Trying to control a simple "character bar spinner" with Start and Stop buttons

I would like to control the starting and stopping of the spinning action of the characters '| / - \' with Start and Stop buttons. In the below code, the Start button works just fine to start the spinning; but the Stop button (which I hoped would break the While loop, and thereby stop the spinning action) gives an error. Not sure why this is not working. Any illumination would be greatly appreciated. -Leith

handles.fig=figure;

handles.pb1=uicontrol('style','pushbutton','position',[100 200 100 40],'callback',@start_spin,'string','START');

handles.pb2=uicontrol('style','pushbutton','position',[250 200 100 40],'callback',@stop_spin,'string','STOP');

function start_spin(~,~)

idx=1;

j=1;

spin(idx,j);

end

function stop_spin(~,~)

j=0;

spin(idx,j);

end

function spin(idx, j)

ix = 1;

chars='|/-\';

while(idx==1)

if j==0

break;

end

fprintf(1,'\b%c', chars(ix));

ix = mod(ix,4)+1;

pause(0.05)

end

end