Listening an event when another function of the class is running?

Hello,

My class looks like:

classdef linescan99 < handle
 methods
        function obj = linescan99(obj_surveilleur_de_contact)
           addlistener(obj_surveilleur_de_contact,'Contact',@obj.handleEvnt);
        end
        function follower(obj)
            while 1
                %computations
            end
        end
    end
end

I've noted that when the follower function is executing, the class is unable to listen a possible event 'Contact'. What's unfortunate: this event would lead to a break of the follower function while 1 loop...

Do you know how to force the function to listen the event? maybe inside the loop?

Thank you very much

Answer 1

[Guillaume]

I think the problem is that your function suiveur is called by the event handler, so your while loop is executing while an event is already being handled. I'll be very surprised if event handlers are reentrant in matlab, so no new event can be handled until you've finished dealing with the current one (in effect until you've quit your loop).

You're trying to do some multithreaded (or asynchronous) programming here which is not really supported by matlab. A possible workaround would be to use a timer to periodically check whether or not the event has been raised:

%Warning: completely untested code. There may be typos:
classdef linescan99 < handle
    properties (SetAccess = private)
       suiveur_en_cour = false;
       minuteur;
       tracking = false;
    end

      methods         
          function this = linescan99(obj_surveilleur_de_contact) %constructor
             addlistener(obj_surveilleur_de_contact,'Contact',@obj.handleEvnt);
             this.minuteur = timer('BusyMode', 'drop', 'ExecutionMode', 'fixedRate', 'Period', 0.1, 'TimerFcn', @obj.suiveur);
             this.minuteur.start();
          end

          function handleEvnt(this, obj_surveilleur_de_contact,~)
             if obj_surveilleur_de_contact.State %contact
                disp('linescan lancé')
             end
             this.suiveur_en_cour = obj_surveilleur_de_contact.State;
          end

          function suiveur(this, ~, ~)
            if suiveur_en_cour
               if ~tracking %this is just to make sure disp is only called once, when tracking begins
                 tracking = true;
                 disp('tracking en cours');
               end
               %do whatever you wanted to do in your while loop
            else
               tracking = false;
            end 
          end        
     end     
  end

If this works for you, you could also simply store a reference to obj_surveilleur_de_contact in linescan99, check its state with the timer and not bother with the listener or events.

[Michaël]

Thank you very much!

Which "ideal" language are you thinking of for this kind of programming?

[Guillaume]

With the design you have, what you want is to launch a background or asynchronous task from the event handler, so that the handler can return immediately.

Matlab parallel processing toolbox would probably let you do that easily but since I don't have and have never used that toolbox, I can't help you with that.

The 'ideal' language is the one you know, really. Among the ones I know, asynchronous programming is very easy with .Net (so C#, VB.Net or C++/.net) .

Answer 2

[Geoff Hayes]

Michaël - without seeing more of your code, in particular the handleEvent or all of follower, it is difficult to know for sure what is going wrong. But if I were to guess, it may be because of your while loop

 while 1
     % computations
 end

Is this a "tight" while loop that does not allow itself to be interrupted? Are there any calls to drawnow or pause? If not, then this could mean that the while loop will continue to run and monopolize all the processing thus not "giving a chance" to any other call until it has been completed. If I ever write something similar, then I put a short pause in the code to temporarily halt execution and allow other events (callbacks etc.) to occur.

 while 1
     % computations

     % pause for fraction of a second
     pause(0.01);
 end

Try the above and see what happens!

[Walter Roberson]

The documentation implies in various places that timer interrupts can occur between any two lines, but that all other interrupts can only occur when you encounter a drawnow() or pause() or waitfor() or uiwait() or figure() or reach the command line.

[Michaël]

Thank you for your reply.

My detailed codes can be found here.

I thought the event/listener options allowed to run in parallel several while 1 loops, what's apparently false. Matlab doesn't seem to be an appropriate language to optimally do what I want to do: permanently monitoring a possible event (via a while 1 loop), to start or break another while 1 loop.

Thank you again

[Michaël]

I've added a pause as you suggested and that doesn't work: the while 1 of my first class doesn't seem to be executed during this pause. Matlab doesn't seem to allow a function to continuously "gate" the execution of a function, what involves two while 1 loops...

[Geoff Hayes]

...the while 1 of my first class doesn't seem to be executed during this pause

Isn't the pause in the while loop? Can you post the code (i.e. attach) so that we can see the changes?

[Michaël]

Yes, it's in the while loop.

Answer 3

[ARPIT]

Hey Michael,

Can you kindly suggest how you finally implemented it because I also want to do exactly similar thing.

Thanks in advance

[Michaël E]

Hi,

Matlab isn't able to run concomitantly two (while) loops; a workaround is to use the "timer" object.