MATLAB: Aborting callback execution in GUI – Drawnow error

drawnowguiinterrupt callback executiontry/catch

Hi all,
I'm working on a GUI, where I have one task that is very computationally intensive. Therefore I want to add a button that enables the user to cancel the current calculation. I read a lot of posts that tackled the same problem, and therefore I implemented a try catch statement. Pressing on the cancel button creates an error in its own callback function and I try to catch this error in the other callback which contains my computation. This compution consists of a loop that runs over different frequencies, thus in the catch block, I use the break function to get out of this loop. Inside the loop I also place a few drawnow commands to be able to see when the cancel event is triggered. When pressing my cancel button during a computation, I see the error that I generate, but matlab also generates another error stating: "Error using drawnow" / "Error when evaluating uicontrol Callback". And I have entirely no idea where it comes from. Any hint is welcome … At the time I get this message, matlab is still in the loop, meaning that the break statement is not reached yet …?
Thanks!
Bart

Best Answer

I am not sure why you are using errors to try to cancel an operation. It seems simpler to just have the cancel button set a flag that your operation checks. Here is a simple example of what I mean:
function [] = gui_breakloop()
% How to stop a while loop with a toggle button.
S.f = figure('name','gui_breakloop',...
             'menubar','none',...
             'numbert','off',...
             'pos',[100 100 300 150]);
S.pb = uicontrol('Style','toggle',...
                 'Units','pix',...
                 'Position',[10 10 130 130],...
                 'CallBack',@pb_call,...
                 'String','Start Loop');
S.pb(2) = uicontrol('Style','toggle',...
                   'Units','pix',...
                   'Position',[160 10 130 130],...
                   'CallBack',@pb_call2,...
                   'String','Cancel',...
                   'enable','off',...
                   'userdata',0);
movegui('center')   
guidata(S.f,S);  % Store the handles in GUIDATA.         
function [] = pb_call(varargin)
% Callback for looping button
S = guidata(gcbf);
set(S.pb(1),'string','Looping!')
set(S.pb(2),'enable','on')
pause(.05)
while 1
    A = sort(rand(1000));
      pause(.05) % Forces graphics refresh
      if get(S.pb(2),'userdata')
          set(S.pb(1),'string','No Loop')
          set(S.pb(2),'userdata',0)
          break
      end
  end
set(S.pb(2),'enable','off')
function [] = pb_call2(varargin)
S = guidata(gcbf);
set(S.pb(2),'userdata',1)
Related Question