Interrupting a Callback with a Push Button

I have a while loop inside a certain callback function. Inside the while loop, I want to check whether a certain push button has been pressed. I tried using drawnow but it didn't not interrupt the current callback and process the push button code. Any ideas ?

Answer 1

[Adam Danz]

"Inside the while loop, I want to check whether a certain push button has been pressed"

The callback function for the pushbutton should toggle a stored value from 0 to 1 (for example). How you store that value will depend on how your app was developed (guide, app designer, or uicontrol). Within the while-loop you can add a condition that checks the value of the variable altered by the pushbutton. If the value is 1, you can use "continue" to skip to the next iteration and you can set a flag that ends the while loop upon its next iteration. Alternatively, use "break" to exit the loop. You can then reset the stored value back to 0.

Note that the placement of this conditional within the while loop is important and you may want to place this condition at sevearal areas within the while loop. All execution of the while loop prior to the condition will still be executed.

If any of that is unclear, please add some detail so additional suggestions can be more concrete.

[update] Here's a functional demo you can run. It creates a simple GUI with two push buttons: Go & Stop. Press Go and watch the command window count to 1000 within a while-loop. Press stop to stop the process. Then press go again to restart from the beginning.

% Create GUI
h.fig = figure();
h.but1 = uicontrol(h.fig,'Style','PushButton','Units','Normalize','Position',[.1,.8,.2,.1],'String','Go');
h.but2 = uicontrol(h.fig,'Style','PushButton','Units','Normalize','Position',[.1,.6,.2,.1],'String','Stop');
h.but2Val = false;  % Default stop-button-off
guidata(h.fig, h)

h.but1.Callback = {@button1CallbackFcn,h};
h.but2.Callback = {@button2CallbackFcn,h};

function button1CallbackFcn(hObj,event,handles)
% Responds to "go" button; counts to 1000 in command window
c = 1;
handles.but2Val = true;
while c < 1000
    pause(0.1)
    handles = guidata(hObj.Parent);
    
    % Check if STOP buttons has been pressed
    if handles.but2Val
        c = 1000;                 % Short circuit the while-loop
        handles.but2Val = false;  % reset stop-button to off
        guidata(hObj.Parent,handles)
        continue                  % skip the rest of this loop
    end
    
    disp(c)
    c = c+1;
end

end

function button2CallbackFcn(hObj,event,handles)
% Responds to "stop" button.  Toggles the but2Val to TRUE
handles.but2Val = true; 
guidata(hObj.Parent, handles)
end

For app designer

Here's a simple implementation of this idea in app designer. It requires a button that starts a process and a "state button" used to interrupt the process.

   % Button pushed function: Button
        function ButtonPushed(app, event)
            % Count to 1000 unles interrupted
            c = 1;
            while c < 1000
                pause(.1)
                disp(c)
                c = c+1;
                % Check if state button was pressed (from false to true)
                if app.StateButton.Value 
                    c = 1001; % short circuit the while-loop
                    app.StateButton.Value = false; %rest the button state
                end
            end
        end

[Joseph]

I am using the app designer to create the gui. The defined functions don't include the 'handles' parameter, is it possible to use this method or is there another way for the app designer ?

[Adam Danz]

It's definitely possible to adapt this model to appdesigner. The "app" variable is analogous to the handles variable.

Give it a try and if you get stuck I'd be glad to help out.

[Joseph]

The problem with my app is that the callback function of the stop button is completely ignored when the original callback is being executed. So if I make the stop callback alter a certain variable, the value of the variable will only change after the loop finishes, and the loop won't break when the stop button is pressed. How can I check whether the stop button is in queue ? Is there an event log I can check ? Is there another trick you recommend for this issue ? Thanks.

[Bruno Luong]

You can always control the callback entering and exit displaying something from the callback function (example in the video link).

Note that for GUI you have two properties that is important BusyAction and Interruptible

See This Video and this page

In your case I recommend to set the primary button Interruptible to 'on' and BusyAction to 'cancel', the Stop button Interruptible to 'off' and BusyAction to 'queue'.

However you cannot control finely the priority level of the tasks, and it entirely possible that the callback is triggered with large latency and too late for you to catch it.

Don't for get to call "drawnow" now and then within your calculation-loop. I believe the drawnow command might trigger some callbacks in the queue.

I can't speak for App Designer, I haven't yet use it (our GUI are fairely complex and use often java stuffs from Yair's undocumented blog so I'm not ready to jump to the new boat).

Beside using GUIDATA, or nested function suggested by Adam and Stephen, you can use SETAPPDATA/GETAPPDATA, SET(hObject,'UserData'), PERSISTENT variables or GLOBAL variable to set/check the state of the button and whereas user click on it. All those are common technique of GUI variable communication, and you select whatever the technique that you feel comfortable with.

[Adam Danz]

@Joseph, a very simple implementation of this idea to App Designer has been added at the bottom of my answer. It only requires a regular button to start the process and a STATE BUTTON to interrupt the process. See the comments within the code for more detail.

State buttons take a value of TRUE or FALSE so when the state button becomes TRUE, the process is interrupted and the state button is set back to FALSE.

Answer 2

[Stephen23]

" Is there another trick you recommend for this issue"

The simplest solution is to use nested functions: simply define a logical value in the main workspace, check it on each loop iteration, and change it using a (nested) callback function. Depending on how the loop is defined, you might also need to consider making the appropriate callbacks interruptible:

https://www.mathworks.com/help/matlab/creating_guis/callback-sequencing-and-interruption.html

I use very similar methods to interrupt demo modes of GUIs, e.g. check out the colornames_cube GUI demo mode:

https://www.mathworks.com/matlabcentral/fileexchange/48155-convert-between-rgb-and-color-names

Important: nested functions do not work with GUIDE, I have no idea if they work with App Designer, but really writing your own GUI code gives much simpler code with much better control over your GUIs anyway...