Duplicate instances of methods executed while using parfeval and backgroundpool

I have the following function that sends regular updates

function countUp(q)
%countUp Counts up 1 every second
    for i = 1:5
        disp(i);
        pause(1);
        send(q, i);
    end
end

The above function is called by the mlapp

properties (Access = private)
            q = parallel.pool.DataQueue; % Description
            L;
            f;
    end
    
    methods (Access = private)
        
        function myDisp(app, data)
            log = ['Recvd data update: ', num2str(data)];
            disp(log);
            app.Label.Text = num2str(data);
            if(data == 5)
                cancel(app.f);
                app.StartButton.Enable = true;
            end
        end
    end
    

    % Callbacks that handle component events
    methods (Access = private)

        % Button pushed function: StartButton
        function StartButtonPushed(app, event)
            disp('Start button clicked');
            app.StartButton.Enable = false;
            app.L = afterEach(app.q, @(data) myDisp(app, data));
            app.f = parfeval(backgroundPool,@countUp,0,app.q);
            disp('Start button cb complete');
        end

        % Button pushed function: StopButton
        function StopButtonPushed(app, event)
            delete(app.L);
            cancel(app.f);
            app.StartButton.Enable = true;
        end

First run returns

Start button clicked
Start button cb complete
Recvd data update: 1
Recvd data update: 2
Recvd data update: 3
Recvd data update: 4
Recvd data update: 5

Each time I start it it looks like duplicate instances of the function is executed

Start button clicked
Start button cb complete
Recvd data update: 1
Recvd data update: 1
Recvd data update: 2
Recvd data update: 2
Recvd data update: 3
Recvd data update: 3
Recvd data update: 4
Recvd data update: 4
Recvd data update: 5
Recvd data update: 5

It never looks like cancel(Future) is working as intended. What am I doing wrong?

[Walter Roberson]

For debugging purposes, I would do something like add a random number to the output. That would allow you to distinguish the case of the code executing twice (two different random numbers appear) from the case where the output is (for whatever reason) being displayed twice (same random number will appear for the pair of outputs)

Answer 1

[Jaynik]

Hi Harish,

Before setting up a new listener with afterEach, ensure that any previous listeners are removed. This can be done by deleting app.L if it exists. You can edit the StartButtonPushed function as below:

function StartButtonPushed(app, event)
    disp('Start button clicked');
    app.StartButton.Enable = false;
    % Delete existing listener before creating a new one
    if ~isempty(app.L)
        delete(app.L);
    end
    app.L = afterEach(app.q, @(data) myDisp(app, data));
    app.f = parfeval(backgroundPool,@countUp,0,app.q);
    disp('Start button cb complete');
end

Hope this helps!

[Harish Ananthakrishnan]

Thanks for the response. Looks like I was deleting before cancelling

            delete(app.L);
            cancel(app.f);

I should have use this and it works. Any reason why one has to cancel a future before removing the callback?

            cancel(app.f);
            delete(app.L);