do not work ishandle and ishghandle in a loop?

Hello all,

I have a program which read values from sensors. The program must stop reading when the figure is closed. I had a program where I used a callback linked to the serial to execute when a terminator was received, then I evaluated the contition of ishghandle and worked properly.

Now, I would like to have the reading code inside of a loop, which execution condition is ishghandle. Well, it seems that this function or ishandle do not work inside of a loop, because value once figure is executed is 1 and it does not change to 0 when the figure is closed. I tested placing the condition in the while or inside the while as below.

My version is 2018b

function read(a)

figure;
drawnow;
delete(instrfindall);

s = serial(a);
fopen(s);
p=ishandle(gcf);
while(1)
   
    if p == 1
        n = s.BytesAvailable;
        if n == 6
            m = fread(s,6);
            disp(m);
            disp(p);
        end
        p=ishandle(gcf);

    else
        break;
    end
end

fclose(s);

end

Answer 1

[Walter Roberson]

If there is no current figure, then gcf will create a new figure... which would have a valid handle.

function read(a)

fig = figure;
drawnow;
delete(instrfindall);

s = serial(a);
fopen(s);

p=ishandle(fig);
while(1)
   
    if p == 1
        n = s.BytesAvailable;
        if n == 6
            m = fread(s,6);
            disp(m);
            disp(p);
        end
        p=ishandle(fig);

    else
        break;
    end
end

fclose(s);

end

[Carlos Molina]

Yes, you right, but if you execute this sentence in a if-else condition out from the loop, ishandle or ishghandle return 0 if figure is closed and they do not open a new figure.

figure
h = gcf;
ishandle(h)
%Here the result will be logical 1

%Then if close the figure and execute again
ishandle(h)
%Result is logical 0 and any new figure appears

So, I do not know why this different behavior. Any solution for this?

[Steven Lord]

The code in the comment calls gcf once and uses the output from that call, which will not change. It always refers to the same figure (which may be a "handle to deleted figure" if the figure has been closed.)

Your original code calls gcf repeatedly, and as Walter indicated if there is no figure open (because it's been closed), that call will create one. So the figure that your gcf call before your while loop references may not be the same as the figure that your gcf call inside your while loop references.

Be very, very careful about using gcf, gca, etc. in your code. The c stands for current and what's current can change over time.

[Stephen23]

"Be very, very careful about using gcf, gca, etc. in your code."

Specifically: if you want to write efficient, robust code, do NOT use either gcf or gca.

Always obtain and refer to explicit graphics handles for all graphics objects that you need to refer to.

[Walter Roberson]

gcbo is hypothetically an exception, as it does not change according to which object is "active". On the other hand, I have rarely used gcbo.

clf() and cla() do have uses, but only when you pass a graphics object to them instead of relying on the current object.