MATLAB Answers


Cannot Break A Function Loop With GUI Button

Asked by Beril Sirmacek on 31 Jul 2018
Latest activity Commented on by Beril Sirmacek on 1 Aug 2018
I have a GUI which has START and STOP buttons with functions below. START button starts a live camera stream. However, I cannot break the loop by pushing to STOP button. How can I stop the camera acquisition? It seems like the STOP button is ignored until the WHILE loop ends. The STOP button is not read during the loop process.
I'd be grateful to hear your suggestions to make the STOP button working.
%%Start button
function pushbutton1_Callback(hObject, eventdata, handles)
global vid;
global stop_pressed;
handles = guidata(hObject);
stop_pressed = get(handles.pushbutton2, 'Value');
disp('START LIVE STREAM button pressed')
% set camera settings:
vid = cameraSetup();
max_frame_no = 10000;
frame_no = 1;
while (frame_no < max_frame_no)
if stop_pressed == 1
img = getsnapshot(vid);
frame_no = frame_no + 1;
%%STOP button
function pushbutton2_Callback(hObject, eventdata, handles)
global stop_pressed;
disp('STOP LIVE STREAM button pressed')
stop_pressed = get(handles.pushbutton2, 'Value');

  1 Comment

Check stop_pressed in your pushbutton2_Callback. If you are using a pushbutton instead of a togglebutton your pushbutton Value will be 0.

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2 Answers

Answer by Adam
on 31 Jul 2018

The attached files give an example of a start-stop GUI which works relatively simply. Don't use global variables to pass data around though - they may or may not be contributing to the problem here, but they bring so many problems they are really not worth it and are pretty much never a good solution to any problem.

  1 Comment

Thank you for sharing the example. It indeed works with this demo, however it doesn't stop the camera stream when I put imshow function in the while loop.

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Answer by Beril Sirmacek on 31 Jul 2018

Looks like I fixed it.
All I do is that I have added a "drawnow;" comment after imshow within the while loop.
I am still not sure what "drawnow" is doing differently, but it somehow makes the loop breakable.
Thanks for your attention.


As stated in
doc drawnow
drawnow updates figures and processes any pending callbacks so it is like having a pause instruction in that it provides an interrupt opportunity in code (as well as updating the UI).

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