on 10 Aug 2022
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

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