ros subscriber is interrupted by ginput

I have a ginput command in my ros subscriber callback, which causes an interruption of the subscriber. The callback is entered once, and then it stops spinning. When I comment out ginput command, the callback is called continuously.

[Geoff Hayes]

Jin - is this the ros subscriber from https://www.mathworks.com/help/robotics/ref/rossubscriber.html? Can you show some of your code?

[Jin Dai]

Hi Geoff, nice to hear from you. The code is attached below:

 classdef EdgeFollow
    properties
        pcl_sub;
        initialized;
    end
    methods
        function this = EdgeFollow()
            rosshutdown;
            close all
            rosinit
            this.initialized = 0
            this.pcl_sub = rossubscriber('lead_laser/pointcloud', @this.extractEdge); 
        end
    end

    methods(Access = private, Hidden = true)        
        function extractEdge(this, src, pclMsg)
            disp('enter cbk')
            if(~this.initialized)
                figure(1)
                hold on
                xlabel('x','fontsize',15)
                ylabel('y','fontsize',15)
                axis equal
                [x,y] = ginput(1);
            else

            end
        end
    end
end

The callback is only called once.

If I comment line: [x,y] = ginput(1), the callback is entered continuously.

Thanks.

[Geoff Hayes]

Jin - doesn't the call to ginput "block" and so you are waiting for the user (?) to choose a point on the axes? What are you expecting to happen with the call to ginput?

[Jin Dai]

I deleted most codes to show you only the code structure. I clicked a point and found nearest neighbors of this clicked point. After this, it exits the callback, and never enters it again.

Answer 1

[Sebastian Castro]

This is expected, sorry to say.

Recall that the MATLAB environment is single-threaded, with the exception of optimized math functions (which you can't control as a user), or if you're using the Parallel Computing Toolbox to explicitly assign tasks to multiple cores.

So, a blocking operation like ginput means your subscriber won't be receiving any messages until you free up computational resources.

- Sebastian