function ret = gui_active(optional_input)

%

% gui_active - used to implement an abort function in a GUI

%

% initially : gui_active(1)

%

% in the aplication, occaisionally 

%  EITHER  1) if (~gui_active) then ... do abort action

%  OR      2) polling gui_active(-1) will cause an error with the value 'abort_requested'

%             that can be caught with a "try-catch" block

%

% to initiate the abort (for both cases) call:  gui_active(0)

%



% the call to drawnow enable other controls to "patch-in" before the

% rest of this code execute and possibly change the "is_active" state



persistent    is_active;



if (nargin > 0)

    if (optional_input == -1) 

        drawnow;

        if (~is_active)

            error('abort_requested');

        end

    else

        prev_is_active  = is_active;

        is_active       = optional_input;

        if ((prev_is_active > 0) & (is_active == 0))

            disp('operation aborted');

        end

    end

else

    drawnow('expose');

end



ret = is_active;